Managing Tonal Range

There are a number of ways to modify an image so that it is less vulnerable to the ravages of dot gain, dot disappearance, tonal jump, and other quirks of screen printing. Which strategy you choose depends on the image.

Compressing Tonal Range

Images that have a wide dynamic range can suffer from blowout in the highlights and blotchy fill-in in the shadows when screen printed. In order to avoid such degradation, it's advisable to decrease the contrast for full range scans, setting the white point at 10-15% and the black point no higher than 85%.  Examine your image before manipulating it - if there isn't any information in the bright highlights or dark shadows, little or no tonal compression may be necessary.

Clipping the Shadows

If an image is dominated by shadows, it may not make sense to limit the darkest areas to an 80% dot.  The desired effect may not be a dense halftone, but a smooth, continuous layer of ink. However, if one chooses to work with the darkest ranges, it's important not to have small fluctuations in the 90-100% range.  On screen, it's very difficult to distinguish details in the shadows, but at an 85 line screen, pockets of 96% density in a larger zone of 100% density manifest themselves as distinct pinholes in the film - usually not a desirable effect.

The solution is to completely eliminate subtle detail above 85% by "clipping the shadows." Using Photoshop's "curves" function, adjust the range so that as little of the image as possible is in the 85-99% range.  It will probably take some twiddling to get the image to look right.