DPI, or Dots Per Inch, is a very misunderstood topic in digital photography. I have seen many "professional" organizations ask for image submissions to be no less than a certain DPI, for which I am about to show you is a completely useless requirement. In fact, I am going to give you everything you need to know about DPI right now, in this first paragraph. Basically, DPI ONLY matters when printing images. I'll say it again--it ONLY matters when printing images. It has zero effect on images displayed on the web, and has zero DIRECT effect on file size. So, are you with me so far? Do you need some poof? Are you interested and want to learn a little more about this often misused characteristic? Do you want to know what that cute picture has to do with DPI? Then please keep reading.
First, an explanation of what DPI is. Like I said earlier, DPI is Dots Per Inch. As it was designed, it is information stored in your image "alongside the pixel data" that is an instruction to the printer (the hardware, not the person) of how big to print the given image. It is nothing more than an instruction to the printer--it doesn't change your pixels at all if you go from one DPI to another. However, like I said in the first paragraph, it does nothing for images displayed on your monitor. The only thing your monitor cares about is the number of pixels (across and down). That's it. Just the total number. That's because the monitor just wants to know how many pixels on the screen to dedicate to the display of the image. Your image is 640 pixels wide by 480 pixels tall? Guess what? On **ANY** monitor, at a 100% zoom, your image will be 640 pixels wide by 480 pixels tall. So how many inches will that be? Hopefully that sounded like a really strange question, because that's exactly what DPI would have answered. As you know, if you are viewing that same image on your computer geek friend's 24 inch super monitor, that image will be bigger in INCHES than it would on a 15 inch monitor. Just like it would be bigger on a projection screen. But, for paper, now we are talking real inches here, where size does matter. If you print an image and you expect it to be 8 inches by 10 inches, you don't want to have to worry about what size printer is being used, or what size paper was loaded. You want the printed image to be 8x10, no questions asked. DPI to the rescue! Still don't believe me? How about some proof?
The two images above are both the same, except that one was saved at 72DPI, and the other at 300DPI. Why those two particular DPIs? Well, let me do the easy one first. 300 DPI is very popular for the maximum resolution most printers today are capable of handling. There's no reason to go any higher than the maximum resolution that your printer can actually print at--you are just wasting disk space and processing time. So, where did the 72 DPI come from? A long time ago, many of us had 14 inch monitors (if you are still using one raise your hand and repeat after me: "I'm too cheap to buy a new monitor"). Back to that 14 inch monitor. 14 inches was the diagonal measurement of the screen, from one corner to the opposite. The actual screen dimensions were 11.2x8.4 inches. Back then we all mostly ran our monitors at 800x600 resolution, which works out to 800/11.2=71.42 pixels per inch--pretty close to 72. What that does for you is if you are looking at an image in PhotoShop that is set to 72DPI, the image on your screen really is actual size as it will be printed. Pretty handy, huh?
So, back to the two images above. Could you tell which one is 72DPI and which one is 300DPI? They are both being displayed at 100 resolution, 100 crop, full zoom, whatever you want to call it. Does one look sharper than the other? What about the file sizes? I can tell you that there is exactly two bytes difference between the size of these two images. Not kilobytes, or megabytes, but "BYTES". One image is 149,200 bytes and the other is 149,202 bytes. Still can't tell? That's because there's no way for you to tell the difference other than look at the exif information in the images. If you did, you'd see that the top image is indeed the 300DPI image. By the way, it's also the image that's two bytes bigger. Wanna guess why it's bigger? It's because it takes an extra two bytes to store the extra digit in the size, 72 vs. 300. If you do think that one image looks darker or or the colors look a little different than the other, I'll bet that you are looking at them on an LCD monitor. Try this: move your head up and down while looking at the pictures, or scroll the page up and down while looking at them. You'll see that they are the same. Trust me on this--they're the same. So, what have we learned here? The DPI of an image does not affect the pixels. It is in fact, just an exif tag, stored in the image and says nothing about the actual image quality.
So, what would happen if we printed those two images? Well, the 72DPI one would print at 6.69 inches on a side (482/72) and the 300DPI image would print at 1.6 inches on a side (482/300). But since they both have the same number of pixels, the 72DPI image would look "blown up" if you were to view it up close. You would definitely be able to see individual pixels. But guess what? You could give me the 72DPI image, just like it is, and I could change the DPI and print it at 300DPI (it would print at 1.6 inches on a side). Again, there's no difference between those two images--that DPI is just a code in the exif information. Now, if I wanted to print either of those image at 6.69 inches and have it look good, like it was printed at 300DPI, well then I've got a problem. I don't have the pixels to do that. I could tell PhotoShop to resize the photo, but that will certainly add artifacts to the image.
So, like I said, changing the DPI of an image does not alter any pixels, or add or delete them. Now, I know what you are about to do. You are going to open an image in photoshop, click on the "Image" menu, then "Image Size...", and try changing the DPI of an image and click OK (try something ridiculous like "1 DPI"). Your image is ruined, right? You forgot something. When you changed that DPI, PhotoShop did you a favor and adjusted the pixel dimension to keep the PRINT SIZE the same. Remember, if you change the pixel dimensions, you are most definitely changing your image. Press ctrl-z to undo that DPI change and let's try it again. Click on the "Image" menu, then "Image Size...". Note the initial pixel dimensions (write them down if you are prone to senior moments like I am). Now change your DPI to something, like 1. Note that PhotoShop kindly adjusted your pixels dimensions for you. Thanks, but no thanks--you want them back the way they were. Change the pixel dimensions back to the original and press OK. Your image looks the same. Save it and the file size should be the same as before. Now, if you were to save it, there is one side effect if you were using a jpg image. Remember, every time you resave a jpg, the image gets recompressed. So you might get a slightly different image. So, what did change? Well, if you were to print that image at 1 DPI, you'd make a nice billboard. Your image would be huge (in inches that is). File size stayed the same.
Still don't believe me? Here is another page with four PNG images, each saved with different DPI settings. PNGs use a lossless compression, so I didn't have to worry about the re-compression issue when resaving the files. The images on that page range from 1 DPI to 600 DPI. Yes, ONE dot per inch. In other words, each dot is an inch wide (that will be a big dot, when printed). You can easily see that they are all the same size on your screen, they all look the same, and if you check the file size, you'll see that they are all EXACTLY the same size. Believe me now?
Things also get interesting when taking pictures and saving them to your memory card or when transferring images from your camera to your computer. Obviously, your camera has no knowledge of how you intend to print your image. Hopefully by now, if I were to ask you "what DPI should your camera save it's images at?", you'd say something like "it totally doesn't matter". Because it doesn't matter. It could be 1DPI or 1000DPI. It just doesn't matter. Hmm, I see a new marketing scheme: "The new PhotoPhlex 2000. Better than the rest because it has 1000DPI resolution." I hope you are laughing right now. The reality is, different camera manufacturers do apply different DPI resolutions to their images, but they have to apply something, right? Don't get worked up over it.
So, there you go. Everything you ever needed to know about DPI and digital imaging (unless you want to read this Wikipedia article, which does have a little more detail). Now go out there and take some pictures!