In the first part of our look at image resolution, we examined the first section of the Image Size dialog box in Photoshop, "Pixel Dimensions", which, as a quick recap, tells us the width and height of our image in pixels, and tells us the file size, which is usually in MB (megabytes, or "millions of bytes"). Nothing terribly confusing here.

The second section of the Image Size dialog box is "Document Size", which can be a bit more confusing but really isn't much more complicated than the Pixel Dimensions section. In fact, the two of them go hand in hand. Let's take a look at the Document Size section, and by the time we're done, you should have a pretty good grasp on the difference between the two and on image resolution itself.

Document Size

Document Size goes hand in hand with Pixel Dimensions, yet is also completely separate from it. "Gee thanks, that really cleared it up," you're saying. I know it sounds confusing, but bare with me for a moment. Notice at the bottom of the Document Size section, it says "Resolution", and in the Resolution box, it says "72". Notice also to the right of that is another box, this one saying "pixels/inch".

What this is telling us is that when we go to print the photo, 72 pixels out of our 3456 pixels from left to right in our photo (the width), and 72 pixels out of our 2304 pixels from top to bottom in our photo (the height), will be printed for every one inch of paper. That's what "image resolution" means - how many of your image's pixels left to right and how many of the pixels top to bottom will print in every inch of paper. Of course, an inch is a square, which means the number of pixels from left to right and top to bottom will always be the same, and that's why the Document Size section contains only one number for Resolution. That number (72 here) represents both the left to right and top to bottom number.

So, if we have 3456 pixels from left to right in our photo, and 2304 pixels from top to bottom in our photo, and we have 72 pixels per inch listed for the resolution of our image, how large will our image actually be if we were to print it? Well, to figure that out, all we need to do is divide the width and height of our image (in pixels) by the print resolution (also in pixels). So let's do that:

3456 divided by 72 = 48
2304 divided by 72 = 32

After our simple math (yes I know, math sucks but this one's easy), we find out that at a print resolution of 72 pixels per inch, our photo is going to be 48 inches wide by 32 inches high. That's a huge photo! But wait a minute, didn't we see those numbers 48 and 32 somewhere before? Why yes we did. Take a look once again at the Document Size section:

Look what values it's giving us for the width and height of our image - 48 inches for the width, and 32 inches for the height. Exactly what we came up with ourselves when we divided the number of pixels wide and the number of pixels high by 72 pixels per inch (the resolution). And that's really all that image resolution is. It's how many if your image's pixels will print inside every inch of paper, which then tells us how large the image will be when it's printed.

Keep in mind as we're going along that I keep saying "printed". I can't stress enough, and this is the number one reason why so many people have a difficult time grasping the concept of image resolution, that resolution means absolutely nothing until you go to print the image. It has absolutely nothing to do with how your image appears on your screen.

Just to prove there's nothing up my sleeve, let's change the resolution value of the photo from 72 to, oh, let's make it 300, which will mean that for every inch of paper when we go to print our image, 300 of our image's pixels will be printed from left to right and 300 again from top to bottom. You can see the change in the screenshot below:

Now, since 300 of our image's pixels from left to right are going to be fitting inside every inch of paper as opposed to only 72 pixels, it stands to reason that it's not going to take 48 inches of paper to fit the entire width of our photo into. Likewise, since 300 of our pixels from top to bottom are going to be fitting inside every inch of paper as opposed to only 72 pixels, it shouldn't still take 32 inches of paper to fit the entire height into. Just for fun, let's do the simple math ourselves. Once again, all we need to do is divide the width in pixels and the height in pixels by the resolution in pixels. So let's do that:

3456 divided by 300 = 11.52
2304 divided by 300 = 7.68

According to my math, when I take 3456 pixels wide and divide them by 300 pixels per inch, that gives me 11.52 inches. Likewise, 2304 pixels high divided by 300 pixels per inch gives me 7.68 inches. In other words, when I take my photo that's 3456 pixels wide by 2304 pixels high and print it at a resolution of 300 pixels per inch, my photo will be 11.52 inches wide by 7.68 pixels high. Let's take a look at what the Document Size section is telling us. Am I right?

Looks like my math skills are stronger than ever (okay, so I used a calculator). Photoshop is showing us exactly what we expected, that at a resolution of 300 pixels per inch, it will take 11.52 inches wide and 7.68 pixels high to print our entire photo.

To summarize then, all "image resolution" means is how many of your image's pixels will print inside every inch of paper. Again, it has no effect at all on how your image appears on your screen, since your monitor has nothing to do with your printer.

Pixel Size

There's one other aspect that image resolution has to do with, and that's the size of the pixels when you go to print the image. It makes sense, really. An inch is an inch is an inch. The size of an inch is always the same. It's, well, one inch. So, since the size of an inch can't change, the size of the pixels has to change. For example, in order to fit 300 pixels into an inch, you would need pixels that are considerably smaller than if you only wanted to fit 72 pixels into an inch. Sort of like how, if you wanted to fit 10 people into a phone booth, you'd need people who were considerably smaller than if you only wanted to fit 3 people in there. Fortunately, you don't need to worry about that. Photoshop takes care of resizing the pixels for us. I just wanted to explain that "image resolution" really means two things - the number of pixels per inch that will be printed on the paper, and the size of those pixels. As I said though, Photoshop takes care of sizing them for us. All we need to do is make sure we're using the correct value for "pixels per inch".

"Hey, wait a minute!" you're thinking. "Correct value?! What correct value? There's an actual number I'm supposed to use for resolution that's 'correct'?!"

Yep, there sure is. Well, if you're concerned about image quality, anyway (and of course you are). We look at the "correct" resolution values to add for maximum image quality when printing in our next section - How Image Resolution Affects Print Quality.