PPI, or Pixels Per Inch, is a very misunderstood topic in digital photography and is often confused with Dots Per Inch, or DPI. I have seen many "professional" organizations ask for image submissions to be no less than a certain PPI or DPI (only adding to the confusion), for which I am about to show you is a completely useless requirement. From now on, we will use the correct term, "PPI". In fact, I am going to give you everything you need to know about PPI right now, in this first paragraph. Basically, PPI 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 PPI? Then please keep reading.
First, an explanation of what PPI is. Like I said earlier, PPI is Pixels Per Inch. A better term might be Pixels Per Printed Inch. It is the number of pixels in your digital image that will be squeezed into one inch of your printed image, therefore determining how big the overall print will be. It is nothing more than an instruction to the printer (the hardware, not the person)--it doesn't change your pixels at all if you go from one PPI to another--so long as you are not resampling your image (adding/subtracting pixel information), PPI does not change the number of pixels in your image. Whether your image is 1 PPI or 1000 PPI it’s the same image. The only thing that changes is the output size.
However, like I said in the first paragraph, PPI 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. What may be different is screen resolution (which is measured in dots per inch). The higher the screen resolution, the smaller your image will appear to be. But a 640px x 480px image is the same no matter what screen resolution you are using. It only appears smaller with higher res monitors as they pack the pixels closer together.
So how big will the image be on screen (in inches)? Hopefully that sounded like a really strange question, because that's exactly what PPI 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 (assuming that both monitors are running at the same resolution, say 1280x1024). Just like it would be bigger on a projection screen. Fortunately, in practice we don't measure our images on screen in any hope that we are going to get a specific size. Could you imagine? "I want that image to be 5x7 on everyone's monitor, no matter what size monitor they have and what resolution they are running at." 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. PPI 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 72 PPI, and the other at 300 PPI. Why those two particular resolutions? Well, let me do the easy one first. 300 PPI is the de facto standard output resolution for digital images. Printers are capable of printing at much higher resolutions (output described as dpi as printers output dots not pixels). The human eye though cannot interpolate image resolutions higher than around 300 PPI so this is why it’s generally the maximum resolution that is asked for by many pro labs. There's no reason to use higher resolutions as your eye will not be able to see the difference. You are just wasting disk space and processing time.
Now that I have spilled the beans on DPI, let's discuss it for a moment. DPI is used generally as an “output” resolution. Printers and monitors - scanners also use DPI even although it’s an input device but what is does is scan physical prints which are measured in DPI (hope I’ve not confused you)! So, in general, as photographers we generally do not need to concern ourselves with DPI issues and could probably drop the term from our vocabulary unless we were buying a new printer or monitor, or correcting someone for misusing the term DPI. You may want to read this Wikipedia article for more information on DPI.
So, where did the 72 PPI 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 DPI (DPI because it is an output device). What that does for you is if you are looking at an image in PhotoShop that is set to 72 PPI, the image on your screen really is actual size as it will be printed. And when you view an image in Photoshop at 100% you are viewing the pixels mapped 1-1 (1 pixel for 1 dot on your screen). Pretty handy, huh?
So, back to the two images above. Could you tell which one is 72 PPI and which one is 300 PPI? They are both being displayed at 100% resolution, 100% crop, full zoom, which you may think is odd (unless you have understood everything so far--yeah me!). 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. 2 bytes is nothing and in real terms both images are the same image.
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 300 PPI 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 PPI 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? First let me introduce a small simple equation that can be looked at in three ways to work out any variable you require. · Px = Pixels (on one axis) · R = Resolution (ppi) · Ps = Print Size Ps = Px/R Px = Ps x R R = Px/Ps
So lets say we want to print the above images. What size print would the 300 PPI image print? Ps = Px/R so 482/300 = 1.6” Now with the 72ppi image – Ps = 482/72 = 6.694" Because the higher resolution packs the same number of pixels closer together, the image appears much better in print (although smaller). The larger print may show the individual larger pixels making the image look blocky and pixelated. But guess what? You could give me the 72 PPI image, just like it is, and I could change the PPI to 300 and it would now print out at 1.6 inches on each side (remember this assumes image resample is switched off). Now, if I wanted to print the image 6.69 inches and at 300 PPI, well then I've got a small problem. I’d need a larger image! I could add pixels in photoshop (resampling switched on) but that will certainly add artifacts to the image (it may still appear blocky). How many pixels would I need to print the image at 300 PPI? Px = Ps x R so Px = 6.69 x 300 = 2007 pixels on each axis! Way more than the 482 we have ;)
So, like I said, changing the resolution 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 resolution of an image and click OK (try something ridiculous like "1 ppi"). I mentioned previously about “Image Resample”. With resampling switched on, Photoshop WILL change the number of pixels in your image to maintain the print size chosen. Generally this should be off unless you need a specific print size and a specific resolution. For example you have an image that is 3000 x 2400 pixels. At 300 PPI, what size will be able to print an image? Well (axis a)Ps = Px / R = 3000/300 = 10”. (axis b) Ps = 2400 / 300 = 8. My image will print a 10 x 8 Lets say I want it to be a 20x16 image. If I left everything the same except the resolution, I’d be able to print a 20x16 at 150 PPI (20 x 150) = 3000 & (16 x 150) = 2400. Is 150 PPI acceptable for a 20x16 print? Probably. But that's for you to decide. Generally people view larger prints from farther away, so a lower PPI is usually acceptable. Oh, and don't forget, the printer itself is still printing at whatever DPI it is set at (perhaps 300).
If I want to print at 300 PPI, how many pixels would I require? Px = Ps x R = 20 x 300 = 6000 and 16 x 300 = 4800 – So 6000 x 4800 pixels!! You could resize your image in photoshop, but be careful. Photoshop "randomly" adds pixels and doing this could ruin a good image. Better to print at the default resolution where you can. Large images can be printed at lower resolutions because they are not viewed in the same way as small prints. 150 PPI would have been fine for the 20x16 although labs may still request a 300ppi image. If so just resample and check the sharpness of your image. You may need to sharpen a little. There is also commercial software that you can use to upconvert your images to make them larger, and there are also photoshop tricks that you might look for. Just remember that you need to watch out when you make your image bigger in pixels. You are permanently altering your image and you may not like the results.
So, like I said, changing the PPI 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 PPI of an image and click OK (try something ridiculous like "1 PPI"). Your image is ruined, right? You forgot something. When you changed that PPI, 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 PPI 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 PPI 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 PPI, 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 PPI 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 PPI to 600 PPI. 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 PPI 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 1 PPI or 1000 PPI. It just doesn't matter. Hmm, I see a new marketing scheme: "The new PhotoPhlex 2000. Better than the rest because it has 1000 PPI resolution." I hope you are laughing right now. The reality is, different camera manufacturers do apply different PPI 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 PPI 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!