The Beginner's Guide to Image Editing, Pt. 3

What is resolution?

Image resolution is the number of pixels displayed per unit of printed length in an image. It’s a measurement used for printing (hence the phrase “of printed length”) and it’s usually stated in pixels per inch (ppi—used when referencing stuff onscreen) or dots per inch (dpi—used when referencing something that’s about to be printed because printers print dots).

How do resolution and pixels work together?

The way resolution affects an image is thus: For a given number of pixels in the width or height of an image, the higher the resolution, the closer the printer packs those pixels together in order to make a beautifully smooth and sharp print. The closer the pixels are packed together, the smaller the image will be physically when printed. Conversely, the lower the resolution, the more loosely those pixels are packed and the larger the image can be physically—though it loses definition and the print will look extremely crapified.

Because our monitors can only display a certain amount of information our eyeballs/brains can process, resolution really only matters to printing devices. That’s right, it doesn’t mean a hill of beans until that image is headed for a printer, because only a printer can do anything with the resolution measurement. Your monitor just doesn’t care: A 72 ppi image will look exactly the same as a 3000 ppi image, though the latter will appear physically huge on your screen and you’ll need to zoom way out to see it all.

So how much resolution do you need to make a gorgeous print? It depends on the printer. While the current crop of consumer-level inkjet printers do a fine job at 225-250 dpi, a professional service bureau may require 300+. If it’s your own printer, run a few tests. If someone else is printing it, ask them what resolution they need.

Important things to note in the screenshot above:

1. At the bottom of the document window, you can see that the file is indeed large—it’s 14.4 MB (circled in red).
2. Note the pixel dimensions shown in the upper portion of the Image Size dialog: 2599 x 1949 (also circled in red). Remember these numbers for a few seconds.
3. Note the honking big physical dimensions of this image. If I were to try printing the thing at its current resolution it’d be over 36 inches by 27 inches... INCHES!

Now, watch carefully as I increase the resolution of the image without messing with the number of pixels. Here’s how in either Photoshop CS or Elements.

Step 2: At the bottom of the dialog, uncheck Resample Image. This all-powerful, magic option locks the number of pixels contained in the image, thereby locking the quality. Enter 300 into the resolution box and look what happens:

The image now has a resolution of 300 ppi and the physical size has decreased to about 8.5 x 6 inches. See how the physical dimensions of the image changed but the pixel information didn’t? I have exactly the same number of pixels I started out with: 2599 x 1949; they’re just packed more tightly together. The file size didn’t change at all—it’s still 14.4 MB, as evidenced in the document window and in the Image Size dialog (another clue that the pixel data didn’t change). I haven’t changed the quality (pixels), I’ve just changed the measurement (ppi) which only affects the printed image.

Admittedly, this is confusing because the onscreen image didn’t change a bit (see earlier paragraph on monitors, eyeballs, and brains). The truth is only revealed by the Image Size dialog. The thing to remember is that as long as you 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. Ever. If you leave the Resample Image box checked, though, you’re messing with photo quality.

Step 1: Press C to select the Crop tool in either software package.

Step 3: Choose File > Print to print it now, or save the photo to print later. If it’s being sent to someone else to print, choose File > Save As and pick TIF from the file format pop-up menu. TIFs can be opened in most image editors and is a lossless graphic format, which will touch on a bit later. You can also print directly from a native Photohop or Element document.