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In the camera the first and most important irregularity is
the non-uniform illumination of the field of the camera lens which varies from
100% in the center to less than 50% in the corners as small apertures and to
less than 20% with the lens wide open. In the print this is manifested by a
progressive darkening toward the corners, and in the multipanel projection by a
"scallop" effect in the skies. (Fig. 1). In very dark or very light scenes the effect is not too noticeable due
to over or underexposure effects, but in fully exposed and printed material the
effect is pronounced and disturbing. Due to the considerable range of exposure
some changes in color no doubt accompany this error, but since similar effects
happen in the adjacent panels the effect is not serious. This defect can be corrected in part by
redesigning the camera lens so as to have as little vignetting as possible at
the corners of the field. Even so,
because of the well known "cosine fourth" law, the illumination will be no better
than about 50% in the corners. However,
the furthermore serious drop to less than 20% at wide apertures can be reduced.
It should be noted that two different optical principles are
operating here. First, is the "cosine fourth" law (Fig. 2) which states that in
an ideal optical system free of distortion in the image and in the pupil (the
"hole") through which the light passes, the illumination at the film plane will
vary as the fourth power of the cosine of the angle of the rays make with the
optical axis. It implies that for our camera lenses, where the angle is about
35 degrees to the corner of the frame, no more than 45% illumination can be
expected.
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