A Through the Lens Light Meter (TTL)
I build a extinction type light meter that projected a lit image of a gray scale
to the eye piece. It was a pelicle in the Arri light trap opening that showed
the gray scale the same distance as the ground glass. With the shutter half
closed the iris could be adjusted so that the brightness on the ground glass
could be matched by eye with the projected gray scale. It was sort of an extinction
It worked great with a 25 to 250 and long lenses but was way off for some short
hard lenses. The device went to the failed idea pile. Years later I discovered
the problem with the wide hard lenses. Light on the ground glass is determined
both by the "F" stop and the angle that the light is reaching the ground glass.
A ground glass does not catch all the light rays of a lens that has elements
close to the film plane but the film gets all of these rays. If you remember
the Angenieux 14.5 gave a dark image to the eyepeice, but the exposure on the
film was OK. Some Cooke wide angles also were dark.
This is also a problem with tilt and shift lenses where the light rays are at
an extreme angle to the ground glass. A stronger diopter lens at the ground
glass would help, but that would require some re-engineering of the viewfinder
Most wide angle lenses are retrofocus and the last group of lenses closer to
the film are often 30 or so mm. focal length. The lens width is accomplished
with retrofocus elements on the front of the rear group. (See Sidney Ray's Applied
Summary: Wide angle and shifted lenses give darker images to the ground glass,
but not to the film. Judging exposure by apparent brightness on the ground glass
can be risky.
Extreme shifts and tilts do change the exposure if the lens to film distance
is quite different on one side, top or bottom.
© Copyright 1999-2004 Ron Dexter. All Rights Reserved.