Image resolution is complicated, and can be difficult to wrap your head around. So I’m not surprised at this common mistake: answering “300 DPI” to the question, “at what resolution would you like your image file ?” But that’s like answering “60 miles per hour” to the question, “how far is it from Detroit to Chicago?” Both answers tell you part of the story, but don’t supply enough information.
Look at the screenshots below, from the image size dialog box in Photoshop. Both show 300 pixels per inch images, but the top one is 1 inch square and the bottom one is 10 inches square. Notice the file size is much bigger in the bottom picture; they’re both 300 dpi, but one’s half a megabyte and the other is over 50 megabytes. The bigger file will print nicely at 10 inches, the top one will look terrible printed that size.
The Solution (short version)
When specifying digital image resolution, you don’t have to mention DPI. If the image is for print, just say how many inches wide or tall you want it, like “I need the photo to be 10 inches high,” or, “send me a photo sized for 8x10s.” Or, “the image will be printed 15 centimeters high.” Forget about DPI, just talk about inches and millimeters.
If the image is for use on the Web or for video, it’s best to specify dimensions in pixels. The big picture at the top of this post is 1000 pixels square, at 72 pixels per inch, but the “pixels per inch” part isn’t so important. If you’re talking to a photographer or web designer or videographer about the size of a screen image, you can just say how many pixels wide or tall you want the image, without getting into PPI.
Images for Print: More Info
So what’s with “300 DPI”? Isn’t that the proper resolution for print? Well, yes, sort of. But here’s why it’s not such a useful specification:
1) This one’s a little nitpicky. DPI stands for “dots per inch” which is an old printing term having to do with halftone dots (like you see if you look closely at a photo printed in a newspaper). With digital files, when you’re setting image resolution (in Photoshop, for example), you’ll be dealing with pixels, not dots. So really PPI (pixels per inch) would be more appropriate.
2) 300 PPI is the most common resolution for printing (inkjet, sending to a photo lab, printing press, etc), but not always. Some printers do better with 400 PPI resolution, and sometimes decent prints can be made from files at 240PPI. So 300 PPI isn’t always the best print resolution. But unless you’re doing the printing yourself, you don’t have to worry about this. What you do need to do is specify how big you want your photo printed. So forget about DPI and PPI, and just specify how big you want the printed image to be.
Another solution is to refer to file resolution in pixels. Notice in the screen grabs above, 10″ square at 300 PPI is the same thing as 3000 pixels by 3000 pixels (300 x 10=3000). This is actually simpler, because you only have two variables (width and height) rather than three (width, height, and pixels per inch). Also, pixel dimensions also apply to web sizes. Web designers specify height and width of images in pixels, not inches, and generally don’t use DPI or PPI numbers.