For a quick visual judgment about how much sharpening to apply, it is a common practice to use View | Actual Pixels in Photoshop. If your image is destined for a desktop inkjet printer, View | Actual Pixels is a poor choice.

Let's consider a 6MP image from something like a Canon 10D or Digital Rebel. If you do not resample or crop the image, you could find yourself scrolling over an image that is more than 28 x 42 inches. This is the result of the impedence mismatch between typical monitor resolutions and typical printer resolutions.

The rule of thumb is that monitors are 72 ppi. Actually, it depends on the video mode and the size of the monitor. 1280*1024 will have more or fewer pixels per inch, depending on thr size of the monitor.

Prints on a desktop inket printer are typically printed at 180-300 ppi.

Take that 6MP image, which is likely 3072x2048, and those pixels result in a much larger image when viewed on a monitor. The solution is to reduce the viewing size of the image in PS. For a 6 MP pixel image, 40-50% will give you a better judgment about whether the image will likely be under-/over-sharpened when it is printed. Larger images, like the 8.3 MP images from the Canon 20D or 1D MkII, should be viewed with even lower magnification.

Here's a 100% crop of an image that has not yet been sharpened:

While nothing looks out of the ordinary, the image is actually magnified relative to how it would appear as a 12x8 print. The image below is a screen shot of the same image viewed at 40%. Still not sharpened.

If I relied on the 100% view, I would be inclined to over-sharpen the image. Why? The 100% view on the monitor appears more like an image that has been resampled 150%. It is softer.

Here is the same image, after three-pass sharpening with an added round of local contrast enhancement:

In practice, when sharpening I move between extreme magnification -- on the order of 500% or 1000% -- and reduced magnification -- around 40%. With my monitor, I find 40% gives me an on-screen display for a 6MP image that is nearly identical in size to an 8x12 print. I use extreme magnification to count the pixels in the sharpening halo.

I aim for a sharpening halo of 1/50th of an inch. I divide my intended ppi by 50. So, for a desktop inkjet printer, I look for a sharpening halo of 5 to 6 pixels. Remember, the halo includes both light and dark boundaries. If you use either light or dark pixels alone, aim for 1/100 of an inch (1/100 + 1/100 = 1/50). Here's a crop at 1000%, with a portion of the sharpening halo highlighted in red. For more on sharpening halos, see my tutorial, "Put a Fine Edge on Your Sharpening Skills."

Viewing the image at the right magnification and counting pixels in your sharpening halos will likely get you very close to optimum sharpening settings. It is important, however, to make a test print and look at it from the intended viewing distance with appropriate lighting. You may still need to refine your sharpening settings. Monitors, after all, emit light. Photographs reflect light. Not at all the same thing.

Counting pixels in the sharpening halo and then reducing the image to judge how it will look when it is printed is all well and good. However, there is no substitute for a test print.

Cheers!