Written By Steve Patterson

Have you ever downloaded an image from the internet and then printed it, only to get results that were, well, less than you expected? The image looked great on your computer screen, but when you printed it, it either printed at the size of a postage stamp or it printed at a decent size but looked blurry and/or "blocky"? The culprit is image resolution.

Actually, that's not really fair to say. Image resolution didn't purposely set out to make your life miserable when you printed your downloaded internet photo. The problem was simply that most photos on the internet have very small pixel dimensions, usually in the neighborhood of 640 pixels wide by 480 pixels high, or even smaller, and that's because images don't need to be very large in order to appear at a decent size and good quality on your computer screen, and also because smaller images download much faster on websites than larger images do. But that's getting into a whole other topic of optimizing images for the web, and we don't need to worry about that here.

So what can you do to make photos you download off the internet appear just as high quality when printed as photos you took yourself with your digital camera? The answer - absolutely nothing. There simply are not enough pixels in most internet images to allow them to print at high quality, without printing them at the size of a postage stamp, that is. Let me explain why.

First of all, let's get off the topic of downloading images off the internet (since we really shouldn't be doing that anyway without permission from the copyright owner) and look at image resolution in general. We covered it in much more detail in the "Resolution, Part 1" and "Resolution, Part 2" sections of the website, so this will be a short recap.

The term "image resolution" means how many of your image's pixels will print inside every inch of paper. As I stated in those two main "Resolution" sections of the website, image resolution has everything to do with printing your image and nothing to do with how it appears on your computer screen, which is why images you download off the internet usually appear much larger and/or higher quality on your screen than they do when you print them.

Let's use a photo as an example:

I can't help but laugh every time I see this photo of a horse I took while driving around the countryside one day. Normally this horse stands proud, beautiful yet powerful, full of grace and dignity. Yet I seem to have caught him in a rather unflattering moment. He's standing on a bit of a strange angle, he has a piece of straw dangling from his hair, and he seems to be in the middle of chewing his food. Either that, or he's desperately trying to crack a smile for me, I'm not sure. However, since this guy is already embarrassed, as am I for being such a great photographer, let's use this photo as an example.

On your screen, this photo looks decent enough as far as image quality goes. It's large enough to make out that it is in fact a picture of a horse, and it doesn't appear blurry or blocky. Overall, a pretty good quality photo. Let's take a look at what Photoshop's Image Size dialog box is telling us though.

As we saw in the "Resolution, Part 1" and "Resolution, Part 2" sections, Photoshop's Image Size dialog box (which you can access by going up to the Image menu at the top of the screen and selecting "Image Size" from the list of options) is divided into two main sections, "Pixel Dimensions" and "Document Size". The Pixel Dimensions section at the top is telling us that this photo contains 500 pixels from left to right (the width) and 333 pixels from top to bottom (the height). It's also telling us that the file size of the image (before I optimized it for the web) is 975.6K (kilobytes, or "thousands of bytes"). That may sound like a lot of pixels (500 wide x 333 high), and it is for displaying on a computer screen, but let's take a look at the Document Size section to see how large our image is going to be if we try to print it.

The Document Size section of the Image Size dialog box tells us two things - what the current print resolution is of our image, and what the width and height of our image will be when printed based on what the print resolution is. Currently, it's telling us that our print resolution is 72 pixels per inch, which means that of the 500 pixels we have in our image from left to right, 72 of them will print in every inch on paper, and of the 333 pixels we have in our image from top to bottom, 72 of those will print in every inch on paper. The number value in the Resolution box is for both width and height, not the total number of pixels that will print. Think of it like this - what the Resolution section is saying is, for every square inch of paper, 72 pixels from our image will be printed from left to right and 72 pixels from our image will be printed top to bottom. The total number of pixels in every square inch on paper then would be, in this case anyway, 72 x 72 (72 for the width times 72 for the height), which gives us 5184 pixels.

Let's quickly do some simple math ourselves to make sure that the width and height being shown to us in the Document Size section is correct. We know from the Pixel Dimensions section that we have 500 pixels from left to right in our image and 333 pixels from top to bottom. Our print resolution is set to 72 in the Document Size section, so to figure out how large our image will be when printed, all we need to do is divide the number of pixels from left to right by 72, which will give us our width, and the number of pixels from top to bottom by 72, which will give us our height. Let's do that:

500 pixels wide divided by 72 pixels per inch = 6.944 inches wide
333 pixels high divided by 72 pixels per inch = 4.625 inches wide

Based on our own simple calculations, we see that at a print resolution of 72 pixels per inch (ppi), our image will be 6.944 inches wide by 4.625 inches high. And if we look at the Document Size box again:

That's exactly what it says! Wow, that's going to print large enough to give us a standard 4x6 inch print, with a little extra to spare! That's great!

Sadly, no. If only life were that simple.

The fact is, 72 pixels per inch is not enough to give us good quality, professional-looking images when printed. It's not even close. To show you what I mean, here's the photo of the horse again as it looks on your screen:

And here's an approximation of how this same image will look if you try to print it as a 4x6 inch photo at its 72 pixels per inch resolution:

Doesn't exactly look good, does it? The problem is that at 72 pixels per inch, there simply isn't enough image information being printed for the photo to look good. It's fine for looking at an image on your computer screen because computer monitors are generally referred to as "low resolution output devices". They don't require high resolution images in order for the images to look great on them. Printers, however, are high resolution devices and do require high resolution images to maintain image quality.

So how high of a resolution value do you need for professional quality printing? The generally accepted number is 300. Printing an image at a resolution of 300 pixels per inch gives you enough image information for professional quality prints. In fact, 300 is usually a bit more than you need. You can often times get by with 240 pixels per inch for resolution without noticing any loss of image quality. The professional standard though is 300 pixels per inch.

Let's take our same image then at 500 pixels wide by 333 pixels high, change our resolution from 72 pixels per inch to 300, and see what we get. Here's the Image Size dialog box again showing the new resolution of 300 pixels per inch. Notice in the Pixel Dimensions that we still have 500 pixels for the width and 333 pixels for the height. The only thing that's changed is our resolution, from 72 to 300:

And here's an approximation of how the image will look when printed at 300 pixels per inch:

Hmm. Wait a second, that can't be right, can it? I mean, sure, the quality of the image looks much better now, but look how small it is! Well, actually, that's exactly right. Look closely at the Document Size section again:

Notice how it confirms that our image resolution is 300 pixels per inch, but check out the values for width and height. Our width is now only 1.667 inches, and our height is only 1.11 inches. That's because our image only contained 500 pixels from left to right and 333 pixels from top to bottom, so by printing at 300 pixels per inch, it didn't take much to fit the entire image on paper, and we're left with an image that looks great as far as quality goes, but is probably much smaller in size than anything you'd have a use for.

So okay, we've proven that downloading images off the internet and trying to print them isn't going to work. But forget about internet images. What about the photos I've taken with my digital camera? How can I make sure I'm using the right image resolution to get professional print quality from my printer? An excellent question, and the answer comes to us once again through some boring yet simple math.

Let's say you've taken some photos of your recent family vacation using your digital camera and you want to print out some 4x6's on your printer. We know now that in order to achieve professional quality prints, we need set the resolution of our images to a minimum of 240 pixels per inch, although 300 pixels per inch is the standard and is the ideal resolution to use. Let's look at both of these resolution values though to see how large of an image, in pixels, we'll need out of the camera in order to print 4x6's with good image quality. First, let's look at 240 pixels per inch:

To figure out how large, in pixels, our images from the camera need to be in order to print 4x6's at professional quality, all we need to do is multiply 240 times 4 for the width, and then 240 x 6 for the height (of course, that's assuming the photos are using "portrait" orientation. "Landscape" orientation photos would have 4 for the height and 6 for the width). Let's do that:

240 pixels per inch times 4 inches wide = 960 pixels
240 pixels per inch times 6 inches high = 1440 pixels

Based on our simple math, we can see that in order to print a digital photo as a 4x6 at 240 pixels per inch resolution, which should give us good quality, our photo's pixel dimensions need to be 960x1440. We can see exactly how many pixels that is by multiplying 960 by 1440, which gives us 1,382,400 pixels. Let's round that up to 1.4 million pixels. That may sound like a lot of pixels but it really isn't, not when you consider that 1.4 million is the minimum number of pixels you'd need to print good quality 4x6 photos using the minimum resolution we can to achieve good quality, which is 240 pixels per inch. The good news at least is that these days, most digital cameras on the market are 4MP ("mega pixels", or "millions of pixels") cameras and higher, so they'd have no trouble printing good quality 4x6's even using 300 pixels per inch for the resolution.

Of course, we haven't actually looked at how many pixels we'd need to print professional quality 4x6's at 300 pixels per inch, so let's do that now. We'll use the same simple formula as above, where we'll multiply 300 by 4 and then 300 by 6 to give us the pixel dimensions we'll need:

300 pixels per inch times 4 inches wide = 1200 pixels
300 pixels per inch times 6 inches high = 1800 pixels

Let's do one more quick calculation to see how many pixels we need in total:

1200 pixels wide times 1800 pixels high = 2,160,000

So, in order to print a photo as a 4x6 using the professional standard of 300 pixels per inch for resolution, our photo needs to be 1200 pixels wide by 1800 pixels high (again, depending on whether the photo is in portrait or landscape orientation, the width and height may need to be flipped), which means we'll need a total of 2,160,000 pixels, which again should be no problem for most digital cameras on the market today which are 4MP and higher.

What if you have a photo you absolutely love and feel it deserves an 8x10 print rather than a 4x6? How large of an image in pixels do we need to print a good quality 8x10? The answer is as easy as when we needed to find out how large of an image we'd need for a 4x6. All we need to do is multiply the resolution value in pixels by the width in inches, and do the same thing for the height. Let's first use 240 pixels per inch as our resolution:

240 pixels per inch times 8 inches wide = 1920 pixels
240 pixels per inch times 10 inches wide = 2400 pixels
Total number of pixels = 1920 pixels wide times 2400 pixels high = 4,608,000 pixels

From our little bit of math, we can see that in order to be able to print a photo at good quality as an 8x10, our photo needs to be 1920 pixels wide by 2400 pixels high, for a total of approximately 4.6 million pixels. Now we're starting to push the limits of lower end digital cameras. A 4MP digital camera doesn't use quite enough pixels to be able to print an image at 8x10 at 240 pixels per inch resolution. It's about 600,000 pixels short. You can still print an 8x10 image of course, but you're not going to get professional image quality anymore.

Let's do the same calculations for an 8x10 at 300 pixels per inch resolution:

300 pixels per inch times 8 inches wide = 2400 pixels
300 pixels per inch times 10 inches wide = 3000 pixels
Total number of pixels = 2400 pixels wide times 3000 pixels high = 7,200,000 pixels

Now we're really pushing the limits as far as digital cameras currently on the market. In order to be able to print a photo as an 8x10 using the 300 pixels per inch resolution standard, our photo needs to be 2400 pixels wide by 3000 pixels high, for a total of 7.2 million pixels! Now that's a lot of pixels! This means you need at least a 7.2MP digital camera in order to be able to print your photos as 8x10's and still get true, professional quality prints.