Sharpening needs to occur in two stages, the first needs to be done on the raw digital image file to account for any flaws in the camera’s optics. This is called an ‘initial sharpen‘, and it is generally done is a light fashion.
The second sharpening is done on the image file once it is sized for printing. This is called the ‘sharpen for output’. At this stage you can pretty much go as hard as you wish with sharpening. We advise that after resizing the image to print size at 300dpi in Adobe Photoshop, you view it at 100% (actual pixels), and then use filter-sharpen-unsharp mark to apply the sharpening.
If you are using Adobe Lightroom, view the image at 100% when you are in the develop module and apply sharpening. Beware, Lightroom is generally used in handling raw files, and is not showing you the 100% information for the print size, so treat this as an initial sharpen, unless you make a copy of the file during an edit in photoshop, and downsize it for printing.
The problem with over sharpening the raw file is that as you down size the image for printing in a print RIP (printing software), you can generate some nasty effects such as moiré. Moiré is where a regular pattern such as a grill of a car , brick wall , is distorted and often discoloured, sometimes in a very distracting way. The sample here is meant to be black and white image but you can clearly see the grill of the car has both a distorted pattern on it and discoloured.
This problem is becoming more prevalent as photographers give clients the DVD of the professional shoot with the images over sharpened at high resolution. The only way to get this super-sharp look: size your files to the print size you require at 300dpi, then sharpen to taste.
So beware when pushing things towards a sharp edge!