A few scanning tips


The First Fundamental Concepts of Digital Images

Scanning and Printing, and Video screens too

Digital images are new to some, and there certainly are things that we definitely need to know, about how it works. If you have a question, the idea is that it's hopefully answered here (at this site somewhere). There is quite a bit about using digital images and resolution here.

This is the "digital in a nut shell" view, and then there are also more pages here starting at the next following page. This is just a summary of the First Basics about using images that you just gotta know. It's fairly simple, but this much is pretty critical to know about using digital images. A digital image is one that has been scanned (in a scanner or in a digital camera), so that the colored scene image areas have been "digitized, meaning millions of tiny colored dots in the scene are replaced with numbers , which are called pixels. A pixel is a numeric representation of the one single color of a very tiny dot of scene area (24 million of them in 24 megapixels). Image files contain millions of pixels (megapixels). Each pixel has numeric specification of the Red, Green, and Blue components (called RGB color) of the color of each tiny scene dot area. RGB color is here. An image file does have some size in bytes of storage, but to use it, the image size is specified in pixels. Repeat, image Size is in pixels, NOT bytes. Bytes are not useful numbers in using the image, except for storage needs.

There are two very different ways needed to use images (including digital camera images): 1. printing or scanning or 2. video screens (meaning all types of video screens, like desktop monitors, cell phones, television sets or projectors. I call it all video, but meaning a screen instead of paper. My own usage is that viewing the image either involves paper (scanning or printing), or it doesn't involve paper (video screens). The difference is that paper is dimensioned in inches (images needing more conversion work), but video screens are already dimensioned in pixels (like a 1920x1080 pixel video monitor screen).

We scan and print for the capability of our output device (video screen or printer). We choose the scan resolution based strictly on the needs and possibilities of the output device that will use that image. That output device is normally a printer or a video device, and there are limits of their capabilities.

The term dpi is used to mean "pixels per inch" for resolution of an image. Dpi is actually technical jargon for "dots per inch", but dpi definitely means "pixels per inch". Some want to call it "ppi" now as more specific (and everyone uses either one they choose, but you must recognize either term as "pixels per inch"). I am old school, so it's dpi here. But a pixel is in fact a dot of one color, and dots per inch (dpi) has always been used for printing or scanning resolution. Probably your first question is how much dpi? The more pixels that are in an inch means smaller finer detail can be seen, however the device showing it (a printer or a video monitor) has definite limits of how much resolution they can handle. So we scan for the purpose of the device that will use the image.

A different subject, but to get it out of the way, there is also a different usage of "dpi" by ink jet printers to mean "ink drops per inch" used by their print head hole-spacing geometry, but this is NOT image resolution. Instead in ink jet printer print heads, it is about color precision when trying to reproduce one of 16.7 million colors using only 3 or 4 colors of ink. Ink jets printers must print several dots of 3 or 4 ink colors in the area of a pixel to reproduce the color of that pixel. Even if printing B&W photos, there are 256 shades of gray to reproduce (with only dots of black ink and blank spaces on white paper). But this ink drops per inch number has no exact meaning to us, other than as a relative color quality number. So printers today no longer are showing that ink drop dpi term, replacing it by simply asking a "Good, Better, Best" type of choice about specifying color quality, which is all we need to know, instead of a number like 4800 dpi that no one actually understood.

Scanning typically involves scanning paper, probably photos or documents or magazines, etc. Printing that image typically involves paper too. And the fact is that paper is dimensioned in inches (or mm or cm, same paper concept).
Basic Concept 1: If you specify to scan a 6x4 inch area (like a photo) at 300 dpi, the scanner will create an image of size (6x300)x(4x300) = 1800x1200 pixels (simply 300 pixels per inch). That's all there is to the dpi resolution term... pixels per inch. The more pixels across an area, the more detail can be distinguished in it. Detail in digital work is seen as just a color change, represented by at least two pixels of different colors making an apparent "edge". Pixels are similar to the way we perceive images in tiles arranged to make images... just a dot of color.

Printing:   Basic Concept 2: Our printers really cannot reproduce detail of more than about 300 dpi (due to needing to combine several ink dots of 4 ink colors (Cyan, Magenta, Yellow, Black, or CMYK) in the tiny space of one 300 dpi pixel). So printers are clearly designed to print color at a maximum 300 dpi (because that's what the human eye is able to see, and more would have no purpose). There are other scanner modes, like line art (scanning black ink on white paper, like a text document or newspaper cartoon drawings maybe, but line art pixels are either black ink or a blank on white paper) that can be scanned at up to 1200 dpi, and printed at 1200 dpi (and up to 2400 dpi commercially), because this mode has no colors to reproduce with only a few inks.
Basic Concept 3: Photos are highly detailed, and best printed at 300 dpi (which is what our printers are designed to do), but we scan with dpi based on the degree of enlargement we intend to show.
Basic Concept 4: If you scan a photo at 300 dpi, then also print it at 300 dpi, it will print the same original size scanned, like say same size on paper as the 6x4 inch photo scanned. That is because 300 dpi means to the scanner that it scans 300 pixels per inch, in every inch. A 6x4 inch image will create (6x300)x(4x300) = 1800x1200 pixels. To the printer, 300 dpi means it spaces the pixels 300 to an inch, so 1800x1200 pixels print (1800x300)x(1200/300) = 6x4 inches size on paper. This is just how things work, dpi is pixels per inch, and understanding that will be critically important to your image work.

My purpose of showing all these numbers is that they are important, so shown so you can think them out to fully understand pixels per inch, how to use it, and then remember it, because if you work with digital images, you will be doing a lot of it. It is all pretty simple, but important, and easy to understand.
  4 inches scanned at 300 dpi is 1200 pixels.
  1200 pixels printed at 300 dpi is 4 inches on paper.
That's really about it, and other than 2+2, there is not much math more simple than this. 😊

If you wanted to print at enlarged double size, then scan at 600 dpi and print at 300 dpi and it will print double size. Because 4 inches scanned at 600 dpi is 2400 pixels, and 2400 pixels printed at 300 dpi is 8 inches. We can do these even numbers in our head. If you want to print 10x size, then scan at 3000 dpi and print at 300 dpi, however ...
Basic Concept 5: most color photo prints really don't have more than about 300 dpi of detail to give to the scanner (photo prints are designed to be viewed by human eye, and are NOT designed to be enlarged). If enlarged too much (more than the detail you actually have), they are just blurred stretched over a larger area. Enlarged 10x means seeing 1/10 the resolution. But if you had 3000 pixels per inch, that 10x ends up still at 300 dpi, so that one looks great. But if your 10x only started with 300 pixels, it ends up at 30 dpi, and you won't like that.
Basic Concept 6: Scanning film does have much more resolution (designed to be enlarged), and can use up to about 3000 or 4000 dpi, and film is typically small, so then 10x enlargement is not so ridiculous. The high resolution is used for enlargement of film. Same with digital camera images, they also have many pixels for much higher resolution, so can also be enlarged well. But paper prints don't enlarge well.

Video monitors and printers work very differently from each other, and must be discussed and used one at a time. Video monitors have no internal use for the dpi term. Printers do use the term dpi, meaning pixels per inch to scale the size of printed images (so 1200 pixels printed at 300 dpi would scale to be 1200/300 = 4 inches size on printed paper). Which is simple, but it is unique.

But video monitors don't know the term dpi, because monitor screens never heard of inches, and don't know their size in inches (not internally... for example, you can show a TV image on any size monitor, 12 inches or 72 inches, and both screens show the same 1920x1080 pixel screen). Video is already dimensioned in pixels, for example 1920 x 1280 pixels screen size is common today, and so pixels are simply shown one for one in those pixels). So because the screen size in inches varies (from cell phones to 72 inch wall TV), the 1920x1080 pixel screen varies in "apparent dpi". But approximations of apparent dpi are shown, perhaps near 300 dpi for cell phones or 100 dpi for desktop monitors... meaning only as a way to predict viewed size in inches on your own video screen. But images on the internet web are seen by viewers with all kinds of monitors.

The digital camera creates the image of the size of its sensor, typically pretty large, which is of course then already scanned (digitized, represented in an image file by digital numbers for pixels). The scanner creates the image according to the pixels per inch term dpi, and of course according the size of the area scanned. A 5x7 inch photo scanned at 300 dpi will create a (5x300)x(7x300) = 1500x2100 pixel image. (the point is, if necessary, you think out those numbers until understanding them is second nature). A video monitor or a television set screen might be purchased as some size in inches, but internally, it shows a screen dimensioned in pixels, like 1920x1080 pixels. Pixels are all that exists inside the video monitor. A 1920x1080 pixel image would fit perfectly on at 1920x1080 pixel screen. A 500 x 300 pixel image will fill 500 x 300 of those 1920x300 screen pixels, one for one. Image Size in inches on the screen is rarely the way to think of it (because a different screen may see it very differently), but those numbers will fill roughly 1/4 of the screen width and height in pixels. There are exceptions, if you show a camera image too large to fit on the screen, it normally will be automatically resampled smaller to better fit on the screen. But once you have created the image, then nothing in video is about inches.

Enlargement of images is important: An example is scanning 35 mm film at about 2700 dpi would create an image size enlarged to print 8x10 inches. 2700/300 = 9x enlargement, but the 8x10 paper has two unequal dimensions. Let's say 35 mm film is roughly 1.4x0.9 inches, so 2700 dpi would create (1.4x2700)x(0.9x2700) = 3780x2430 pixels, larger than your 1920x1080 screen, except showing it as is on a video screen would likely be automatically resampled to a smaller copy so it will fit the screen better, but probably not exactly, because the shapes of image and screen are likely different. Then printed at 300 dpi would be (3780/300)x(2430/300) = 12.6x8.1 inches, which you could fit on 8x10 inch paper.But showing it (screen or printer) larger than the film size would show increased fine detail, film does have it to give (up to about 3000 or 4000 dpi).

Too much enlargement: You can enlarge anything just a bit OK, maybe by 20%, but scanning paper (a photo print, or a document or a magazine or newspaper) simply does not have detail to give. A photo should be good up to 300 dpi. You can scan a color photo at 600 dpi and print 2x size at 300 dpi, and it will be enlarged, but noticeably degraded somewhat, possibly acceptable for some noncritical purposes, but far from your best work. At 900 dpi to print 3x, a photo will be very seriously degraded. Because enlarging 2x original size reduces its resolution to half. Film does have it to give, at least up to 3000 or maybe sometimes 4000 dpi. Digital camera images at 300 dpi and even 240 dpi will likely be fine. My 36 megapixel DSLR is 7360x4912 which if at 240 dpi is 30.7x20.4 inches. We don't view large prints at hand-held distances, and standing back a bit reduces the necessary resolution. 240 dpi would be fine at such size.

Digital Camera images: Camera images are the same size (in pixels) as their sensor, but 6000x4000 pixels is a good example (6x4 is 24 megapixels). But if you print 6000x4000 pixels at 300 dpi (3:2 Aspect Ratio), dividing by 300 dpi will need 20x13.3 inch paper. If you tell the one hour shop to print it 6x4 inches, they will, but will first resample it smaller so they can print it at 250 or 300 dpi. If you tell them to print 5x7 or 8x10 inches, they also will, but that paper is likely not the same shape as the image, so some part of the image will be cut off. But today, likely no human eye ever sees it to judge how to best crop it, the machine just gets it onto paper.

If that may matter to you (and it will), you will need a photo editor (I suggest better than a free one), but then my advice is to do this preparation yourself, so it will come out as you want it to come out. It's easy to make it perfectly match the paper size. Actually, Step 1, you first ought to consider cropping away unwanted areas, removing empty areas with nothing in them to show, and certainly removing any distractions that harm your image (this really can improve many pictures, and makes your desired subject larger in the frame). But if extreme, cropping reduces your pixel count. Then Step 2, crop image to match the paper shape you intend to print (Speaking of paper SHAPE, Aspect Ratio, Not about paper SIZE). Example, 8x10 inch paper is SIZE, but its SHAPE is Aspect Ratio 4:5) by choosing area and size to keep, and the area to let go. Step 1 and 2 might conflict, removing too many pixels to still print well, but if so, you can Undo the crops and try again, compromising. Then Step 3, resample to 300 dpi size (or to print size in inches), and Step 4, write a JPG file using a fairly high JPG Quality. This page has those full instructions on its second page. First page is just introductions. You possibly may want to print it on two shapes of paper, so you would do Steps 2-4 twice, for each paper shape. But without exceptions, NEVER overwrite your original image. That ruins it for future needs yet unknown. But do think ahead and edit the file names to tell you which shape to print which one. There are several very good editors, but for great convenience, you need one that specifically offers cropping to fit a specific paper shape (aspect ratio). Adobe Elements is one that does, without an annual subscription cost.

In closing this summary, the rules are different for images intended for printers or video screens, as described, so that's a big deal, but its easy, and many get enjoyment from it, photography is a good hobby. If you acknowledge and understand and remember these basics, you're almost there, but you will need to understand dpi. The basics really are just about this simple, but the following site pages detail the significance of these differences. There is lots here.

Basic Properties of Printed ImagesBasic Properties of Video Images
Image size is measured on paper in inches or centimeters (paper size is also measured in inches) Image size is measured on the screen in pixels (screen size is also measured in pixels)
Image size in inches does NOT necessarily vary with scanned resolution Image size in pixels DOES vary with scanned resolution
Image size in inches is modified on paper by scaling (by declaring a different dpi value) Image size in pixels is modified on screen by resampling (changing size in pixels)
Image pixels are spaced on paper using specified scaled resolution (dpi) Image pixels are located at each screen pixel location, one for one
Several printer ink dots (of usually only 4 CMYK ink colors) within a pixel are dithered (mixed) to represent any color of that image pixel One screen pixel location contains one image pixel, and can be of any RGB color value

So because of these fundamental differences, when this text says "it's this way" or "it's that way", then notice that it also says "for printing" or "for video". Don't get them out of context, because the two modes are very different, with different properties and concerns.

Questions most often involve dpi resolution

There are generally two different goals for creating an image (in scanner or camera), either to show it on the video screen, or to print it on paper. These uses have different rules. These digital basics are summarized here. Some Printing Guidelines are here. The bottom line is that we do have to know about pixels, at least that they do exist. Our digital images are dimensioned in pixels. Pixels are ALL that there is in a digital image.

The following specifically speaks of scanning and printing to make a copy an image or a document.

The purpose of high scan resolution is to create more pixels to provide for enlargement, so an important way to think of it is this way:

Print Resolution

Paper is dimensioned in inches (or mm). Images are dimensioned in pixels. Paper has a fixed spatial dimension. Pixels do not, we can print images any size (dpi), or show on any size of video screen.

To help distinguish your scanner menus about this: Line art mode is one bit data, only two colors, one black ink and white paper, which is ideal for inked line drawings or printed text documents. Only two colors allows higher resolution, and eliminates unwanted intermediate colors, like pink or blue color casts. Some scanners call it line art mode (and I do), but other scanners may call it B&W mode (meaning those two colors, NOT meaning photos).

Scanners have 3 Scan modesSome scanner models have
3 Source modes to scan
Color, or may be called RGB mode,
with 16.7 million possible colors
Reflective mode (photo prints or paper documents)
Grayscale, or called B&W Photo mode,
with 256 possible gray tones
Film slide (positive film)
Line art, or may be called B&W mode,
2 colors, either Black ink or blank White paper, for text or line drawings,
but definitely NOT for photos
Film negative (inverts and removes orange mask if Color mode)

Some flatbed scanners can scan both paper (photos and documents) or film (possibly as large as 4x5 or 8x10 inch film). Most other scanners scan only either paper or film. Film sizes may be limited, probably at 35 mm size, but a few film scanners used to include 60mm Medium 120 film. Reflected mode for paper has both the sensor and the light on one side of the paper (light is reflected from the paper). Film mode has sensor and light on opposite sides of the film (light travels through the film to the sensor).

What we really need to know first is how to use our images. How to scan what we need for our purpose, and how to print them right, how to properly make the best use of them. We have to learn that pixels exist, and that images are dimensioned in pixels. We have to think in terms of pixels. Then it all becomes easy.

We normally always want to print photos at about 300 dpi on paper. 300 dpi is simply the magic number representing the maximum detail that a good eye can normally see at close distance. Paper is dimensioned in inches, and images are dimensioned in pixels, so pixels per inch is a concern.

Honest, 300 dpi is an optimum and desirable maximum goal for printing color photos. B&W photos might offer just a bit more, but color prints are not designed to offer more. This is a given here, not for debate. Except actually, many one-hour photo print shops set their machines to print 250 dpi maximum, and it is adequate too. Dpi is pixels per inch, same thing (which is jargon perhaps, but naysayers see below).

Line art mode: You should know that Line Art is black ink on white paper (two colors, no gray tones), for example printed text or line drawings (like cartoons). See Scanning Line Art mode, but other than line art mode, we are mostly speaking of color or grayscale photo images here. However an exception is that B&W documents in line art mode (not photos, but text or line drawings without tonal data) can scan and print better at 600 dpi, or 1200 dpi line art for best commercial printing, meaning, with smaller "jaggies". But 300 dpi line art is still more then fair (fax is only 200 dpi line art). But for more complex tonal work (color or grayscale photos), our printer cannot print 600 dpi tonal detail. It is not designed to try, because our eye at normal viewing distances cannot resolve more than 300 dpi detail (Epson printer users may argue for 360 dpi, but the scanned photo likely does not have that much to offer).

So printers typically have a limit of about 300 dpi for how color images can be reproduced (but higher resolution is used for line art work). For ink jets (typically with four CMYK ink colors), several ink dots have to fit within one color pixel (mixed inks to reproduce the pixels color), which is quite a stretch even for 300 dpi pixels. Scanning a color photo print at more than 300 dpi is rarely useful, it only makes the file size larger.

However make no mistake, scanning film at very high resolution is necessary for enlargement, so that it could be printed large at 300 dpi. Scanning film at 3000 dpi and printing at 300 dpi, will be ten times larger dimensions than the film size (and film is designed to provide high resolution for enlargement) However photo prints and printing technology is designed to be viewed by eye, and enlarging photo prints does not work well. Prints are not designed with more to offer. Scanning prints at 600 dpi to print double size at 300 dpi will show signs of image quality suffering, and printing 3x size will be considered pretty poor quality. If you should have doubts, you should try this, actually printing that enlarged copy, so you will believe it.

First Basics of Scanning, Video, and Printing Resolutions

You really need to know this much about showing images. It will go so much easier when you know. And it's very easy too.

Scan resolution of 300 dpi means that the scanner will create 300 pixels for every inch scanned. Scanning five inches at 300 dpi creates 1500 pixels across it (to reproduce the five inches).

Showing a dimension of 1500 pixels on a monitor video screen simply will fill 1500 pixels of monitor space (or if the screen is smaller, a copy of the image will likely be resampled smaller to fit). The dpi number creates the scanned pixels, but then the dpi number is ignored by the video screen. Video measures in pixels instead of dpi.

Print resolution of 300 dpi means that the printer will space the available pixels at 300 pixels per inch of paper. Printing 1500 pixels at 300 dpi will cover five inches.
Or you can simply change the dpi number (called scaling, which is NOT resampling, and changes only the dpi number itself, and does not change any pixel), and then printing the same 1500 pixels spaced at 200 dpi will cover 7.5 inches.
Another way to print is that if you instead specify to print it ten inches, it will print 10 inches at 1500/10 = 150 dpi. This includes at the one hour printing shop, and most home printers can do it too. You should determine the final dpi result first.

A common purpose is to create a scanned copy to print at the original size of the scanned photo print on the home printer. Simply scan photos at 300 dpi, and print them at 300 dpi. Then the same dpi number means the source will be copied at same original size, at the optimum 300 pixels per inch.

Digital camera images are high resolution, which varies with sensor size, but perhaps more than 4000 pixels per inch or more (full frame) to maybe 2000 in cell phones. But camera does not know your goal, so the printing dpi number that the camera provides is likely not a useful number. Camera images are already digitally scanned in the camera file, so for maximum image quality, it's better to use that original file instead of scanning a print of it. The camera image (in the original camera file) might be for example about 6000 × 4000 pixels. At 300 dpi, it would print (6000/300 × 4000/300) = 20 × 13.3 inches (50.8 × 33.8 cm). See the Image Size Goal calculator below for the necessary minimum size to print or  send it to the one hour shop. Odds are good that it may not be the correct SHAPE to match the print paper, so to avoid bad cropping surprises, it is best to first crop it yourself to the proper SHAPE to correctly fit the paper size. See Resizing Images for an optimum way to crop to the proper SHAPE.


To create sufficient pixels to view in a print enlargement, the ratio of (scanning resolution / printing resolution) is the enlargement factor. For example,

Scan at 600 dpi, print at 300 dpi for 600/300 = 2X size (prints 200% double size)

Scan at 450 dpi, print at 300 dpi for 450/300 = 1.5X size (prints 150% original size)

Scan at 300 dpi, print at 300 dpi for 300/300 = 1X size (prints 100% original size)

Scan at 150 dpi, print at 300 dpi for 150/300 = 1/2X size (prints 50% half size)

If the original is 6 inches and you want to print it 4 inches at 300 dpi, scan at 300 x 4/6 = 200 dpi, print at 300 dpi for 200/300 = 2/3X size

If the original is 4 inches and you want to print it 6 inches at 300 dpi, scan at 300 x 6/4 = 450 dpi, print at 300 dpi for 450/300 = 1.5X size

Relax, most scanner software should easily compute any of this arithmetic for you. The Preview scan where you have marked the cropping to be scanned will know the Input size (scan size), and you can specify the Output size (printing size) at 300 dpi. Or you can simply set the Scale factor (the desired enlargement percentage) and the Output 300 dpi, and either way will do what you want. It will also mark your scanned image with your desired Output printing resolution. If you don't do it this way, and just say scan at X dpi at 100% Scale, then it will scan and set the printing resolution to be the same as scanning resolution, which you will need to change when you are ready to print. But whatever the print dpi number is, it will have no effect at all on images shown on a video screen.

A Beginners probable First Mistake

Beginners assume maximum resolution must be an advantage. But Scan Resolution is about Enlargement. It will be a big headache and a bigger mistake to scan at the highest resolution just because the scanner can do it. Scanning anything paper at maximum dpi will be a far-too-huge image. You will never have a use for that much size. Only scanning small film might possibly need near a maximum scan. We might imagine high resolution is about showing greater detail, and of course it can, but an image on paper cannot have nearly that much detail. Even if it did, the only way we can see that detail is to enlarge it bigger to see it better. We don't need more than 300 dpi for printing, and normally even less for the monitor screen (unless maybe it was something tiny).

But Enlargement is an important tool, sometimes necessary (especially from small film). We use high resolution for enlargement. We need more pixels to show it larger to see the detail. This is NOT speaking of resampling larger, which cannot increase image detail, instead it reduces resolution. This enlargement is due to greater scan resolution, creating more pixels reproducing finer image detail from the original image.

So scanning small film needs more enlargement just to view it. One example is to scan 35 mm film at 2700 dpi, and print at 300 dpi, for 2700/300 = 9X size enlargement. 9X is about 8x12 inches (about A4 size) from full frame 35 mm (about 1.4 x 0.9 inches, about half the dimensions of wallet size). The ratio of (scanning resolution / printing resolution) is the print enlargement factor. There are possible exceptions if it is tiny, like if you expect to crop small film seriously and still enlarge considerably, or maybe it is really tiny film like 8mm movie, then higher resolution may be needed, but those are special events. Try first planning what size image you will actually need.

The general use of film is to enlarge it for viewing, and it has fine grain designed to be enlarged, so a reasonable effort enlarges well. Photo prints are of adequate quality at original size, but prints are not designed to be enlarged again (and attempts will be degraded). 2x enlargement of a photo print might be still be acceptable for some uses, but it seems likely you won't like 3x (detail resolution will be low, spread out, not sharp). Working from the original digital image or from the film will have detail to enlarge better. Prints are already enlarged, and are designed as the final purpose, not designed to be enlarged. In contrast, the film is the original master source, with fine detail designed to be enlarged. Or digital cameras provide the original source file, even better (depending on image size). Either way, it can be better image quality to go back to the original source than to plan to enlarge a print copy. Copying prints at original size should work out well though.

Note that to scan a full page size at high resolution like 4000 dpi is nonsense, surely a misunderstanding. You're not planning a huge enlargement of that full page, and will have no use for such a large image. The concept is about desired enlargement, a goal for a purpose. A full page size is already large. When you just want to print a copy of a photo print or a document page at original size, just automatically scan and print at 300 dpi. One exception, if and only if the scan is line art mode, then maybe scanning and printing line art at 600 dpi can be a little better (no tonal colors to dither, and 600/600 is still original size).

Again, if scanning something small, like a 35 mm film frame, and wanting to print it large, like full page size, then the idea is to scan it at maybe 2700 dpi at 100% (or 300 dpi at 900%, which is the same result), either of which will create enough pixels so that then printing it at 300 dpi will enlarge it to 2700/300 = 9x, like to A4 or 8x12 inch size.

But if wanting to scan something and print it at same original size, then scan at 300 dpi, and print at 300 dpi, and you will reproduce original size at optimum image quality.

This is very easy, and very important to know, and this little bit to know may answer most questions.

The numeric requirement is that if you want to print an image as 8x10 inch size at 300 dpi, then this printing obviously will require an image size of:

(8 inches x 300 dpi) x (10 inches x 300 dpi) = 2400x3000 pixels.

Fill in your own numbers, but for example, if our film source to be scanned is say 1 x 1.25 inches, and if we want 8x enlargement (8 inches / 1 inch = 8x enlargement), then scanning at 300 dpi print resolution x 8x enlargement = 2400 dpi scan resolution will do it.

(1 inch x 2400 dpi) x (1.25 inches x 2400 dpi) = 2400x3000 pixels.

which is calculated to be the exact same requirement for pixels. Simply scan at (300 x 8) = 2400 dpi to enlarge for printing 8x original size at 300 dpi.

Or if we want to print a copy at the same original size, then scan and print at 300 dpi (which is simply 1x enlargement).

Or if we want to print as half size of original, then scan at (300 dpi x 1/2) = 150 dpi (and print at 300 dpi).

That little formula is the basic concept of digital scanning and printing. How hard is that?

Again, scanning 6x4 inches of paper at 100 dpi will produce
(6 inches x 100 dpi) x (4 inches x 100 dpi) = 600x400 pixels of image size.
Plug in the appropriate numbers to get the size image you want (in pixels) from what you are scanning (inches). dpi = pixels per inch.

That dpi concept is true for printing goals too, meaning that if you plan to print 8x10 inches at 300 dpi, then in preparation, you need to create in the ballpark of
(8 inches x 300 dpi) x (10 inches x 300 dpi) = 2400x3000 pixels.

Procedures we use

The scanner software tool will compute this math for you, and will help with this. It provides more calculator than this little one  , to make work easy for you. The concept is INPUT from scanner, and OUTPUT to printer. The typical scanning menu will show in advance, the size of the INPUT area to be scanned, the resolution and any enlargement factor, and the size of the OUTPUT image, in pixels and/or inches. Output for video screens is only concerned with output size in pixels, ignore output inches then. You can just make the numbers show your output goal.

The scanner tool converts an INPUT scan area to an OUTPUT print area, using dpi and enlargement, same as described here. Unless 100% scale (1x enlargement), the dpi value entered will be assumed to be the desired OUTPUT printing resolution value, and the actual scanning resolution is computed according to enlargement, which not many scanners will show. The two dpi are equal Only if at 100% scale, and the dpi seen will be the printing resolution goal (see this page). Otherwise, the scanning resolution will be Enlargement x Printing dpi, which you should know this even if not shown. The scanner may also have a beginners automated menu to hide much more of this detail.

Repeating concept, if you scan a slide at 300 dpi 9x enlargement, the output file will be marked with 300 dpi, to print 9x size, as is.
If you scan it at 2700 dpi 100%, you get exactly the same pixels, but the output file is marked 2700 dpi, to print original size (at excessive resolution specified). We scale this file to 300 dpi later, before we print it (which simply only changes the dpi number in the file, so then these are equivalent in that way).

In practice, the computed scan resolution number might come out like maybe 1037 dpi. Which will work, but good practice for critical work might increase it to the next even number the scanner menu offers, like when 150 300, 600, 1200, 2400, 4800 dpi is offered, choose the next larger one, like 1200 dpi in this case. Then the final scan can be cropped and resampled as necessary (see this page). Not much actual difference today, but the even menu value can be more a critically precise scan (the scanner carriage motor moves in certain steps, and the scan sensor also has certain intervals, which these defaults will match. Then the photo editor can better resample the larger full image).

There is a qualification about printing. There are two methods that should be mentioned:

Any time when "fitting the paper", your job is to provide sufficient pixels to match your desired actual print size. Optimum is about 300 pixels per inch for best quality photos (or 250 dpi works well too). If you want 4x6 inches, that's 1200x1800 pixels. If you want 8x10 inches, that's 2400x3000 pixels. (Large wall posters probably have too few pixels for their size, and must be less dpi, but they are viewed at greater distances.)

There is another procedure often necessary, because camera images and print paper are often Not the same SHAPE, and so cannot be fitted to the paper as is. For example, a 4x6 image will not fit 4x5 paper. The print is not going to work out well unless they are the same shape (see this about cropping to fit the paper). But the most essential basic fact is that images are dimensioned in pixels, and that is how we must think of it. That makes it easier, not harder. And it is necessary, and it's very important to plan your pixels.

Scanning and Printing Goals

Sufficient pixels to print at least at 250 dpi, and optimally at 300 dpi is a proper goal to print photo images. This is true for images sent to online printing services, or true of your own ink jet printer too. More than 300 dpi really cannot help the printer (in color mode), but very much less is detrimental to quality. This is very simple, but it is essential to know and keep track of. This simple little calculation will show scanned or printed image size.

Image Size Goal for
desired Scan or Print Size

To scan or print x

   at dpi resolution

This little calculator has these purposes:   (or there's another fancier dpi calculator)

3000 pixels spaced at 300 pixels per inch (dpi) is 10 inches. This easy fact is about the least that we all should know about using our images. There is a larger dpi calculator that knows more details about scanning, printing, and enlargement.

So repeating to make a special point. We know that to scan a 35 mm film slide to print at A4 or 8x12 inches, we need to scan at about 2700 dpi, to achieve about 9x enlargement. However, we do not specify BOTH 2700 dpi and 9x numbers at once. One of them at a time is plenty.
  1. We can scan at 300 dpi and 900% (9x enlargement), meaning it will scan at 2700 dpi and will create enough pixels to print about 8x12 inches at 300 dpi, about 2400x3600 pixels.
  2. We can scan at 2700 dpi and 100% (1x), and it will scan at 2700 dpi. The file will come out marked as 1.4x0.9 inches at 2700 dpi as specified, and it is the same pixels, about 2400x3600 pixels. Then we easily scale it later to 300 dpi ourself, which is perfect too.
  3. Scanning at 2700 dpi AND 900% is absolutely NOT what was intended, a beginners mistake, enlarging 9x twice. It would scan at 2700x9 = 24300 dpi, about 21600x27000 pixels (from a slide), and if it is able to complete, it would be 81x enlargement at 300 dpi. Choose either method A or B, but not both.

When scanning paper prints, it's important to realize that an area scanned at 300 dpi will create the pixels necessary to also print the same size at 300 dpi. The concept either way is pixels per inch. 300 dpi is likely what you want for a photo copy job (but a line art mode scan of black text or line drawings can use 600 dpi well).
But film has to be enlarged, so it has to be scanned at higher resolution to create sufficient pixels for enlargement.

This printing dpi number does NOT need to be exact at all, ± 10% of 300 dpi won't matter much, but it should be ballpark. But planning size to have sufficient pixels to be somewhere near this size ballpark (of 250 to 300 pixels per inch) is a very good thing for printing.

The Tricky Difference about Video Screen Resolution

The role of scan resolution for the video screen is to create the appropriate image size, the dimensions of image width x height in pixels. Scanning dpi and enlargement do affect the final video image size (pixels), but on the screen, only the output image size in pixels has any importance (not inches). Video screens and printing on paper are just simply very different concepts. Video is easier actually, but we have to understand the difference. It should all be obvious, and second nature, once we know.

Your digital camera does not use paper either. Film was dimensioned in mm like paper, but a digital sensor directly creates an image dimensioned in pixels. Yes, the sensor is dimensioned in mm (like film), but the "scanning" job is complete now, and what we get out of the camera is pixels. However, your camera will also stick in some arbitrary dpi number, maybe 180 or 240 or 300 dpi. At best it is just a bad guess, because the camera has no clue what size you might print it, if at all. This might indicate a huge print size of a couple of feet? But that has no meaning there, don't worry about it yet. You will determine a useful print size and dpi number when you are ready to print. This dpi number does not affect the pixels that the camera creates. Dpi is just a separate lone number stored someplace in the image file to later tell the printer how to space the pixels on paper, and this number can simply be changed at will (called scaling, to match the paper size).

Digital home movies were 640x480 pixels in the past, and today are 1920x1080 or 1280x720 pixels, to match HDTV screens which are also one of the same two sizes. Digital is about pixels.

Ifs and buts ... (confusion factors)

  1. Your photo editor or photo viewer shows pixels. Then images too large for its window are typically automatically resampled smaller to fit in the window. Your photo editor should then indicate the relative new size, like 1/4 size, or maybe new pixel dimensions of the copy. Then our screen then shows that smaller copy, one for one with its pixels. The image file data you are editing remains full original size (unless you manually resample it), but the video view you see may be much smaller, in order to fit on the screen. Editors will provide an option to see full 100% Actual size (camera images are normally much larger than the screen).
  2. Document software (like PDF or word processors like Word) are different than photo editors. Designed to reproduce paper, so the images in those documents do use image dpi (or your scale of desired print size) to be sized relative to the paper page size specified. That's special, showing inches of paper on a video screen. If the image is supposed to be four inches wide on paper, it will fill half the width of paper that is supposed to be eight inches wide. Actual size seen on screen will be resampled to whatever size your screen is, or whatever size you zoom it.
  3. Web browsers are different than photo editors. Typically they do show a 400x300 pixel image at 400x300 pixel size. Default settings probably reduce the size of images too large for their window. However, similar to paper documents, when we zoom the web screen for larger text, the browsers today also zoom the images, to maintain the same web page proportions, to still appear like it would print on paper. That's special too.

But otherwise, video screens normally show pixels as pixels, one image pixel on one screen pixel. That does make converting paper documents dimensioned in inches be a problem on the screen, which has no concept of inches or dpi. See method about video "logical inches" (on its 2nd page).

Scanning with a camera

The digital camera can work very well to copy photos, but (like any other photo), copy work depends on your procedures.

The scanner is carefully designed to control the lighting, the position of the light, and the color and white balance of the light. It holds the media square and flat. These are big jobs. The scanner is designed to do the copy job well, and automatically.

However, all of this is left up to the camera operator, who needs to have some understanding of how to do that part well too. This camera copy setup is Not automatic, and the details of what you do are important. To use a camera for scanning documents, see Google for the standard copy procedures. It will be a closeup, so you surely will need to use a macro lens.

Cameras don't use dpi, their sensor creates a fixed image size in pixels. However, the sensor is composed of pixels (pixel size dimensioned in micro-mm), and the pixel spacing does determine a dpi resolution, which is typically a very large number, because even a full frame size sensor must be enlarged about 8x to print an 8x10 inch print. A cell phone or compact camera image is enlarged 30x or 40x to print the same 8x10. The computed scan dpi is pixels shown / inches shown. So photographing a 6x4 inch printed photo copy into (if 24 megapixels) 6000x4000 pixels, which is 6000 pixels / 6 inches (pixels per inch) = 1000 dpi, which is scan resolution. Then this 6000x4000 pixel image printed at 300 dpi is 6000 pixels / 300 dpi = 20 inches wide on paper. You'll likely want to resample it smaller to 1800x1200 pixels to print a copy at original 6x4 inch size at 300 dpi.

See copying slide film with the digital camera.

The next page will start with "What is a pixel?", but before we get started, a note relating to context usage of "dpi":

DPI, PPI, SPI - What's in a name?   DPI vs. PPI?

Any controversy in terms is that ink jet printer ink drops and image pixels are very different concepts, but both use the term dpi in their own way (dots per inch).

ink jet printer ratings can refer to printer ink drops per inch as DPI which is NOT AT ALL the same thing as printing image pixels per inch. Ink jet ink "drops per inch" will be a number several times larger than pixels per inch. Ink jet printers typically have only four colors of ink (CMYK), but need to reproduce color photos containing possibly millions of different colors. Combining several colors of tiny ink drops (the four colors of CMYK ink) into each pixel is just their way of mixing the primary color inks to be one of the millions of image colors that one pixel represents. Each image pixel is only some ONE of millions of possible colors. The ink jet "ink drops per inch" is a color quality (accuracy) specification, NOT about image resolution detail. A pixel's RGB color has 256 shades of Red, 256 shades of Green, and 256 shades of Blue. So possibly a pixels color could be one of 256x256x256 = 16.77 million different colors. The printer cannot possibly put up to 256 dots of each of RGB ink into one pixels space (at 300 dpi pixels, that is 0.085 x 0.085 mm areas). These are such different concepts that some people imagine we ought to reserve the term dpi for those ink jet ink dots, and reserve use of ppi only for image pixels. Not really a bad plan, except that this view fails to recognize existing real world usage, which had always used dpi for image resolution, so some think ink jets should have found a different term. And FWIW, today's ink jet printers are in fact minimizing their usage of the term "4800 ink drops per inch" (which was not understandable anyway), and are now instead adopting a "Good, Better, Best" system of specifying color quality settings (accuracy of the reproduced color of the pixel).

Image files simply have NO capability to specify printer ink drops per inch. Only the printer driver does that (Print menu). Image files can only specify the pixel's color, and the pixel spacing when printed. Image parameters are only about pixels, with no concept of ink drops. If the image file specifies dpi or ppi (interchangeable), it is the image resolution for printing, but it can only be about pixels. Ink drops is a different issue, about how the specific printer tries to reproduce the ink color of those pixels.

We may hear scanning resolution called spi (Samples Per Inch), and that is indeed what it is. We often hear image resolution called ppi (Pixels Per Inch), and that is indeed what it is. The spi and ppi terms are correct. However historical and common usage has always said dpi for image resolution, meaning pixels per inch, and that is indeed what it is. Dpi is fully interchangeable with ppi. Pixels are conceptually a kind of colored dot too, and resolution has always been called dpi, for years before we had ink jet printers. Dpi is just jargon perhaps, but it is a fact of life, and make no mistake, it always has been. Scanners and scanner ratings say dpi too, meaning pixels per inch (see dialog pictures here, here, here, and here). I habitually always say dpi myself, an old habit. Image resolution has always been called dpi (referring to scanning or the printed image, pixels per inch of printed paper).

We may use either term of our own preference, but we certainly need to understand it both ways. You will see each used somewhere. Some scanners and photo editor programs have even switched to saying ppi now, and there is nothing wrong about that. But others have not switched, so insisting on conformity requiring all others to only say ppi will necessarily encounter much opposition, because the real world simply isn't that way, and obviously not all are willing to switch yet. Their view is that dpi has always been the term for image resolution, regardless of any whippersnaper's opinion. 😊

Continuous Tone printers (dye-subs, and the chemical photo paper printers) don't print discrete ink dots of four colors like ink jet printers must. Instead they mix the color of the pixel directly, and they print pixels (called continuous tone). There are no dithered ink dots then, just pixels. But these printer ratings still refer to the spacing of those image pixels with the term dpi, simply because dpi has always been the name for "pixels per inch".

Scanner ratings also always called it dpi, also referring to Optical Resolution of pixels of course (scanners don't use ink dots). You have never heard of a 4800 ppi scanner (and there are no ink drops used in scanners — scanners create pixels).

The technical specifications at the very heart of our digital imaging definitions say dpi for image resolution:

  • JPEG file specifications See page 2 and 5, dpi, "dots per inch", meaning pixels (revision 1.02 is the final version).

  • TIFF file specifications See page 38, "dots per inch", meaning pixels (revision 6.0 is the final version).

  • EXIF 2.2 specifications See pages 19, 90, 101, 104, 108, 112, "dpi", meaning pixels. Exif 2.2 is the final revision. The number 72 dpi there has no significance, except in Adobe programs which assume blank dpi information is considered to be 72 dpi. The origin was that the early Apple Macintosh video 512x324 pixel screen in 1984 was called 72 dpi, meaning pixels. It only showed text back then.

There are no ink drops in these specifications.

But there is no harm done if you want to call it ppi. It is understood both ways.

I'm OK with either term you choose to use, I understand it either way. My point here is that we must understand it both ways, because we will see it both ways, often, in the real world.

It's easy, not a problem — the idea of printing digital images is always about pixels per inch, so when the conversation context pertains to images instead of printers, all of these terms, spi, ppi, and dpi, are exactly the same equivalent concept — they all mean pixels per inch. That's the only thing it could mean. If the conversation pertains to ink jet printers, then you need to read it closely.

There is no problem understanding any use of dpi if you know the context. Context always means the only thing it can possibly mean. If the context pertains to images or resolution or printing pixels, dpi means "pixels per inch". If the context pertains to ink jet printer color quality ratings, dpi means "ink drops per inch". There is no other meaning possible. This should be clear and no big deal — the English language is full of multiple context definitions.

So yes, ink jet color quality rating dpi is something entirely different than printing resolution in dpi, referring to ink jet printer ink drops per inch instead of image pixels. Ink drops per inch is only about color quality (dithering quality, about color accuracy), and is NOT about printing resolution. Ink drops per inch is NOT data stored in the image file. It is NOT asked in any photo editor EXCEPT only as a question that the print driver asks as you go to print it (important to ink-jet dithering, but actual print shops won't ever ask, they know what to do). The ink jet printer likely only has four colors of ink — or maybe a few more in some cases — but nowhere near the 16.7 million color possibilities of 24 bit color, one of which the pixel might be. The concept of a pixel is in fact a colored dot, and the printers goal is to reproduce the color of that dot. But the limited ink jet printer cannot directly reproduce the color of a pixel. The ink jet printer just tries to approximate the color of a pixel, with multiple ink drops chosen from four colors of ink (maybe 5 to 8 colors in a few cases). This method of simulating colors by combining multiple individual ink drops is called Dithering. If it was, say, a 250 dpi pixel (size, 1/250 inch), it must make several tiny ink dots of its few CMYK ink colors, which are located with the perhaps 1200 or 1440 dpi spacing within that pixel's 1/250 inch space (not very exactly, see the Printer Basics section). These are multiple dots of ink (in one pixels space) attempting to reproduce the color of one pixel, which itself is a dot of color in the image. Any color error is carried over to adjacent neighbor pixels, which are then intentionally tinted the opposite way to compensate. The printer is trying the best it possibly can to reproduce the pixel's color, but ink jets with four inks cannot reproduce colored pixels directly. The point is, image pixels and ink jet printer ink drops are NOT the same thing at all. Yet the spacing of both is called dpi, with very different meanings, understood in context of use. If it is about ink drops, it is about ink drops. If it is about pixels, it is about pixels. Image files do not contain any notion about ink drops, so any discussion about images is about pixels. Seems simple.

Dwelling on the obvious, but the reason for all of the above is because "dpi" has simply always been the name of printed image resolution. That's plenty good enough for me. There are no ink drops in image files. No ink drops in scanners or cameras either. Those fundamental and elite specification documents written by the experts do not use ppi one time — dpi has simply always been the name of it. I always say dpi too, for same reason, simply because that's always been the name for pixel resolution. Some users today think that sounds wrong, and their notion is to call it ppi. But dpi has always been the real world name of it, however what's important to you is that you do need to understand it either way. Basically, if usage context refers to images, dpi means pixels. If usage context refers to printer printheads, dpi means ink drops.

Yes, sure, the term dpi does have multiple meanings, same as most English words do. Now we have many newbies getting into digital, and this term confuses some of them who dream up their own imagined restrictions, demanding the term dpi ought to be reserved only for printer ink drops (simply their notion, about how they wish things were instead). They apparently don't know the real world term for image resolution has always been dpi. (Sorry, yes, I am going too far here, just for them)

Also, search Google for the phases "72 dpi" and "72 ppi" (using the quotes to be a phrase). We all know "72 dpi" is about video pixels, and has absolutely nothing to do with ink drops, but look at the count of the hits... in 2023, I see 28.8 million for "72 dpi", and only 341K for "72 ppi". That's 84 to 1, nearly two orders of magnitude, an overwhelming difference in popular usage. I am just teasing the troops, but some of them should "get over it". Dpi is the name of it, always has been, and we might as well get used to it.

Repeating, but fortunately, ink jet printers are phasing out their use of the pirated term dpi, and now the printers color quality selection menu choice is usually stated as Good, Better, Best (HP), or Fast, Standard, High (Canon), or maybe Draft, Standard, High (Epson), regarding choosing color quality. Not confusing us with X dpi ink drop density seems good to me, and color quality is what it is (color accuracy). Many beginners have never even seen the idea of ink drops per inch today.

Ppi is a relatively new term, we never saw it until recent years, but we are seeing ppi used some now, and it seems a perfectly fine name too, since dpi with respect to images does mean "pixels per inch". It might even have been a good plan, but it was not the plan. It may sound a bit silly to pronounce ppi, but recent photo editor software often does say ppi, while scanner software generally says dpi (but we see some exceptions to both). But usage makes either term correct now, even if the long established name for image resolution has always been dpi, for many years. Nothing makes ppi mandatory.

I am NOT arguing we must use the term dpi. My choice is dpi because that's how I learned it. But I don't care which you use, I understand it either way. I am just saying you absolutely should understand both too, because both is what you will see today, it is used interchangeably. I learned it as dpi (not that my choice is important, but dpi is what it's always been called). Mainly, my rant is simply explaining why you need to expect to see it used both ways. It's OK with me if you want to use your own preference, since both terms mean pixels per inch. We all understand it either way, and you should too. BUT again, regardless of your own preference, you definitely will often see both dpi or ppi used, so for your benefit, in the real world, you MUST understand what you read both ways. It should be second nature for you, not confusing. If about images, it can only be about pixels. If about printers, it may be about ink drops in the print menu, but that won't be in any Edit menu. Both can be dots. That's simply how things are (and you already know that). Think of this as training to understand what you will see elsewhere.

I am not arguing which is better, or how things could have been, or ought to have been. It simply wasn't, and I am observing what actually is. Basic Concepts are about what is, which is both ways today. The important thing to me (and my big peeve) is that it certainly does the beginner no favor for the "holier than thou" to stand up and incorrectly shout that dpi is wrong, and that everything we read everywhere is therefore wrong. That is confusion indeed, harmful, not helpful, since it does not promote understanding of the real world that actually is. That's my fight. The only reasonable action is to instead simply state that both terms are used, they mean the same thing if about image resolution. (To those so-called "advisers" — remember, when everyone except you is wrong, you probably don't understand the situation.)

The only proper instruction is that both terms are used, interchangeably. Expect to see either. To understand what we read, we must understand it either way.

There is really no problem understanding the two uses of the word dpi if you know the basics, and realize the context. It always means the ONLY thing it can possibly mean in context. This should be no big deal, the English language thrives on multiple context definitions. If it is about ink drops, it is about ink drops. If it is about pixels, it is about pixels. If it is about the images themselves, it is Not about ink drops. If it is about the printer, it might be about spacing the image pixels, or it could be about the spacing of the ink drops to color each pixel (but ink jet printers today are instead adopting a "Good, Better, Best" notation about specifying ink drop color quality, instead of saying dpi).

If the usage context pertains to images or printing pixels (and it almost always does), then dpi always means "pixels per inch". So does ppi, same thing exactly. It cannot mean anything else, printing is about spacing pixels on paper. The two terms are fully interchangeable, use either according to your whim (but we gotta understand it both ways). If we have a 300 dpi image, both terms mean it will print at 300 pixels per inch (pixel spacing on paper), so that 300 pixels will cover one inch.

If the usage context pertains to ink jet printer ink dot ratings, dpi means "ink drops per inch" (but since the ink drops are actually larger than their spacing, the rating is more specifically about carriage and paper motor stepping intervals). If the printer rating is 1440 dpi, it means its motors can space 1440 ink dots per inch while trying to simulate the color of the pixels in that 300 dpi image. The pixels still remain 300 dpi size (as best as they can be reproduced). Most printer drivers have renamed this now anyway, as Quality, offering Good, Better, Best Quality, or maybe Fast, Standard, High Quality. This ink drop spacing is a quality parameter, about reproducing those pixels to the degree possible — it is not an image resolution parameter.

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