Some members of the Leica Reflex list found that their manual focus Leica-R lenses were having slight exposure problems when mounted (with an adapter) on their Canon EOS 20D cameras. This was a problem that hadn't shown up in previous EOS digital or film cameras. Bob Palmieri on the list asked Chuck Westfall of Canon USA, who quickly responded with the cause and solution to the issue:
(for Chuck Westfall):
My EOS 20D in aperture priority mode with manual diaphragm lenses exposes properly at 2.8 and 4, begins to overexpose from 5,6 to 8, then comes back down around 11.
How can this be possible?
Is there a solution besides using an external meter?
The EOS 20D focusing screen is optimized for superior brightness at moderate apertures from about f/3.5 and smaller, compared to conventional ground glass designs. This makes the viewfinder image brighter and easier to focus at those moderate apertures, but the trade-off is that it passes disproportionately more light to the metering system. When a Canon EF lens is mounted to an EOS camera, a variable exposure compensation factor (a program curve, not just a fixed compensation factor) for this phenomenon is fed through the system in order to provide correct metering for all apertures. However, when using a non-coupled manual diaphragm lens as you describe, no such communication takes place, so the responsibility for exposure compensation reverts to you. It's unnecessary to use an external meter. Instead, you can take a series of test shots at the working aperture(s) you plan to use, then analyze the test photos to determine the most desirable exposure compensation factor for each aperture. The 20D's auto exposure bracketing (AEB) function speeds up the process of taking the test photos, and you can use the Info palette in Photoshop to determine the most accurate exposure. If you can standardize on one particular aperture you plan to use (for maximum sharpness, desired depth of field, etc.), that will simplify the calibration process by eliminating the need for tests at other apertures.
Director/Media & Customer Relationship
Camera Marketing Group/Canon U.S.A., Inc.
Many thanks to Bob for documenting the issue and to Chuck Westfall for his quick and candid response. Please note that this issue only comes up when using older non-electronic manual focus lenses (Leica-R, Pentax screw or K-mount, etc.) lenses on Canon EOS 20D cameras using adapters.