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SCREEN BRIGHTNESS 153
the center of the picture ami to make no readjustments when readings
are made in the corners or elsewhere.
In all likelihood, 70% or more of the 16-mm projection objective lenses in use are of the 2-in., / 1.6 variety. In a typical lens of this class (made by Bell and Howell and tested in 1!'41 I, the resolving power measured 100 lines per millimeter at the center of the field, and was less than 20 lines per millimeter in the corners of the field. Such a lens, when sharply focused at the center, is very fuzzy at the corners; when rotated slightly to focus sharply at the corners, the center loses a major percentage of its resolving power. Operators using such a lens "split the focus" (as they call it) and focus most sharply at some indeterminate point intermediate between the two settings. Such a lens is bad, since the quality of the projected image varies over an excessively wide range because the operator cannot focus twice at the same point. Slight changes in focus adjustment are usually required with even the best projectors after the machines have warmed up. and although the picture may be satisfactorily sharp at the start, it will be very difficult to make readjustments of focus setting satisfactorily while the film is running.
It is not necessary for lenses to have such bad geometric distortion. A very good lens type such as the Bausch and Lomb 35-mm Super Cinephor projection objective lens is quite satisfactory at all parts of its field ; it is quite large in diameter compared with 16-mm projection lenses of comparable rating. Incidentally, its price is about $150. while the price of the Bell and Howell lens previously mentioned was only about $15. Needless to say there have been considerable improvements in the quality of 2-in. / 1.6 Bell and Howell projection objective lenses, but it is still imperative for a purchaser to test a lens carefully before accepting it.
Designing a good 2-in. lens of wide aperture is not a simple matter and manufacturers have chosen different design compromises resulting in price differences and performance differences. One group of manufacturers has chosen to retain the same number of lens elements and to increase the resolving power in the corners by lowering it somewhat in the center; this has been characteristic of many of the lenses supplied by Ampro, Victor, and RCA. Other manufacturers such as Eastman have added an extra lens element, a field flattener, which materially improves the flatness with relatively little sacrifice of resolving power at the center.
As an illustration of the differences in resolving power (in lines per millimeter) to be expected when different focus adjustments are made or