IBM 1725 SLS - Manual (Page 131)

Using Gamma
Correction
4-5 Print Quality
mines the number of spots used to represent gray levels in a given
area. A 71 lines per inch (lpi) screen frequency has 71 lines of dots
for each inch of the halftone screen.
The following illustration shows the relationship between screen fre-
quency, gray levels, and print resolution. As the screen frequency
increases, the number of gray levels decreases.
The number of gray levels that your printer can produce depends on
the screen frequency and the print resolution. The default value for
screen frequency using 600 dpi is 71 lpi. This typically allows 73 dif-
ferent levels of gray. The default value for screen frequency using 300
dpi is 53 lpi. This typically allows 33 different levels of gray.
You can change screen frequency through your application or by
using the PostScript setscreen operator.
The extra gray levels available through higher resolution provide a
smoother shift from the darkest black to very light grays while higher
resolution provides sharper image outlines.
300x300 dpi
600x600 dpi
42 53 71 106
Screen Frequency (lpi)
Gray Levels
250
200
150
100
50
0
51
33
19
9
201
129
73
33