By Carl Weese
When I look at a picture onscreen at more than 100% and up to the limits of what the software will present, I see a nice clear grid of pixels, but I literally haven't the slightest idea whether the mush of color and tone they present as a fuzzy mosaic would be a crisp presentation in a print.
It's like trying to tell whether a 35mm negative is sharp using a grain focuser. No matter how soft the picture might be, you can always find a nice crisp grain pattern with enough magnification. You have to back up and look at the projected image with the naked eye to tell whether the picture itself is sharp. At magnifications well over 100% on screen, files will show a crisp grid of pixels whether the optical image is sharp or not.
For what it's worth, Adobe recommends 100% view only for special technical tasks like using the chromatic aberration tool. Jeff Schewe, who I think is the best tech writer out there on digital imaging, recommends using 25% view for examination of files. The theory is that the amount of interpolation PS does to present a monitor image at 25% is analogous to the interpolation the printer will be doing. I think he recommends that if you are examining a RAW file that you plan to enlarge, you might use 50%, but 100% and over is just a rabbit hole.
Side tip: if you fiddle with the size of the ACR window, you can force it to present the default "fit screen" view at exactly 25%. Convenient.
Another parallel occurs to me. You can't focus a view camera with a 10x Loupe. Nothing ever looks sharp. It's too critical for the task. 3x or 4x is plenty.
One last note. I do some scanning of stock photos my book packager client acquires. One guy in particular has a fantastic old file of chromes going back 30 years of work with architects and magazines. The subjects are great, but the film is in poor condition. I've learned that it's best to retouch at 50% view. (That's 50% for a file at the size it will run in the book, 300 ppi.) I won't retouch until the client can tell me it will run 4", or 8", or whatever, because I could spend double or more the (billable) time retouching a shot that will run large than if it runs small. At that size I can work fast, and the minor problems all disappear at print size. If I tried to retouch these at 100%, I'd never finish doing completely unnecessary work.
Featured Comment by Moose: "If you want a serious treatment of this area, the late PS guru, Bruce Fraser, treats it as an integral part of the practice of preparing images for printing. He outlines measurement and calculations to find the right magnification for your own situation and several excellent illustrations of how bad 100%+ images can look on screen that are nevertheless just right for printing. The book is Real World Image Sharpening with Adobe Photoshop CS2. I think I first heard of it here? I'm very happy I found it."