Most people when first working with digital images have difficulty with the relationship between resolution and image size. Indeed, many have difficulty grasping the concept of resolution. This article seeks to clarify these concepts.
First off, let's get some terminology down. Digital images are commonly described in terms of "dots per inch" (DPI), however dots are really things that appear in printing. The image contains, rather than dots, pixels. ("Pixel" is a contraction of "picture element", indicating the smallest unit into which a digital picture may be divided.) Whereas dots tend to be round, pixels are typically square. So the more accurate term for describing digital images would be pixels per inch, or PPI.
You should be able to see the pixels very easily in these two photographs of my dog, Tigger. The image on the left consists of 36 pixels across by 25 pixels high, for a total of 900 pixels.
The image on the right is 18 by 12 pixels, for a total of 216. And, while we're on the subject, the image at the top of this page is 286 by 200 pixels, totaling 57200. Of course, the more pixels the more detail. In the top image you can see the catch lights in Tigger's eyes, whereas on the lower right image you may not even recognize it as a picture of a dog.
In the right-hand picture, notice that there is no detail within a pixel. A single pixel is a single, solid color. (It is the smallest unit of information in a digital image.) All digital images are comprised of nothing but pixels, no matter how detailed the image is, and each pixel is a single, solid color. (That color is represented by a number in the digital file. That's how computers keep track of colors. It's all numbers to the computer.)
"Image size" simply refers to the size of the image, right?. But wait. What do I mean by 'image"? It could mean several different things. It could mean how large it displays on your monitor, or how large it prints. Or it could mean how many pixels make up the image.
How large the image is on the monitor or in a print can vary. I'll explain why below. But how many pixels the file contains is a fundamental property of the image. That is the only "size" that is stable. (Yes, in image editing software you can change the number of pixels, but this is a rather significant alteration to the file, rather like taking a picture of a picture. You're really creating a new image file when you do this.)
So, when I talk about image size, I'm thinking about the number of pixels in the file. I can't guarantee you that everybody will use this term in this way. (Thus some of the confusion.)
To add to the confusion, the term "resolution" is used in different ways in different contexts.
Digital cameras are categorized in terms of resolution. The resolution of a digital camera is the number you get when you multiply the number of pixels across by the number of pixels high that the camera produces. A "megapixel" is a million pixels, so a four-megapixel camera creates an image containing four million pixels. (The image at the top of this page we determined contains 57,200 pixels. That is 0.0572 megapixel. Pretty small!)
Computer monitors are said to display at a certain resolution. Monitor resolution is not specified in terms of a single number, but rather of the width and height, in pixels. Some common monitor resolutions are 800 by 600, 1024 by 768, 1280 by 1024 and even 1600 by 1200. You can multiply those numbers together to produce a single resolution number. For example 1280 x 1024 = 1,310,720, or approximately 1.3 megapixels. That means that a 1.3 megapixel image would fill the screen of a monitor set at a resolution of 1280 by 1024 pixels.
But you can change the resolution of your monitor! I could set my monitor to any of the above listed resolutions. (On a PC, right-click on your desktop, click "properties" then "settings". There is a slider on the left that you can move to change display resolution. In Mac OS X, it's listed in System Preferences, Hardware, Displays.)
Now, what's interesting about this is that when you change the resolution on your monitor, although you're changing how large the image displays, you're not changing the image size (i.e. number of pixels) so you must be changing the size of your pixels, to make a different number of pixels fit the same size space. My monitor has a display area that is 14.5 inches across, so if my horizontal pixel count is 1280 then I'm fitting about 88 pixels into each inch. (1280 divided by 14.5) However, if I change my monitor's resolution to 800x600, I've only got 55 pixels in each inch. (800/14.5) Monitors are said to have a resolution of 72 pixels per inch, but you can see now that that is not necessarily true. In fact, it is seldom true.
On my monitor the image at the top of this page is a bit over three inches across. The ones below it are around five and four and a quarter inches across, respectively. If you have a different size monitor or have it set to a different resolution than I do, you're displaying a different number of pixels per inch, so a given number of pixels will take up a different number of inches. If you have a ruler handy, compare the width of these images on your monitor with mine.
Digital images may be viewed on a monitor or printed. If you're asked to supply an image for use on the Web, it'll typically be requested at a particular size, in pixels. You might be told that the larger dimension must be 300 pixels, for example. Usually when an image is put on a Web site one pixel in the image occupies exactly one pixel on the monitor. (The Web page can specify otherwise, but quality and/or efficiency is lost by doing this.) We know now that those 300 pixels might occupy a different amount of space on your monitor than mine, but such is life on the Web.
When you prepare an image for print, you can specify the number of pixels you want to occupy each inch. And, just as with the monitor, if you change the number of pixels you're fitting in an inch, you're also changing the number of inches the image will occupy. For example, if your image is 600 pixels wide and you specify that you want to print at 300 pixels per inch, the image will be two inches wide. If, instead, you specify only 150 pixels per inch, the image will be twice as wide, occupying four inches. Notice that the image itself is no different in these two situations. It still contains 600 horizontal pixels. You're just determining how big you would like each of those pixels to be printed. You might decide that you want the image printed really big, like, say, 37.5 inches wide. (This size makes the math work out well.) Well, 600 pixels divided into 37.5 inches means that you'll only have 16 pixels per inch. That means each pixel is 1/16 inch wide. That's pretty big for a pixel. You'll definately see the pixels.
The image to the left has 48 pixels across, and on my monitor the image occupies three inches. So each pixel is 1/16 inch across. (48/3=16) As you can see, the pixels are pretty big. So, while we could print our imaginary image at 37.5 inches wide, each pixel would be so large that it would be clearly visible. We refer to that phenomenon as pixelation. We would say that that print is quite pixelated.
Because you can specify the number of pixels you're going to put in each inch when printing an image, images for print are usually described as a certain size at a certain number of pixels per inch (PPI). So, back to our imaginary image of two paragraphs ago. The very same image can be said to be two inches wide at 300 PPI, or four inches wide at 150 PPI, or 37.5 inches wide at 16 pixels per inch. It's the same image. We haven't changed it a bit. We're just deciding how big we're going to print each pixel.
I started out by distinguishing between dots per inch and pixels per inch. Dots per inch is a measurement used in printers. If you're using a laser printer, you will get good results if you match the PPI to the DPI. If you're using an ink jet printer, you typically do not need to have as high a PPI as your DPI. My printer, for example prints at 1440 dots per inch, but I find that I see no difference with this printer between a print from an image with 150 pixels per inch and one with 300 pixels per inch.
If you're going to print your image using an offset press, a whole new variable is introduced, i.e. the line screen. But that is beyond the scope of this document, because I suspect your brain is already full.
I hope this has been helpful and you have not been too confused by all the numbers.
April 11, 2005