Details on this page:
Most camera images are usually originally too large to use (but which does allow many possibilities).
If preparing a scanned image, a common procedure that the meticulous use instead of scanning at a precise non-standard scan resolution (like maybe 328 dpi) is (if a standard menu resolution is not sufficient) is to intentionally scan a little larger by using the next larger standard scanner resolution menu choice. The reason is that the scanner hardware (the sensor pixels and the carriage stepping motor) can only do certain steps, only those specified standard values offered in the menu. Any other value is approximate resampling, not actual sampling. It is a small and hard-to-see difference (maybe a geeky thing), however the photo editor can do this resample better, after it has all the larger data.
We may not print the image on paper, in which case its shape may be more arbitrary. Or printing a smaller image on larger paper or viewing on a video screen is possibly not a shape issue. But if filling the print paper, it is very necessary to fit the paper.
For printing, FIRST, crop as desired to both fit specific paper SHAPE if important, while also adjusting crop size to improve artistic composition, meaning specifically adjust location of the crop box to its best location (keep important detail, and crop only unimportant). With
Then SECOND, resample that image to the smaller desired SIZE to print or view on monitor.
But save this edited image to a new file name. ALWAYS keep and preserve the archive of the original image, you may need to go back some day for some new plan.
Please realize that an easy "Crop to fit paper shape", and then a simple resample calculation (a calculator in the resample portion below) is all you need to know to print photos.
Cropping to print shape is a popular option, but not a universal feature in every program. If you have no tools yet, then just to mention one in the Free category is:
Faststone Image Viewer is free for home and educational use. It is a decent editor, certainly plenty for this procedure which is pretty easy to do. For deeper work, it has standard necessary tools like a Levels histogram. It does like to start images in full screen mode (without menus), which is good for viewing images, but working on images needs menus. Hitting the Tab key clears that, and menu Settings - Settings changes image launch to be Window mode. This option to fit image to paper shaper is menu Edit - Crop Board, and you move its location with the Right mouse button and size it with the image border marks.
Adobe Photoshop Elements (cost is $100 US one time) and Lightroom and Photoshop ($120/year subscription) also do this, and surely many others too. Matching the shape of the print paper is a basic and necessary printing procedure.
Adobe: The image shows the Photoshop Marquee tool (the moving "marching ants", like marching movie theater marquee lights — you have to see it). Elements is the same. The Marquee tool is my choice, and there is also a "Crop tool" too do the same in Adobe Elements and Photoshop and Adobe Camera Raw (ACR).
To Fit the image to the paper shape, these tools when selected show a menu bar where you can select the Fixed Ratio option (seen in above image). Then with an aspect ratio specified, any crop you can draw with the mouse will be that aspect ratio (any variable "Size" that you mark by dragging the mouse right button, but the "Shape" will still continually fit the specified print paper). You can make it any Size you want (as opposed to Shape), and you can place it in any location you want. In Adobe, mark the crop box size, and while still holding the mouse button down, hold the keyboard space bar down, and then the mouse (button still down) will move the crop box to any location. Make your best choice of what the image subject will look like in the final print.
And there are other ways too... better ways. Other crop tools, many will offer a fixed ratio option. I usually do this crop in Adobe Camera Raw (select its Crop tool, then right click in the image for menu to specify its aspect ratio to use). Any crop then will only be the specified aspect ratio shape. Any desired Size or Location, but only the specified Shape. The idea is that we can mark this crop box on the image, and then crop it so that only this area remains in our picture (in the copies farther below). This tool offers the easy way.
There are usually two purposes for cropping.
This image above is 3:2 shape, but the marked crop is 5:4 shape to fit 8x10 paper.
The Marquee tool is the rectangular icon symbol selected and marked here in yellow near the top of the Photoshop tool bar (right click it for other shape options). Then when this tool is selected, we can simply draw a rectangle on the image, by dragging the mouse over the image (dragging: with left mouse button held down). Then menu Image - Crop will do the crop, leaving only the area in this marked box.
Choosing the crop shape to fit the paper print: If we also specify a Fixed Ratio in the tools top menu (like 4:5 here, which is marked yellow here — Note this is A SHAPE, NOT A SIZE... draw it any size you want). Then any crop box we can draw (of any size) can only be this 4:5 shape, which will fit 4x5 or 8x10 paper. You can see the two different shapes here, right? The overall image is aspect ratio 2:3 (taller narrow shape), and the marked rectangle is 4:5 (shorter wider shape). The point is, you can see and adjust what you are cropping, so that the parts you want will exactly fit the paper shape. Free editors like Irfanview or Faststone may not provide this option (Gimp does), but most better programs offer it, like Adobe Elements. It's an important feature.
The really BIG trick in the Adobe Marquee tool is that you can specify the aspect ratio of the crop you intend, and any rectangle you draw will be only that shape. Then draw it the size that you want, then keep holding the mouse button down, don't let up on it, and then you can hold the keyboard space bar while continuing to hold the mouse button, and then moving the mouse will simply move the rectangle around on the image, to exactly where you want it. Otherwise, you have to start over again. Works very well, and this is similarly simple in Adobe Camera Raw crop, where you can just easily do whatever you want to do, with precision.
When you Save it to a file, you should use "Save As" to specify a new file name. It would be foolish to "Save" and overwrite your only master copy of the original, if you expect to ever need it again, when you may want to print a different shape and size. Because the crop and resample operations are Not recoverable (except on Raw images, which is then called Lossless Editing).
Another way - The crop tool in Adobe Camera Raw (included with Lightroom, Photoshop, and Elements) is even easier. Just simply mark a rectangle on the image with the mouse. The rectangle shape is forced to match the selected aspect ratio (you can right click inside that rectangle to select other aspect ratios). Then you can grab the corners of the crop frame to drag it to be other size, and you can just scoot it around to frame the area you want. Couldn't be easier.
So this method can combine this Cropping step and the next Resampling step too, meaning, if you want 8x10 inches at 300 dpi (2400x3000 pixels), just say so, and that's exactly what you will get. It is good to be aware however. If you merely want to crop, and if the Adobe Crop Tool still has the Resolution field in it, then simply DO NOT enter a resolution value in the Crop tool. Just leave Resolution blank if only cropping, so it will not resample. Or, I instead normally just use the Marquee tool above, which simply marks a selection (to be cropped), and will never resample. The Image Size dialog resamples.
Said again: If older Adobe program Crop Tools have a Resolution field, then the Crop Tool will resample if you enter a dpi value (might resample to be smaller, or to be larger, which is good if you intended that, but you should be aware). If you use the Crop tool, but merely want to Crop, then you must leave the dpi field blank, and then the Crop tool will only crop to shape, and will not resample. Playing with it a little will quickly show this.
Don't confuse Aspect Ratio and Crop Factor. They may sometimes have comparable numbers (both maybe 1.5 for example), but Aspect Ratio is about the SHAPE of the image or print, and Crop Factor is about the diagonal SIZE of the camera sensor (which affects enlargement magnification).
Aspect Ratio is the "shape" of the image (it is Not about "size"). Aspect Ratio is the simple ratio of the images long side to its short side (in pixels or inches or mm, the ratio is the same idea). The Aspect Ratio of a 4x5 inch print and an 8x10 inch print are both 4:5, they are the same shape, and 4x5 can be enlarged 2x to also fit 8x10. But for example, 4x6 is long and thin or 8x10 is shorter and wide (Aspect Ratio), so Not the same shape. It is naturally important when printing images on paper that the image shape (aspect ratio) matches the specified paper shape (its aspect ratio). Otherwise the image shape will not fit the paper shape exactly. Then most likely, the photo lab machine will print it so that it fills the paper, but some of the image is cropped away, missing off the edges of the paper (cut off heads, etc). It is far better if you see and plan and do this crop yourself, instead of the print lab machine doing it randomly (We have photo editor tools to make this be easy work). Another possibility if printing at home is that some of the paper area is left as blank white space (if shapes don't match).
This is just saying, a 4x6 image cannot fit 4x5 paper, because they simply are Not the same shape. Aspect Ratio is not about Size, size can be enlarged as necessary, but Shape can only be cropped. It is a very simple concept (like square pegs in round holes), but not all beginners seem to take notice yet. A 2:3 image will enlarge to 4x6 or 8x12, but it will not fit 8x10 paper (unless first cropped to 4:5). It is a big complication that most paper sizes are all different shapes (except ISO metric paper, but which is still different than our images).
Generally, uncropped camera images do Not fit any standard print paper shapes, and standard paper sizes are several different shapes, so attention to image cropping will normally be necessary before printing. One major exception is that DSLR 3:2 images will fit 4x6 inch print paper, but other standard print paper sizes vary differently in shape. But cell phone and compact camera images are 4:3 which is not a standard print size. To prevent surprising image crops by paper shape, it will be advisable to crop them yourself first to fit your selected paper shape.
Aspect Ratio simply compares the image width and height dimensions (pixel dimensions, or dimensions of sensor, or of print or film, inches or mm). Basically, this ratio is just a division of width by height. This represents the "shape" of the image, how square or how elongated is the shape. The only little trick is reducing it to smallest nomenclature. This is done the same way we reduce arithmetic fractions to their simplest representation.
The Greatest Common Divisor (GCD, Wikipedia) is the greatest common integer divisor of the two image sides. For example 6x4 inch paper has GCD of 2, so we divide 6:4 by 2 to simplify its ratio to 3:2 Aspect Ratio. Another popular way to say the same thing is that the division 3/2 = 1.5, for a 1.5:1 Aspect Ratio (relative to small side equal to 1). Or a 3000x2000 pixel image (GCD is 1000) is divided by 1000 to simplify dimensions to 3:2 (or 2:3 would be the same ratio as 3:2, just rotated, but the shape still fits the 4x6 inch paper, rotated too).
The earliest recorded description of GCD method was in Euclid's Elements, 300 BC. Today, the idea in photography is that the image aspect ratio will only fit properly (without cropping) on the same aspect ratio paper, to be same shapes, regardless of sizes. Size can be easily scaled (enlarged or reduced to fit same paper shape), but shape can only be cropped. This shape is a concern for every image you print, because of course, if filling the paper, the paper will already have its own aspect ratio shape.
The process of finding the GCD must stop when a smaller divisor reaches a value not evenly divisible into both values, if the divisor won't go any lower (for example, 7 or 11 is not divisible, but 8 or 9 or 10 or 12 is). In the initial default 5296x4000 value shown is 1.324:1, GCD is 16, and the divisor 16 gives 5296/16 = 331, an odd value which happens to be a prime number, not further divisible by anything (except 1). So it has to stop there. The calculator here also advises how near that result is to a standard Aspect Ratio value. In practice, it need not be exactly precise. This initial 5296x4000 is close to 4:3, so we would still call it 4:3, approximately, close enough, even for the purpose of fitting 4:3 print paper with little issue. 5315 x 3985 (1.33375:1) would have been as close as we could match the same megapixels and a 4:3 shape, but the initial 5296/4000 = 1.324 would be close enough for practical purposes, within 0.7% of 4:3. The calculator finds the closest common nominal, however just in case that was not necessarily the desired goal, the calculator might also show other common choices (those within 3%).
The aspect ratio can also be computed with the small side being 1 and the other side being the division of the larger/smaller dimensions, for example 3:2 is 3/2 = 1.5:1. which is same actual shape if 3:2 or 2:3. The 1.5:1 can specify the precise value. The orientation of the sensor or paper is not defined until it holds a specific image, so we typically always say a ratio one common way, without regard for rotation. For example, we speak of 4x6 inch or 6x4 inch paper, both of which we say is 3:2 aspect ratio. The paper is still the same shape, and it can be rotated too.
Cellular and compact
cameras are 4:3.
Rotated on end is 3:4
Image dimensions of a specific image are normally specified as width x height order, so 6000x4000 is a Landscape image, and 4000x6000 would be Portrait image (speaking of image orientation, not the type of subject). The first number is presumed to be the image width, said as W x H order. So technically, a 3:2 ratio is Landscape orientation, and 2:3 is Portrait orientation (of the same shape). In practice though, we may hear aspect ratio said in a fixed manner regardless of orientation (for example, when speaking of general case of many instead of just a specific one). Camera formats are usually generically stated an landscape orientation, 3:2 or 4:3, before we know of any specific future image. And blank paper size is often said as portrait orientation, but might be said either way. We easily print a 3:2 image on 4x6 paper, just by rotating the image on the paper. These are the same shape. We may rotate the image either way to use it properly. The divided numerical ratios (1.25 or 1.4 or 1.5 to 1) are normally the long side divided by short side, to be a number greater than 1.
When printing the image, we normally need to crop it first, to fit the paper shape (which certainly matters when printing). The image will naturally be limited to the paper shape, but you can do that cropping yourself first, intelligently, to be your choice of what content is cropped away and what is left to see. Even if Not filling the full photo paper shape, and maybe instead preparing a smaller image for a book or document, the available space will have a size and a shape which must be honored. Cropping first to judge the best presentation prevents cut off heads, etc. Because, 4x6, 5x7, 8x10 inch paper are all different "shapes" (and the image is often some other shape). 4x6 is a more elongated shape than 4x5 (so the same one image cannot possibly fit both shapes of paper). 4x5 or 8x10 shapes are a little wider and less long, so to speak. Shape and size are different properties. Images are easily scaled or resampled to different sizes, but the shape depends on the photo paper. 8x10 can print on 4x5 paper, but not on 4x6 paper (because 8x10 shape is 4x5). If the image and paper are different shapes, the paper will necessarily crop it — some of the image will be off the paper and lost, or there will be blank unfilled white space — so the correct shape is important. It seems reasonable and desirable and necessary to decide this yourself about how to crop your image, before you print it. And frankly, a little tighter cropping often helps many images anyway, like the one above, making the remaining subject area larger and more important.
This Cropping step above shows how to crop the image to fit the paper.
Some common Aspect Ratios are shown below. The ratio of the two dimensions is "shape". When the ratio is say 3:2, the sides compare as 3 to 2, which could be 6x4 inches, or 12x8 inches, etc. This same ratio giving Large/Small :1 of say 1.5:1 (3/2 = 1.5), means the images long side is 1.5 longer than the short side. This is a "shape", not a "size". Valid for both inches or mm, it is a ratio.
The formats are ordered here from a more square shape toward the top of the list, to a long and thin shape toward list bottom.
Some are specific paper sizes in inches, other shapes match a few sizes.
Be careful when preparing Wallet size photos for printing, because paper shape and size can vary. Traditionally, standard wallet size photos are 2.5x3.5 inches (measure your wallet picture insert sleeves), which is 1/2 of 5x7 dimensions — meaning, four wallet are cut from 5x7 inch paper. In which case, images cropped for 5x7 can also be printed standard wallet size without needing any additional cropping (the shape is the same).
However, today many shops print wallet as 2x3 inches (four cut from 4x6 paper). Walmart, CVS, Walgreens, Snapfish and RitzPix say their wallet is 2x3 inches, but Costco, Mpix, AdoramaPix, Nations, White House and Shutterfly do specify standard 2.5x3.5 inches. Some shops simply don't specify their exact wallet size. You may want to ask size first, before cropping.
Perhaps cell phone screens have mostly replaced printed wallet size photos today, but anyway ... 5x7 inch images are correct shape to also print standard 2.5x3.5 inch wallet.
The 4x6 inch images are correct shape to print 2x3 inch wallet, which is also a fairly common situation today.
Matching the image shape to the paper shape is a significant advantage, avoiding unexpected cropping when printing.
|Standard Metric Paper Sizes|
|Size Width x Height, mm and inches|
|A0||841 x 1189 mm||33.1 x 46.8 in|
|A1||594 x 841 mm||23.4 x 33.1 in|
|A2||420 x 594 mm||16.5 x 23.4 in|
|A3||297 x 420 mm||11.69 x 16.5 in|
|A4||210 x 297 mm||8.27 x 11.69 in|
|A5||148 x 210 mm||5.83 x 8.27 in|
|A6||105 x 148 mm||4.13 x 5.83 in|
|A7||74 x 105 mm||2.91 x 4.13 in|
|A8||52 x 74 mm||2.05 x 2.91 in|
|A9||37 x 52 mm||1.46 x 2.05 in|
|A10||26 x 37 mm||1.02 x 1.46 in|
The International paper sizes standard is metric sizes like A4, etc. Defined as A0 size having an area of one square meter, but with Aspect Ratio √2 :1. When moving larger, each size step doubles the previous area (long dimension becomes the next step short dimension). This √2 ratio has the special property that all sizes in this series remain the same aspect ratio (1.414:1).
The standard International metric paper size chart is shown several places online, but the inch conversion of the smaller sizes are computed here to two decimal digits to more accurately reflect the √2 aspect ratio.
This International metric Shape is only 1% different than 5x7 Shape, so it can be unknown which standard expected value should apply if they vary a little. The calculators will call it the closest value.
|North American Paper Sizes|
North American paper sizes in inches (ANSI standard, NOT speaking of photo print sizes) are all double size of the previous (with long dimension becoming the next step short dimension), so they also cut or fold in half, but with two aspect ratios alternating 1.294 and 1.545 at every other size (different shapes).
Regardless, the images from the camera are usually a different shape than any print paper (6x4 inch paper matching 3:2 DSLR and 35 film being one exception).
So the point is, these shape differences are the reason people complain that the photo lab didn't print all of their picture area. To prevent the lab from cropping your image in an surprising way (but it is expected, the image shape must fit the paper shape), you should crop to 3:2 shape BEFORE you order 6x4 inch prints, or to 4:5 shape BEFORE you order 8x10 inch prints, or to 5x7 shape BEFORE you order 5x7 inch prints. Then what is cropped is your own choice. If you don't, and if you send it to the lab anyway, the lab will fill the paper, cutting off a little of your expected image. Or sometimes you can request they just print it uncropped, leaving blank white space where it does not fill. It is best if you tend to this before you send it, cropping the way you want it so that the image shape fits the paper shape without surprises. Note the obvious, in these "red" images above at right, we can move our own cropping selection to be at or near either edge, to better center our actual subject in the final area (to choose what gets cut off).
Most paper sizes are each a different shape (except metric is consistent, all actual ISO metric sizes (A, B, C series... A4, A6, etc) are the same shape) but few paper sizes are the same shape as our images.
Historically, the image and paper shape difference was always true of film too. Instead of Crop Factor, we called it Film Format then (size and shape). One difference in the old days of film, a human print operator might watch and select a better print cropping (and corrected exposure and white balance too, at least the major problems). So many of us never even realized there was any problem. But the machines are fully automatic today, so forget that, at least without paying extra. This crop is something you surely want to see and choose yourself first, to be your way, instead of letting the machine do it automatically their way (cutting off heads, etc.) Printing today is usually not watched by human eye. Maybe we do need a bit of computer skill to do that, but it is easy with digital (the purpose of this page).
The Crop Factor here is relative to a 35 mm film frame, as per the usual standard.
So film was all different shapes too. Many compact and phone digital sensors are smaller than even Kodak Disc film, with crop factors of 5 to 8. Only 8 mm movie format approaches that. However, digital does handle the necessary enlargement better than film.
The image above has been cropped now, to show the image that I decided I wanted to show, and also to fit paper shape of 4:5 aspect ratio. This step gets it to 8x10 inch size when printed.
Also shown is the Adobe Image Size dialog box (same in Photoshop or Elements ). It is menu Images - Image Size, or keys CTRL L pops it too (speaking Windows). Most serious image editor programs necessarily do about the same thing, and work about the same way. And BTW, the Best Help available for Adobe stuff is simply to do a Google search for the words you want to know about.
This tool does two very different resize functions: Resample, or Scale for printing. The mode switch on this tool is the Resample Image check box (bottom), checked so it will Resample. If NOT checked, this box will instead Scale for printing (next part below), and then, the top portion of the box (pixel dimensions) is grayed out, image size cannot be changed. But if checked, it will resample, and the pixel dimensions are changed.
Resampling changes the image size, specifically, it changes the dimensions in pixels to be other dimensions you specify, other new pixels. Several reasons why and when we do this, but our camera images have become larger than most uses need, so we generally need to make them smaller, to be the image size needed for our purpose. For example,
1920x1080 pixels - HDTV 1080 (new monitors have become this size too) 2.07 megapixels
1800x1200 pixels - for printing 6x4 inches (300 pixels per inch) 2.16 megapixels
2100x1500 pixels - for printing 7x5 inches (300 pixels per inch) 3.15 megapixels
3000x2400 pixels - for printing 8x10 inches (300 pixels per inch) 7.2 megapixels
4000x3000 pixels - 12 megapixel 4:3 camera image (compact and phone cameras are 4:3)
6000x4000 pixels - 24 megapixel 3:2 camera image (DSLR and 35 film are 3:2)
One thing that will be very helpful to your printing work is that the requirements to print a 6x4 inch print at the ideal 300 dpi resolution computes this following very simple way. To print 6x4 inches at 300 dpi is:
(6 inches x 300 dpi) x (4 inches x 300 dpi) = 1800 x 1200 pixels needed (2.16 megapixels).
Your editors resample tool will do this computation for you.
This simple calculator here will do it, as shown. Or you often can do it in your head (6 inches at 300 dpi is just 6x3 = 18, etc).
Experiment by typing some numbers. It should become very clear, very fast. It is just a simple calculator, about pixels per inch.
If the image is significantly larger than needed for the print, it won't help, it's just a waste, but it won't do harm other than to be a slower upload, and harder to handle. But we can first resample it to closer suitable size. It need NOT be exact, within ± 20% of 300 dpi size is normally acceptable (240 to 360 dpi, but if creating it, why not aim closer to 300 dpi?)
If the image is too small for printing, there's no good solution. Only a larger original image replacement will help the print. Resampling to be larger does NOT add resolution, it just becomes lower resolution (if lacking the necessary original pixels).
The resample box of most editors can also do this too:
(8 inches x 250 dpi) x (10 inches x 250 dpi) = 2000 x 2500 pixels.
The resample tool is a calculator that will do this multiplication. Just enter BOTH 250 dpi AND 8x10 inches, and the pixel dimension numbers change so that this new image size is what you get, designed for this specific printing purpose. It does have to first be cropped to 8x10 inch shape.
Be careful that your numbers do not increase the original size in pixels (which is interpolation, and not a good thing... often better then to uncheck Resample, and just scale the "pixels per inch" number to fit at a lower resolution (see Scaling below).
After the first crop above (done for shape and content), this tool shows this image size now is 2514x3143 pixels (4:5, or 3143 x 4/5 = 2514). Resampling smaller is a drastic operation, which works great, but there is no going back after the excess data is discarded. First archive your largest original data somewhere unmodified (Raw is great for this).
Note the number on the top line of this tool. It is called Pixel Dimensions, but the 22.6M is the data size of those dimensions, in megabytes, 22.6MB. This is not file size, and it is not image size. This is size of the image data when uncompressed and open in computer memory. It is a computed number, from the image size (pixels). For 8-bit images (24 bit color), this number is always 3 bytes per pixel (it is the RGB data for each pixel). Or 6 bytes per pixel for 16-bit images. So any generic 12 megapixel image is 12x3 = 36 million bytes of RGB data. True of any/every 24-bit RGB 12 megapixel image. But this one is now cropped to be smaller, 7.9 megapixels now (2514x3143).
This 22.6 MB number is close, but not exactly (2514x3143 x 3) bytes. Megabytes is an odd situation. One megabyte (1024x1024 bytes) holds about 4.8% more bytes than one million bytes (1000x1000), so the megabyte number is about 4.8% smaller than millions. See More about that. But this size in memory simply depends on the pixel dimensions, as shown, and it is the size of our data, three bytes per pixel for 24-bit images. However, note that megapixels does not use the 1024 multiple, megapixels is simply width x height pixels divided by (1000x1000).
Image data size is not file size. JPG compression makes the data smaller while in the JPG file. File size is variable with the JPG Quality option (High JPG Quality is larger files and better images, Low JPG Quality is smaller files at the cost of worse images). File compression size varies with image content too — featureless image areas (walls, sky, etc) compress smaller than detailed image areas (tree leaves, etc). We speak of JPG Quality losses, which means the addition of JPG artifacts, see More about JPG. The JPG file size might be tolerable quality if around 1/8 to 1/12 of image data size (very vague numbers, but larger files are better images). The image is the full original size again when opened and uncompressed into computer memory (same pixel dimensions), but quality might suffer from JPG compression. The top line of the resize box shows this uncompressed data size, in MB. This is computed at three bytes per pixel (for 24-bit images). That is simply how large our image data is (JPG files tend to give us other false notions about it).
There are three checkbox options in the Adobe Image Size box:
To Resample, the Resample Image check box is checked. We can Resample to change image size (pixel dimensions).
Currently above, this tool's Document Size center portion is showing that this image will print 8.38 x 10.477 inches at 300 dpi. This is simply because:
2514 pixels / 300 pixels per inch = 8.38 inches
3143 pixels / 300 pixels per inch = 10.477 inches.
Enter any other numbers, either inches or dpi, and you will see different results for that goal.
It is just this simple - pixels per inch. For example, if you intend to print 4x6 inches (2:3) at 250 dpi for Walmart (the image will first have to be 4x6 aspect ratio (2:3) for it to come out this shape), you can enter 4x6 inches and 250 dpi in this center portion (Document Size). Any number you type in this Document Size section WILL RESAMPLE THE IMAGE to change the pixel dimensions in the top Pixel Dimensions section. 4x6 at 250 pixels per inch will become a smaller image, it will come out 1000x1500 pixels, and 4.3 MB, but it will be right to print 4x6 at 250 dpi (because 1000 pixels / 4 inches = 250 pixels per inch). You can change either resolution, or dimensions, or both, and it will resample to give you the new pixels for the image size that you specify. Then press OK, and you've got it. If you Save to overwrite the original file, then the original data is gone, unless you archived a copy first, so think that over first — save it to a new file name. Just experiment a few minutes entering different numbers here, and you will get the idea. This is just a simple calculator, but it DOES change the image size (pixels) to be what you specify.
Dpi and inches are related, tied together for printing, according to the number of pixels available. The inches exist only on the print paper (not in video). Dpi is just an isolated number stored in the image file, just a number, used as instruction to the printer, to space the pixels on paper. Dpi is not a property of the image data. Your 12 megapixel JPGs may come out of the camera with this number marked maybe 240 dpi (arbitrary, varies with camera brand), which ONLY means that it then indicates a print size about 12x18 inches. But it is just a made up number — the camera has absolutely no clue or concern how you might print it later. It is just a dummy number — its only purpose is to "not be blank" (so the computation for indicated inches does not blow up). It has no meaning at all, not until you make it show the way you actually want to print it. It is NOT about creation, and it is not about image resolution. The image size and detail depend only on the pixel dimensions, and not at all on this fake dpi number. Yes, it is called resolution, but it is only about printing resolution, how the printer will print it, when you print it, and is NOT at all about image resolution. You can change this resolution number to any number you want (but this Resample box will resample then, where Scaling below will not). DPI is not a property of the image data. Image size is dimensioned in pixels (I may have mentioned that?) Image detail depends on how many pixels we have, and is not affected by this number (until we print it, then dpi only affects how the printer will space those pixels). Video systems do not use this dpi number in any way. Video screens simply show pixels directly. etc, etc, the rest of the site here is about that stuff.
You do want to pay attention to be sure you are resampling smaller, and not resampling larger. For printing, instead of resampling 20% larger, which just makes the image softer (nothing can add detail from the original scene once out of the camera), then it is surely better to simply print it scaled to 20% less resolution (pixels per inch), which increases printed size the same, but leaves the pixels unaffected (which is Scaling, next below).
The last list box at bottom of the tool is the resampling algorithm to use. Resampling makes the image be smaller, but is not otherwise a quality issue if a good resample option is used. Bicubic Sharper is Photoshop's default and a fine choice for reducing a photo image size. Bicubic Smoother can be better for increasing image size, but increasing size is generally not a great option. Note that all images need a moderate bit of sharpening after resampling (all new pixels then). Any sharpening done before resampling is generally pointless, as those pixels will be replaced with different pixels... Sharpening ought to be done last, the final operation. Large images for printing need a bit more sharpening amount than smaller images for video. Very small images cannot stand much sharpening. We sharpen for the viewing goal. Don't overdo it though. For printing, common notions are maybe USM Radius 2, or even 3 pixels. For video monitor viewing, only USM 1 pixel Radius.
This scaling option is used for printing ourself at home or work on our local printer. It is unnecessary for sending it out to be printed. Sending it out to be printed to say 8x10 inches will do the necessary scaling there to print 8x10 inches. It will be 8x10 inches regardless what the dpi number might specify (because you ordered 8x10). But our home printer uses the dpi number in the image file, which will print whatever size it dictates. However, most home printer menus do also offer an option to scale it for you to any printed size. Or you can do it this way, by changing the dpi number appropriately.
This option is ONLY to prepare for printing the image, to cause it to print the size we want on paper. Scaling does NOT affect the pixel dimensions of the image. Whereas... the Resample option above is about changing the pixel dimension of the image, for any purpose, but for example, maybe to make it display smaller on a video monitor. Scaling is very simple, all we do is change the dpi number, specifically without changing any image pixel in any way.
Video and printing are very different. Video monitors simply show pixels directly. If we want to see a different size (maybe the image is too large for the screen), then the image must be resampled, to create a different or smaller image size, the pixels of which are always shown directly on video. Whereas printers operate very differently — printers fill inches of paper by spacing the pixels at different "pixels per inch" rates. Scaling simply changes that dpi number, so the same pixels will cover a different size in inches (on paper). The scaled image will appear unaffected on the video screen, which ignores the dpi number, and shows pixels directly.
Scaling does NOT change image dimensions (in pixels). It never affects any pixel at all, and it has zero effect on the video screen. It is only for preparing to print, and technically, scaling only changes the Resolution field "Number" (pixels per inch), which is just a number. Some new dpi number just causes the printer to compute to print the same pixels to cover a different size in inches on paper (pixels per inch). The dpi number simply "scales" the printed size on paper, by changing the printed pixel spacing to make the SAME pixels cover a different dimension of paper, in inches. The pixels stay the same, totally unchanged. 3000 pixels printed 300 pixels per inch will cover 10 inches of paper, or printed 200 pixels per inch will cover 15 inches (same 3000 pixels). Scaling is very reversible, can be done at will, since there was no actual image change at all (just this one dpi number stored away separately — it is just a number).
We initially cropped this image to 8x10 inch shape (4x5 aspect ratio), so now lets suppose now we want to print it 8x10 inches. If we just feed this image (in first part above, after cropping) to our printer, what this said is 8.38 x 10.477 inches at 300 dpi, and that is how the printer will print it (It is how our local home printer will print it. If we take to Walmart or such, they will print it any size we ask for, i.e., they will scale it for us — they don't look at the dpi value).
Here, same tool, but I unchecked the Resample Image check box, so now it cannot/will not resample. This selects Scaling mode. The top fields become grayed out, the pixel dimensions cannot be changed. All it does is change the separate dpi value, but there is this specific option for it, this is how we do it (it saves the new number in the file, if we save the file). Notice again, the top part (Pixel Dimensions) is grayed out now, we CANNOT change the pixel dimensions in any way. We are not resampling, and we cannot change the image size. All we can do is to just change the dpi number, which changes the printed size, after it is on paper. The purpose is to scale the image size (pixels) to the paper size (inches).
However, we can change the dpi value, or change the printed dimensions in inches, and then the other one will track it. It is just a calculator. I also typed 8 inches for image width in the center at Document Size. This is printing size. The 10 inches automatically changed (because this is the aspect ratio), and the 314 pixels per inch is just the simple calculation: 2514 pixels / 8 inches = 314 pixels per inch. It is still the original pixels, but this dpi value will cause our home printer to print it 8x10 inches.
This 314 dpi is close enough, it is not a critical or exact number. For best printing quality, we would always like it to be around 250 to 300 dpi for printing. Within 10% or 20% is fine (when necessary, when that is the pixels we have), and will be undistinguishable. The places you send it out to for printing normally have capability around 250 dpi, and you would like to have that much. If you don't have enough pixels, then a 150 dpi result is possibly acceptable (not terrible), especially for large prints which are not viewed too closely. My own goal is to provide 300 dpi, "just to be sure"... but 400 dpi is pointless, won't help.
So if we wanted to print this one 4x5 inches, this 2514x3143 pixel size is simply far too many pixels this time. Scaling to 4x5 inches will come out 2514 pixels / 4 inches = 628 pixels per inch, which is simply ridiculous for printing color photos (but is fine and good if for line art). Realistically, printing at 250 to 300 dpi size is all that can benefit the printer (any printer, for color photos), so resampling smaller is often necessary and prudent. It would print if we left it scaled to 628 dpi, but the file and upload would be 4x larger than necessary, for no reason at all.
And when we print, generally the printer driver will use the dpi number to size or scale the printed image. However, many printer drivers today will also allow us to specify the printed size there too. This is the same scaling operation (changes value of dpi to fit the image to the page size). We still need to have the shape right however.
Note this dpi resolution number is just a stand-alone number, which does not affect the image pixels at all (unless we also resample, but this is speaking of scaling — the Resample Check Box is Unchecked). And it only has meaning at the instant of printing on paper. It is arbitrarily chosen to decide print size scaling, and it is unused at all other times. Images viewed on the video monitor do not use this dpi number in any way, therefore images on the internet web also simply do not use this number in any manner. The Adobe menu File - Save for Web strips out this number to save a few bytes, since it is not needed on the web (Adobe reports 72 dpi then, because it is missing, not because it says 72). Video systems simply show the image pixels directly, and in the video system, it simply could not matter less what this resolution number is. It is totally ignored in the video system. It is used for printing on paper, to scale the printed image size to be so many pixels per inch.
Bottom line, if the image size is halfway close to proper printing size, there is no reason to modify and affect the pixels. Just leave them alone, use what you've got, and scale the image to print the exact desired size. If the printing resolution comes out near the ballpark of 250 to 300 dpi, you're good. This Document Size tool is just a simple calculator, which computes: pixels / inches = pixels per inch. And when you click OK, it stores this dpi number in the file, to retain it for printing. Your local printer device uses that number to size the image pixels on paper, so this is how you make it print a specific size in inches. The dpi number is stored in the file (is just a number), but the inch numbers are not stored. There is zero effect on the image pixels if we simply change the dpi number. Then the unmodified existing pixels just print a different scaled size image on paper.