I’m often asked to explain image resolution, and these questions usually surface when digital files need to be used for print. Standard print resolution is much higher than what’s used for web or other low-res images. Hence, images used successfully online usually appear much smaller than they should be when you try to use them for print design.
Let’s step through the process of resizing images — or you can skip to the best solution at the end of this post!
Resizing a Low-Res Image
You already know that resolution is expressed in PPI, or pixels per inch. This is a ratio of print dimensions (inches) to the digital dimensions (pixels).
At 72 ppi, an image can measure 800 x 600 pixels and almost dominate a browser window. It looks as if it would easily be large enough to print full bleed on the front of a brochure, because when I check it in Photoshop, its size is about 11 x 8.3 inches.
However, when I change the resolution from 72 to 300, the size shrinks to 2.6 x 2 inches. Not cool.
Resampling Image Resolution
Let’s go back to the original 72 resolution in this example. Because we know high resolution is important for printing, let’s try to make this higher.
But this time, before changing the resolution, let’s click on the Resample option. Then we’ll change the resolution again to 300.
This time the width and height stayed at 11 x 8.3 inches. And take a look at the pixel dimensions in the above window; they actually more than quadrupled! So where did those extra pixels come from? Photoshop has automatically multiplied existing pixels to try to meet our size dimensions.
Below, here’s how the resulting photos look. The original 72 dpi photo is on the left and the resampled 300 dpi is on the right.
From the info in their windows, we know we’re viewing them at 100%, and the right image has obviously been made much larger. It’s technically 300 dpi, but unfortunately it’s not very good quality. Now you can almost see a rough texture of pixels — a blurry, “pixelated” effect. And because the photo will be printed fairly large, that unwanted texture will be easily seen.
In times of desperation, you can experiment with the other settings within the Resample option. Although it is an option, it decreases the quality of your image. If you’re aiming for a professional appearance in your print work, don’t rely on this procedure.
The Solution to Resolution
The key to having good image resolution is to start as high as you may reasonably need it. For example, I build everything — even images for the web — at 300 ppi. So if the customer falls in love with something I’ve done and wants to use it in a magazine ad, I have a good starting point without rebuilding the graphic from scratch.
If a high-res image is too bulky to work with, save it by itself. Then make a copy of the file to size down and use in your current project. Always preserve your original in high resolution for future purposes.
Stock Photos An exception: If you’re buying stock photos for web use, you’re pretty safe to buy a 72 ppi version at the finished size you need. Stock photos are usually priced by size, so you can plan to go back to buy a larger version if you decide to use it for posters or the like.