How can I work around metering issues with adapted lenses on a Canon DSLR?

Question

I have a bunch of Contax lenses (C/Y mount) and a mount adapter with "dandelion" chips. This should allow me to use my lenses on any Canon DSLR using aperture mode and stopping down the lens. Thanks to the chips I also have focus confirmation.

However, I have metering problems. Wide open it's ok but when I close the lenses I get overexposure (this is aperture dependent).

I have performed tests on the 50D, 5D MII, and 600D. They all present this problem as if the metering was unable to work in low light (with the lenses stopped down). The problem is present for all the lenses I tested (50mm 1.7, 80mm 1.4, 135mm 2.8). Overexposure may reach 2.5 stops with the 80mm totally closed on a 5D while wide open the metering is ok.

Has anyone else also encountered this problem?

Is there an explanation (I mean, there is no coupling: the body does not know if the lens is wide open or stopped down) for how the metering can be off in one case and not in the other one?

Is there any known workaround (apart from a table giving the correction according to the f-number)?

Asked by floqui

Answer

Canon cameras (as most DSLR cameras these days) meter and AF wide-open. There is a limit on the maximum aperture wherein metering and AF can perform properly, and a cutoff where they are unlikely to work at all. In modern Canon cameras, the metering sensor is up in the viewfinder housing, just above the eyepiece you look through. At narrower apertures (lets say smaller than f/8), the amount of light lost between the primary mirror, the focusing screen and the pentamirror/pentaprism is probably so much that the metering sensor has too little light to work with (well less than 50% of the light that originally entered the lens).

Metering sensors are usually rated in a range of exposure values. I think all of the cameras you listed are rated to work between 1 - 20EV and a certain temperature (roughly room temperature, although the exact value sometimes changes from camera to camera.) The 5D II might actually work from 0-20EV. Top-end professional grade cameras and newer mid-grade professional cameras can operate from -1 or even -2 EV up through 20 or more EV, and therefor have up to three stops more low-light metering capability than your average entry-level.

If you stop down your Contax lenses enough in poor enough scene lighting, you might be reducing the EV at the metering sensor below the minimum of its operating range. Since the metering sensor is designed to operate wide open, before an electronic aperture is stopped down for exposure, if you are working in weak light with a stopped-down manual aperture, you'll make it even harder for the metering sensor to operate correctly. You might simply try half-pressing the shutter with the aperture wide-open, meter, use the AE-Lock function to lock in exposure, stop down, and fully press the shutter button to take the photo at the previously metered settings.

Answered by jrista

Check more discussion of this question.