I sit not far from Client Relations here at iStock, and sometimes hear the tell-tale signs of a well-meaning, but somewhat disoriented iStock image purchaser: it's the dreaded 72 dpi question. The conversation usually goes something like this:
This usually goes on for a while until our poor rep gives up and refunds the purchase, or convinces the nice customer that the laws of mathematics do apply to picture resolution. This whole business of Dots Per Inch (dpi), Pixels Per Inch (ppi), or even Pixels Per Centimeter (ppc, for our European friends), can be downright confusing, even for seasoned pros (we had a well-known Photoshop writer give us an email tongue-lashing about this very subject. Was she ever embarrassed when we pointed out her mistake).
We're going to boil it down to the bare minimum. Forget about the dpi of an image. It's pretty much irrelevant until ink hits paper. What is important? Total number of pixels. The resolution can be set to anything (72, 300 or 3000 dpi), and as long as the total number of pixels in the file are kept the same, it's all good. Note here that we are certainly not saying you should up-res or up-sample your images, as it's referred to. This is where people are really getting confused. They've always been told that increasing the resolution of a file is bad--and it generally is*--but that's not what we're doing here. We're scaling the image instead of up-sampling and we're going to do it in a way where the number of pixels won't change one tiny little bit.
Here is the important take-away: the dpi of a file is just a way to measure it. When someone says they need a 300 dpi file, EVERY file can be measured at 300 dpi, whether is happens to be saved that way or not. For example, let's say you just shot a picture with your spiffy new camera. It's 3500 x 3530 pixels, but by default the image was saved at 72 dpi. So if you divide the numbers of pixels by 72 dpi, you will get the total number of inches the picture can be reproduced at.
To drive this point home, let's take a look (literally) at the original image alongside Photoshop's Image Size dialog box:
There are several important things to note in this screenshot:
1. At the bottom of the document window itself, you can see the file is indeed large -- It's 35 MB (circled in red).
2. Note the pixel dimensions shown in the upper portion of the Image Size dialog box: 3500 x 3530. Remember these numbers for a few minutes.
3. Note the honking big physical dimensions of this image if we were to try and print the thing at its current resolution of 72 dpi. It'd be over 48.6 inches by 49 inches people... INCHES!
Now let's change the resolution of the image without messing with the number of pixels, and thereby preserving the quality. Here's how:
Step 1: In Photoshop (or Photoshop Elements), choose Image > Image Size (in Photoshop CS2 just press Command + Option + I on a Mac, or Alt + Ctrl + I on a PC).
Step 2: At the bottom of the dialog box, uncheck Resample Image. This locks the number of pixels contained in the image, thereby locking the quality. Enter 300 into the resolution box, and look at what we get:
The image is now 300 dpi, and the physical dimensions have decreased to a little more over 11.7 x 11.8 inches. See how the physical dimensions of the image changed, but not the pixel information? We have exactly the same number of pixels we started out with: 3500 x 3530. Notice how the size of the image didn't change at all? It's still 35 MB, as evidenced in the document window itself, and in the Image Size dialog box. We haven't changed the quality (pixels), only the measurement (dpi) which affects the physical output size of the image.
As long as you remember to uncheck the Resample Image box, you can tweak the resolution 'til the cows come home and you won't alter the image quality at all. Seriously.
When determining which size image to buy for a particular project, figure out if you need to print the image or not, and if so, what physical size does that printed image need to be. Once you've found the perfect image on the site, take a peek at the image resolution listed in the download box. As you can see, we've already done the math for you.
Even if the file happens to be 72 dpi when you open it in Photoshop, now you're armed with the proper technique for changing it. After all, just because you're printing the image doesn't necessarily mean it has to be 300 dpi. If you're doing commercial printing, talk to your printer about what's required. If it's a color newspaper ad, you'll only need 150 to 200 dpi max. A glossy magazine: 266 to 300 dpi. A high-end coffee-table book using stochastic screening: maybe even 350 dpi.
Inkjets generally require at least 150 dpi (when the resolution of your pinter is 1440 dpi). There are many devices in the current crop of consumer-level inkjet printers which do a fine job at 225-250 dpi. The best thing to do for your own printer is to run a few tests. Can you tell the difference between 200 and 300 dpi?
We hope that this article eliminates any further resolution confusion. Here at iStockphoto, we're all about this wonderful community we've managed to build. We'd never try to pull one over on you, our customers, and if we did, we wouldn't be where we are today. So the next time you think you've been dupped with a 72 dpi image, just relax, take a deep breath, and remember: It's all about the pixels, baby!
*Upsampling is often used to make really, really, really big images—and it's not all bad. If you need to upsample an image--for a billboard for example--use Photoshop and increase it 5 or 10% at a time (with Bicubic Smoother), until you get to the size you're looking for. It works much better than doing it in one step. Alternatively, there are a few great apps on the market that use advanced techniques for even better results: PhotoZoom Professional
and Genuine Fractals
are good examples.
**We often get asked why we don't change the files to 300 dpi. The answer is: we do, when we can. Our system will change the dpi to 300 when it's possible without re-saving the JPEG (since re-saving JPEGs degrades image quality).