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Essentials of Using digital images

Pixels, Printing dpi, Video dpi - What's with that?

These basics are pretty much about the single issue: How do I use my image, how to I make it be the proper size for viewing, for printing, or for the video monitor? All this is really quite easy, but digital may just be a new concept. It is like learning to drive — once you learn an easy thing or two, it's a skill helpful for life. When you know, you will simply just know. But yes, it does seem that we could subtitle this: Details that no beginner wants to know. However the point is: You'll never grasp digital images until you get it ... until you know what digital images are, what to do with them, and how to do it.

Seriously, once we accept that pixels actually exist, then all this stuff is rather easy. It's about pixels.

We just gotta know about pixels, and if any mystery, a very short primer is here: What is a Digital Image Anyway?

This page tries to be a quick summary of the digital concepts, about how things work. The answer to virtually any question about image size starts with one of these basics. To be able to use digital images well, we need this understanding. This may perhaps be written a little like an argument, refuting the dumb incorrect myths we may have heard about how digital works. The concepts below are instead what you need to know to use digital images properly. It is actually rather easy to grasp, if you get started right.


The Most Fundamental Digital Concepts

Color images are commonly RGB data (three values per pixel representing Red, Green, Blue), but there are also other ways of encoding the image data, CYMK, grayscale, line art, indexed color (see more detail about bits). And there are different methods of compressing the data in the file, JPEG for one example.

This was a 4288x2828 (3:2) 12-megapixel Nikon D800 image but in portrait orientation. Then cropped to 2502x3127 pixels (4:5, 7.8 megapixels), and then resampled to 400x500 pixels (0.2 megapixels). We tend to think of these numbers as the "resolution" of the image, but at full size, it is the digital reproduction resolution of the image from the lens (the lens resolution is the limit). The pixels do indicate the "fineness" of the smallest possible digital detail which is a pixel, which is a dot of one color). This example is to show the idea about pixels.


400×500 pixels, 0.2 megapixels

A tiny 32×37 pixel area from the original 2502x3127 pixel version (at red arrow), enlarged 1200% to show the pixels.

This enlargement might look fuzzy, but actually each pixel is sharp. :)  Do realize the original 2502x3127 pixel (7.8 megapixels) image would look great printed 8.3×10.4 inches at 300 dpi, but this 12x that size would print 8.3×10.4 feet at 25 dpi. You would stand well back to view that. The trick is to have enough pixels for your enlargement size. The rule of thumb for printing high quality photos is to have 300 pixels for every inch of print dimension (more or less, the 300 dpi need not be precisely exact, but for prints that will be viewed closely or hand-held, best quality is to stay within about 240 to 360 dpi, or 300 dpi if possible). Specifically, this means 1200x1800 pixels printed at 300 dpi is 4x6 inches on paper.

Pixels are the most basic detail to know about digital images

A digital image is composed entirely of pixels. A pixel is purely numeric data representing some one color, of one tiny dot at its specific location in the image. The main concept of digital is that each pixel is just NUMBERS. A pixel is numeric data describing ONE RGB COLOR of one tiny area. The color in that tiny pixel's area was sampled by the camera or scanner sensor. Then each pixel is ONE COLOR described digitally in numbers, representing the color of one tiny dot at its location in the image. The color of each of millions of tiny dots is so described (a 6000 x 4000 pixel image is a grid of 24 million pixel locations).

A pixel represents the color of a tiny dot with numbers, but conceptually is not unlike a small piece of colored tile in a mosaic tile picture (awesome). The numeric concept may be relatively new today (called digital), but the concept of producing a picture in decorative tile is a few thousand years old. Tile and film both capture tiny samples of the actual color, but digital represents the colors with numbers, necessary because numbers can be written into an image file, but actual colors cannot. And then a pixel that describes a pink color has the similar effect as a small piece of tile the same shade of pink. Our brain recognizes the reproduced image pattern in those pixels or tiles. But enlarge these enough, and all you will see is the pixels or the individual tiles. Pixels are all there is in a digital image, and we must think of it that way. Ignoring pixels will never grasp the digital concept. Digital will make sense when you do think of pixels.

FWIW, a digital pixel in an image file does NOT have a size. A pixel is just numbers representing a color of a tiny dot location. Yes, there was a camera sensor with maybe about 4000 pixels per inch on it. But then a pixels actual final use depends on how we show it enlarged (for example) at 100 pixels per inch on the video screen, or at 300 pixels per inch on the print paper. This "scaling" (using a different dpi number, spacing the pixels differently) is how enlargement is accomplished. (The 4000 pixels per inch might cover the one inch, or 400 dpi is 10 inches, or 100 dpi is 40 inches.) Our digital image does have a size, in pixels, perhaps 6000 x 4000 pixels in data size. But we can view it at any reasonable size. A pixel is just numbers representing a color of a dot at a location in the image.

Pixels are how a digital image reproduces a scene and its colors. The camera lens creates an image. Then to reproduce that image digitally (numerically), the digital camera sensor merely takes millions of color samples (each pixel is a tiny sample of color), at each of every very tiny area of the image, in the way shown above. In contrast, film (which is NOT digital) uses tiny specks of silver or emulsion dyes instead of pixels, which is not digital numbers, but film does a similar sampling idea (actual colors of every tiny area). Film areas actually show the actual color, which we can directly see. However, digital images are totally about pixels, which are simply numbers representing the color at a spot location.

A pixel is three numbers, of the red, green, and blue primary components of the color. For example, the reddest orchids in the picture above have RGB components of about RGB(220, 6, 136), each component on a scale of 0..255, so the RGB components of it are red is bright, green is very weak, near black 0, and blue is about mid-range. A pixel can describe that shade of bluish red of one tiny area. We don't have to know much detail, but see Understanding RGB color.

The "photo detail" that we perceive in a digitally reproduced picture is entirely due to the color differences in the pixels. A pixel is simply a color description. Color is the detail. Pixels show the colored detail. The image detail is shown by the color differences. The colored pixels are all there is in a digital picture.

Pixels are real, they exist, in fact, pixels are ALL that does exist in digital images. There is nothing else in a digital image. We don't need to see each pixel individually, but the image Size dimension in Pixels is the First Thing To Know about using any digital image, because this size in pixels is what is important for any use. The size of a digital image is dimensioned in pixels, for example 6000x4000 pixels is the size of some one image.

FWIW, we see some fanciful things in movies, where tremendously enlarging photo prints provides clues to solve crimes. The resolution decreases as the size increases, so it really does not work well in that degree. Enlarging the original film is by far the best chance, enlarging prints is poor, and enlarging newspaper images is totally hopeless. Enlarging data of a large digital image does show much detail not seen in a small print (see that in images here), but enlarging digital data excessively only shows pixels.

Human eyes have rods and cones which are a similar sampling system of tiny areas. Cones are color sensitive, our eye has red, green or blue cones. Sampling the color of tiny areas is not unlike pixels in that way. The color difference of adjacent areas is how image detail is perceived. We see a black power wire running across a blue sky because the colors are different. Color difference is the detail that we perceive (including slightest tonal shades of same color). In our digital pictures, a pixel is the smallest dot of color that can be reproduced, so we do think of more and smaller pixels as greater resolution of detail.

However, digital reproduction is a "copy" of a lens image. We should also realize that it is the camera lens that creates the image that we will reproduce digitally, and pixels are the detail of reproducing the lens image. For example, in a DX cropped sensor camera, the original is the image from the lens projected onto the 24x16 mm DX digital camera sensor. The image has this 24x16 mm size there, comparing to the size of an APS-C size film image. Then, the camera pixels merely digitally sample that lens image (very much like any scanner samples an image, meaning taking many color samples called pixels) to try to digitally reproduce (convert to numbers) the image that the lens created. A pixel is just numbers, three binary RGB numbers representing the red, green and blue components of the color of the area of that pixel. The pixels do NOT create the image, and cannot improve the lens image detail. The pixel sampling merely strives to reproduce its detail. At best, it can hopefully be a very good reproduction. A 24 megapixel DX image and a 24 megapixel FX image are NOT equal, because the FX image is simply half again larger (36x24 mm), and so does not have to be enlarged as much to show it.

Essentials to Know about Using Digital Images

Sufficient pixels to print at 250 to 300 dpi is optimum to print photo images. More pixels really cannot help the printer (for photos), 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 the image size needed for optimum photo printing. This method is one thing you really need to know, it should be second nature to you, considered when printing any image.

Image Size Goal for
desired Print Size

To print x
 inches
 mm
at dpi resolution  

This simple calculation can serve two purposes:

Call it dpi or ppi as you prefer, but (since it’s about image pixels instead of ink dots), the idea is that this resolution is the spacing of the pixels on paper, pixels per inch.

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 (a line art scan of black text or line drawings can use 600 dpi well).
Or for example, you could scan at 150 dpi and print at 300 dpi for a half size copy.
Or you could scan at 600 dpi and print at 300 dpi for a double size copy.
The concept either way is pixels per inch, in the scanner and in the printer.

But NOT on monitor video screens. Images are shown on the video screen at their actual size in pixels. Image pixels are shown one for one on the screen pixels, so to speak. There are no inches or mm inside video monitors. You might have bought a 23 inch monitor, but its screen is dimensioned in pixels.

300 dpi is likely what you want for printing a high quality photo copy job (a line art scan of black text or line drawings can better use 600 dpi, but 300 dpi is plenty for photo work).

This dpi number does NOT need to be exact at all, 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.

Printing dpi is dependent on the capabilities of the printing process, see a Printing Resolution Guide.

And there is a larger photo dpi calculator that knows about scanning, printing, and enlargement.

Cropping Aspect Ratio to fit the paper size is an important concern too.

Resampling changes the pixels. When the image is too large, resampling entirely replaces the image with a different smaller image, with a different count of new different pixels. Maybe resampling changes an image that is 6000 pixels wide to be only 1000 pixels wide, so it will fit on the video screen. But this is a destructive loss, which may be perfect for the current goal, but destructive meaning that we cannot go back (we discarded pixels, so save this copy with a different file name — always save the original image too). Resampling is a big deal, destructive to the original. But scaling is not — we can instead just change only this dpi number (called scaling) with wild abandon, back and forth, at will. Changing the stored dpi number does not change any pixel in any way. It is just a separate number, a future instruction for printing. It has absolutely no use until time of printing. Then it will control the size in inches when it prints on paper (unless it is scaled again at that later time).

However (a major point), changing this dpi number will cause absolutely no change at all on the video screen (unless resampling is also selected). Video is not concerned with dpi or inches. Video ignores any dpi number, and simply shows the pixels directly, one for one, one image pixel on one video pixel location. No matter what number the dpi says, you will never see any effect of it on the video screen, which simply just shows the pixels directly. See an example of that.

Aspect Ratio: The image itself, and the printing paper size, are commonly different shapes, causing printing problems unless handled. This is not speaking of size, but speaking only of shape, for example, 4x6 paper is more long and skinny, where 4x5 paper is more short and relatively fat. Different shapes, and we cannot print the same image on both in the same way. But 8x10 paper is the SAME SHAPE as 4x5 paper, and 8x12 paper is the SAME SHAPE as 4x6 paper — the proper image shape can be simply enlarged and still fits exactly. To fit our image on the paper, we crop the image shape to match the shape of our paper choice. An image has a property called Aspect Ratio (shape). This is the simple ratio of the two image dimensions. Maybe the image size is 3000x2000 pixels, so the aspect ratio is 3000:2000. We reduce this to the lowest common denominator, and call it 3:2. It just means the two dimensions are in ratio 3 to 2, which is a shape, which can be compared to the paper shape, which normally needs to be the same shape. And 4x6 is also called 3:2 (2:3 paper is simply rotated), but 4x5 is 4:5. Different shapes. More at Aspect Ratio.

Printing paper also has a similar shape, and the same Aspect Ratio applies. For example, 6x4 inch paper is also 3:2 aspect ratio. If we print a 3:2 image on 3:2 paper, it will fit — the shapes are the same 3:2 aspect ratio (3000x2000 pixels is quite excessive though, for 4x6 inches), and really ought to be resampled to about 1800x1200 pixels first (3:2), to about 300 pixels per inch size.

However, if we want to print this image on 8x10 paper, the paper shape is different (4:5 aspect ratio) than the image (3:2), and some of the image will be lost (cropped, outside the paper edge, off the paper — the shapes are simply different). Or we could choose to fit the tightest dimension, leaving blank white borders the other way (we hate that too). We had exactly the same issues with film, not necessarily the same shape as our paper, but digital methods are a bit different. Now, we need to do Crop and Resample and Scale when printing digital images.

Video screens also have aspect ratio. Non-widescreen monitors used to all be 4:3, and HDTV wide screen TV is 16:9. This is equally important if we are trying to fill full screen, but we are more comfortable with blank space bordering our video images, than on paper.

Scanners do use a specified dpi number (scanning resolution) to create pixels from inches on paper, for example creating 300 pixels per inch. If we scan 10 inches of paper at 300 pixels per inch, we create 3000 pixels in that dimension. If we scanned it at 600 dpi, we create a 6000 pixel dimension, which could then be printed double size at 300 dpi (scaling). The basic scanning scaling concept is:

Dpi has necessary meaning for scanners, which create pixels from the paper size in inches.

Scan 10 inches at 300 dpi, and it creates 10x300 = 3000 pixels.
Print 3000 pixels at 300 dpi, and it prints 3000/300 = 10 inches.

See how that works out? The concept is pixels per inch. Scanning at 300 dpi automatically ensures that you will have sufficient pixels to print original size at 300 dpi. Even if not printing, scan dpi still determines the image size in pixels (created from the inches scanned).

Digital cameras create pixels directly, a fixed size image. If the camera sensor size is 12 megapixels, it creates a 12 megapixel image, dimensioned in pixels. There is absolutely no concept of dpi yet (no paper size is defined in the camera, inches are a very undefined concept at that point). The camera cannot possibly guess what size we might print it someday, but we will figure it out later, when we are ready to print, if we even print it. We do not care about dpi otherwise, it is an unused number until we print. However, we are always concerned with image size (pixels), which determines how we can use that image, on the video screen, or when ready to print.

The camera will stick in some arbitrary dummy dpi number, just so some believable printed size can be shown. If they didn't, then Photoshop will automatically call the blank to be 72 dpi, which indicates some unreal print size in feet, so the cameras do stick in a dummy dpi number, maybe 200 to 300 dpi. They don't know what size you may print it later. Camera brands vary in the dpi number they make up, but this value is a meaningless arbitrary number, confusing if we try to make any sense of it. There is NO CONCEPT of inches in the camera (just pixels). The image is dimensioned in pixels. We will change that dummy dpi number when we decide how we want to print it.

But digital basics are all the same for all images, so after image creation, then it is a digital image, dimensioned in pixels. Dpi is only used to control the size of the printed image on paper (paper has inches). Video screens are dimensioned in pixels, and video has no use for any dpi number.

Repeating: Inches only exist on the paper we print on, or the paper that we scan. Inches do not exist in the camera, in the image file, in the video system, or in computer memory. In those situations, only pixels exist. Without inches, there can be no concept of dpi. Instead, digital images are dimensioned in pixels. The single most important thing to know about digital images is their dimensions in pixels. This affects how you can use them.

There are four sizes of a digital image.

Image Size is dimensioned in pixels, which is what determines how the image might be suitably used. The FIRST number you need to know about using a digital image is its dimensions in pixels.

The camera sensor is dimensioned in mm, but it also has dimensions in pixels. For example, a full frame 36x24 mm sensor might be divided into 6000x4000 pixels. The sensor size in mm is all important for computing Field of View or Depth of Field. And the sensor mm dimensions also affect the necessary enlargement factor to print size, but the pixel dimensions are also important for viewing the image on screen or paper.

Data Size is its uncompressed size in bytes when file is opened into computer memory (and image size viewed on the monitor screen is still dimensioned in pixels).

File Size is its size in bytes stored in a file (which is Not a meaningful number regarding how the image might be used. Image size is in pixels). Data compression can affect file size drastically smaller, but it is still the same image size in pixels. So saying “I have an 8 megabyte JPG file” says nothing to describe the image size. Image size is dimensioned in pixels.

Print Size is its size when printed on paper (paper is dimensioned in inches or mm). The size of film is also inches or mm. Sensor size (mm) or film size (mm) must be enlarged to the print or viewing size. By varying the printing resolution (pixels per inch on paper), we can print the image about any size we wish, but the quality will vary. 250 to 300 dpi are usual high quality goals.

In strong contrast to paper, monitor screen size is dimensioned in pixels, and image size is also dimensioned in pixels. The image pixels fit the screen pixels one for one, so to speak. A 600x400 image will show as 600x400 pixels on the screen. If the image size is larger than the screen size, we normally are shown a temporary resampled copy of more suitable smaller size. However, print paper is dimensioned in inches or mm, so images for printing must be scaled to be spaced out so many pixels per inch or mm (often called dpi, jargon for pixels per inch on paper). See basic differences, and more detail between using images printed or on the video screen.

The most common type of color image (such as any JPG file, but Not Raw files) is the RGB 24-bit choice. Note that uncompressed 24-bit RGB data is three bytes per pixel, regardless of image size. However many/most files are compressed into a smaller file size (JPG is normally compressed to unusually small size, which can involve some quality losses). Compressed files are uncompressed again when opened into computer memory for showing (the count of pixels remains unchanged).

The usual and most common type of color image (such as any JPG file) is the 24-bit RGB choice.

Calculate the Four Sizes of an Image

  Specify image size with one of these two options:
Image Size x pixels
Megapixels   and Aspect Ratio
  Data Type
 Add estimated Exif size (optional)  
Bytes
KB
  If Printed at pixels per inch
Image Size
Data Size
File Size
Print Size

Disclaimer: Data Size is the uncompressed image pixels data, and that part is known and simple, but there are also other factors.

Note that uncompressed 24-bit RGB data is always three bytes per pixel, regardless of image size. For example, an uncompressed 24 megapixel 6000x4000 pixel image is 6000x4000 x 3 = 72 million bytes, also 24 x 3, every time. That is its size in computer memory when the file is opened. Fill in your own numbers, but if divided by 1048576 (or just divide by 1024 twice) converts units to 68.66 megabytes. The JPG files will vary in size, because JPG compression degree varies with scene detail level. If you have several dozen widely assorted JPG in a folder (all same uncropped image size written by the same camera), and then sorted by size, the largest and smallest might vary by 2.1 size. Smooth areas of featureless detail (sky, walls, etc) compress significantly smaller than a scene full of highly detailed areas (like many trees or many tree leaves for example). If a JPG in this 24 megapixel example is say 12.7 MB size, then (ignoring small Exif) it is 12.7 MB / 68.66 MB = 18.5% size of uncompressed, which is 1/0.185 = 5.4 : 1 size reduction. That would be a high quality JPG. But JPG file size does also vary with the degree of scene detail, so file size is not a hard answer of quality.

Note that 24-bit RGB data (like JPG) is 3 bytes per pixel, regardless of image size.

Data size is the uncompressed data, the actual data size — how large your uncompressed image data actually is — normally 3 bytes per pixel (for 24-bit RGB, for example JPG files). Compressed File Size in bytes is the least useful number, only of interest for internet transfer or memory card capacity or disk storage. But pixels is the only important number which determines how an image can be used.

Raw files cannot be viewed or printed directly. When in editor memory (or camera rear LCD display), Raw is converted to 16 bit RGB, and processed, and then typically output as 8 bit RGB for viewing or printing. JPG is always 8 bits per RGB channel, or 24 bit color.

The compressed file size will be smaller. JPG will be drastically smaller, variable with JPG Quality setting, but file perhaps only 10% or 20% of data size. We should favor the High JPG Quality settings, that larger file is still very small.

The camera adds Exif data to the file, and a few formatting bytes. Indexed files add a small RGB color table (called the color palette), for each color still used. Camera Raw image files also contain the cameras Large JPG image too (this JPG is shown on the camera rear LCD, and it provides the histogram too). Simple photo editors (not raw editors) which can "open" raw files just show this included JPG as being the Raw image.

Regarding color bit depth (Google), our monitors and printers are 8-bit devices. Many inexpensive LCD monitors have used only 6 bits internally (18-bit color). For photo work, look for the better monitors that actually specify 24-bit color (more common today). Good IPS monitors are becoming inexpensive now (I've been really pleased with a low priced Dell IPS monitor).

Repeating, to be sure it is clear that images have four very different "sizes", of interest in different situations.

If someone tells us they are sending us a 12 megabyte file, that tells us maybe the internet load, or how it will fill our disk, but bytes tells us nothing about the image, or about the image size, or about how we might use it. Bytes involve data compression, another variable. Images are dimensioned in pixels.

For example, if about a 12 megapixel image:

Make no mistake though, Image size is dimensioned in pixels. It is always all about pixels. Digital cameras create pixels. Inches are only about the specific piece of paper. Bytes are only about memory. Pixels are about the image.

Next page is about what you actually need to know to print a photo or document image.

Copyright © 2012-2022 by Wayne Fulton - All rights are reserved.

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