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 meter.

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 optic system.

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 Photographic Optics)

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.

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