B8 Difficulties in Printing

The dynamic range of book reproduction is not nearly the same as the dynamic range of the human eye. When the eye receives the wrong light level, it adjusts. Paper can't do that. The halftone process divides gray scales into dot patterns. An ideal dynamic range for such a process allows a gray scale with 256 levels of intensity. 5

Clearly, the most desirable way of proceeding would be to use an unchanging scale for the printing process. This procedure is impossible with only 256 levels. We saw above that much interesting detail appears with an intensity of 0.0001, particularly with large values of defocusing aberration. Thus, no uniform scale can cover both in-focus images (intensity =1) and defocused images (intensity = 0.0001). We would need a gray scale with 10,000 intensity levels. Also, an image that is self-luminous and one that is reflective have subtle differences. Real stars are viewed in a dark field, with the eye's variable sensitivity tracking the lowered intensity of a defocused image. Images on paper must be lit by a lamp. When paper images get darker, the eye does not follow. The bright corners of the paper are still in the field of view, and the extra illumination subverts the tracking.

Coupled with this problem are the non-linearities associated with printing. Often the dark end of the scale is darker than anticipated through the mechanism of ink spot spreading. The brightest parts of the image are too bright because very small isolated dots fail to pick up any ink at all. The net result is an increase in contrast and a decrease in dynamic range.

We have no choice but to arbitrarily follow the decreased range of a roughly linear printing scale. In the images throughout this book, the brightness of the frame varies considerably. Focused images are compact; they seem to be drawn with just a spot of white coloring. On the other hand, defocused images are rendered much brighter than they actually appear when viewed through the telescope. Both contrast and brightness are under subjective control.

When you look at any image in this book, concentrate on the shape rather than the absolute brightness. The illumination of such an image is the least significant (and most deceptive) of its properties.

5 300 dots-per-inch laser printers theoretically are limited to 30 distinct shadings if they use a 756-inch halftone grid. Most achieve half that in practice.

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