Photoshop CS4 Fundamentals: Changing an Image’s Resolution and Size
Adapted from Photoshop CS4 After the Shoot (Wiley Publishing)
By Mark Fitzgerald
I’ve met many photographers who don’t understand how to change an image’s size or resolution correctly. The problem usually stems from a lack of understanding about image resolution and how it affects image size.
Caution: Always save a master file with all layers before changing size or cropping. Otherwise it may be hard to back up and resize the file for a different output use later.
One of the things that can be the hardest to get your head around when you start down the digital path is resolution. This confusion is compounded by the fact that there are two different kinds of resolution in the digital world. One is dots per inch, and the other is pixels per inch.
If you have a file with a resolution of 120 ppi or lower, you run the risk of seeing the space between the pixels when you print. This causes edge detail, which should be smooth in the print, to look jagged. The goal is to get the pixels close enough together so that these single dots form continuous tones and lines. The illustration below gives you an idea of how this works. As the dots get closer to one another, they begin to form a line. When you zoom out, the individual dots disappear. (It’s similar to the idea of getting ink dots close together on a printer (dpi).
Tip: Keep in mind that some people mistakenly use the terms dpi and ppi interchangeably, saying dpi when they really mean ppi. If they’re not specifically talking about a printer’s output, then they probably mean ppi.
The thing to keep in mind when it’s time to change resolution is that if you have an image file that’s 150 ppi and you simply change its resolution to 300 ppi for printing at a lab, the new image’s dimensions are affected because the distance between the pixels is cut in half. An 8 × 10 becomes a 4 × 5 at 300 ppi.
Doing the Simple Math
I know I just said the dreaded M word, but please keep reading because in the next few paragraphs I show you how to use simple math to understand what Photoshop is doing when you resize your photo files. Begin by getting a better handle on how resolution works. Follow these steps:
Don‘t miss the next Photoshop article on Graphics.com. Get the free Graphics.com newsletter in your mailbox each week. Click here to subscribe.
Adapted with permission from Photoshop CS4 After the Shoot by Mark Fitzgerald. Copyright © 2009 Wiley Publishing