Sharpening for the display is an uncertain endeavor unless you know exactly the display for which you're sharpening. But there's a much bigger issue, which is how to use the display as a guide for sharpening. The easy answer is, unfortunately, that there are no easy answers!
Fortunately, the situation isn't hopelessit's just challenging. The fundamental concern that should always be foremost in your mind is the actual size of the pixels on output. But to understand the relationship between what you see on the screen and what you get on output, some additional information is useful.
True Monitor Resolution
Many of us tend to leave unquestioned the polite fiction that computer monitors have a resolution of 72 pixels per inch (ppi). This is often not the casein fact it's quite unlikely that you monitor's resolution is exactly 72 ppi, though it may be close.
Monitor vendors usually specify the size of the monitor's image area as a diagonal measurement, which makes for a big number but isn't all that useful for figuring the resolution. Determining the true resolution of your monitor is a simple exercise that requires no equipment more complicated than a tape measure.
The horizontal image area of both my LCD displays (an EIZO CG 21 and an NEC 2180 WG) is meaninglessly close to 16.875 inches. I run them at 1600 x 1200 resolution, so the actual resolution is 94.8 ppi. (If I were to run them at 1280 x 1024, the resolution would be 75.9 ppi.) The image area on CRTs is more variable than on LCDs because the geometry controls let you adjust the picture size, but my 22-inch LaCie CRT and my 22-inch Sony Artisan both produce resolutions of around 123 ppi when run at 1920 x 1440.
Table 2-1 shows the approximate resolutions for typical display sizes and resolutions.
Knowing your monitor's true resolution is key in understanding the relationship between the pixels you see on your display and the final printed results.
If your display is around 100 ppi, and you're printing at around 240 ppi, viewing the image at 50% will give you a truer picture of the final sharpening. It still won't be perfectPhotoshop's downsampling algorithms have a different effect on sharpness than do the mechanisms by which printers or platesetters turn image pixels into dotsbut it'll be a lot closer than the Actual Pixels (100%) view.
Likewise, if your display is around 75 ppi and you're printing at 300 ppi, the 25% view will give you a closer idea of final sharpness than any of the higher zoom percentages.
Tip: Avoid Odd Zoom Percentages
The 50%, 25%, and conceivably the 12.5% views preserve sharpness reasonably well, but the odd zoom percentages (66.7%, 33.3% and so on) do not, because Photoshop applies fairly strong antialiasing to those zoom levels, thereby making the image appear softer than it will be on final output.
The key point to all this is that when we look at sharpening on the monitor, we're viewing it "through a glass, darkly" (though the Apostle Paul doubtless had something else in mind when he coined the phrase). At some fairly far-off future date, we may benefit from some technology that compensates for the large variation in the ways different monitors display the same pixels' sharpness, just as color management currently lets us compensate for the way different monitors display the same pixels' colors, but that day won't come soon.
Until that day arrives, you'll have to learn to make that compensation in your head. This is not trivial, but neither is it impossible. First, you need to understand the size relationship between your pixels on screen and your pixels on output. Then it's a matter of learning the behavior of your display, and making constant comparisons between what it shows you and what ends up in print, so that you gain experience as to what will work and what will fail. Throughout the remainder of this book, I'll provide pointers as to the kinds of things to look for.