The abbreviation DPI has become one of the most used, but least understood term in relation to digital photography today, bar none. Every time I teach a photography class I end up having to do a 15 minute spiel on what it actually stands for and why most people should try their hardest to erase it from their brain in relation to what they think it means. It’s particularly confusing because it sounds like it’s being used correctly most times. If you value your sanity and the sanity of those around you, please keep reading.
First off, I want to tell you that it’s not your fault… you’ve been misled, albeit unintentionally by many people, most of whom you have a good reason to think know more than you. So often people use dpi when what they really want to know is resolution. One of the most frequent questions I get is “I’ve been asked to submit a photo to (blank) and they’ve asked for it to be no bigger/smaller than 300dpi, how many pixel wide should I make my final image?”. You’re confused, it looks like a reasonable request, but you just can’t seem to wrap your head around how to go about satisfying it. Or worse, you think you know what they want and solve the problem wrong. Let’s break it down and see where you’re probably getting caught up.
DPI stands for Dots Per Inch, this is a printing term describing how many dots a printer can make per inch on a piece of paper. You want to know how many pixels wide to make your image and here’s the first problem; dots are not the same as pixels and the terms should never be used interchangeably. Your monitor can display 16,777,216 different colors at each pixel site, whereas even the best 7 ink printers can only show 128 possible colors per dot. In order to show the same level of detail and color depth as your monitor, a printer has to be able to produce many more dots per inch (dpi) than the number of pixels per inch (ppi) you want to display; so if you wanted to print a 100px image, one inch wide, your printer would have to be able to print at least 400-600dpi in order for it to have the same amount of detail as the image displayed one inch wide on your monitor.
Ok, that’s sorted, but that still doesn’t answer the question: how many pixels wide should my image be? The problem is that people use dpi or ppi thinking it describes a discrete value instead of term of measurement. Asking someone to submit a photo that’s no more than 72ppi is like asking them to drive to a corner store that’s no more than 10mph away. Without knowing how many inches wide they want the photo, asking for a specific ppi is meaningless, and gives you no clue as to how many pixel wide they actually want the image. Any image, not matter how many pixels wide it is, can be displayed at any ppi, all that changing the ppi does is change how large the resulting image gets displayed. Here’s two extreme examples:
- Example 1) A 1000px wide image gets printed out on an imaginary molecular printer that can literally print on the head of a pin, displaying pixels as small as a single carbon atom. The printer can place a billion dots every inch so the resulting image, printed at 1,000,000,000ppi is only a millionth of an inch wide.
- Example 2) The same 1000px image gets displayed on a JumboTron at a baseball game which uses pixels that are one inch wide, so it has a resolution of 1ppi; the image therefor gets displayed 1000 inches across, roughly 83 feet.
In both examples the resulting image hasn’t lost or gained any resolution, despite the huge difference in size of the resulting image. So changing the ppi of your image will have zero result on the number of pixels or the file size of your image.
Googling around for what others have written about clarifying DPI has shown that even many people writing tutorials on the subject are getting this wrong. Is it really any surprise that many people are confused as well? The overuse of DPI comes from the days of publishing where different publications would print photos at different resolutions; newsprint had a relatively low dpi, whereas a magazine or photo book would have a higher dpi. But since they used standard image sizes and standardized dpi ratings, the combination of the two gave enough information for photographers to size their images. Then internet publishing came along and there were no longer standard images sizes. Somewhere along the line people forgot that, besides the fact that dpi and ppi aren’t the same, dpi alone isn’t enough to tell someone how many pixels wide or tall an image is.
So when someone tells you to give them an image with a min or max dpi, ninety-nine times out of a hundred, what they’re really asking for is either a min/max file size (measured in bytes) or a min/max resolution (measured in pixels). First step is to get that sorted and find out what they really want, once you have that you can actually do something about it.
It’s easiest if they’re looking for a pixel width, because there’s only one way to satisfy that. I’ll explain using Photoshop, but the process is similar in any photo-editing program.
- 1) Open your image and use the top menu to go to Image->Image Size (or press Ctrl-Alt-I) to bring up the Image Size Dialog.
- 2) Make sure “Resample Image” is checked because you want to change the number of pixels in the image, leaving it unchecked will just adjust the ratio of inches wide vs pixels per inch without actually altering the image. Also make sure “Constrain Proportions” is checked, this will force you to keep the same aspect ratio in your image so you don’t accidentally squish or stretch it out of proportion.
- 3) Change either the width or height in pixels, depending on what your requirements are. You’ll notice that the value you don’t change will be changed automatically to fit the proportions of your image. Click OK and you’re done!
There’s only one way to make an image greater or less than X pixels, but if they’re asking for a maximum file size, there’s a couple of ways you can do that. Mostly likely you’re being asked for a maximum file size, in which case you need to shrink your image while balancing resolution with file compression. The more pixels you want to keep, the more compression you’ll have to apply to keep the file size down, but apply too much compression and your image quality starts to suffer no matter how much resolution you keep.
No matter what image format you’re working in, you’re going to be converting it to JPEG as it’s the internet standard compressed image format. When you save as a JPEG in Photoshop it will give you an estimate of how big the resulting file will be for a given quality setting. The scale goes from 1-12; the lower the number the more compression, the lower the file size and the more you’ll degrade your image. On the high end of the scale you have less compression, larger files but little to no visual difference in your image quality. A setting of 10 is the least compression with the greatest image quality. One would think 11 or 12 would give you more quality, but while it will make your files larger, it’s a little known fact that those two setting where included by Adobe for testing and experimentation purposes. They offer larger file sizes but no greater fidelity to the original image.
Check if simply saving as a level 10 JPEG will give you the image size you’re looking for, if not try 8 or 9. If you have to go lower than 8, you’re usually better served by resizing your image a bit smaller instead to keep as much detail in your image as possible. So use the first method above to cut down the number of pixels in your image and try saving again… rinse… repeat… until you get the file size you’re looking for. You’ll very quickly get a sense of what pixel widths and compression settings will give you the image sizes you’re looking for.
Hopefully this helps clear things up. Like I said, it’s not your fault; dpi has been so horribly miss-used and abused lately it’s hard to get a straight answer. I’ve had everyone from editors of major publications and international photo contests get it wrong, so don’t feel bad if it’s left you dazed and confused trying to puzzle out what people are asking you to do to your images.