The key to understanding this percentage stuff is to remember two things. First, at 100-percent view (otherwise known as Actual Pixels), each image pixel is represented by a single screen pixel. This view has nothing to do with how big the image will appear in print (or even on the Web, because different monitors have different resolutions). Second, at any percentage other than 100, you’re probably not seeing a fully accurate view of your image.
At 400 percent, the image is magnified four times. At 50 percent, it’s reduced by half, so you’re only seeing half the pixels in the image because you’re zoomed farther out and Photoshop has to downsample the image on the fly. When you’re viewing at an integral multiple of 100 (meaning 25, 50, 200, 400 percent, and so on), Photoshop displays image pixels evenly. At 200 percent, four screen pixels (two horizontal, two vertical) equal one image pixel; at 50 percent, four image pixels equal one screen pixel, and so on.
However, when you’re at any “odd” percentage, the program has to jimmy the display in order to make things work. Photoshop can’t cut a screen pixel or an image pixel in half, so instead it fakes the effect using anti-aliasing. The moral of the story is always return to Actual Size (100 percent) view to peruse your image, particularly if you’re trying to evaluate the effects of Unsharp Masking. By the way, while it’s tempting to select Print Size from theView menu (in order to see how large the image will be on paper), this setting is only accurate on 72 pixels-per-inch monitors—in other words, on those old 13-inch Apple monitors and hardly anything else. We just ignore it.