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)?
I have made some measures and plots which seems to (at least partially) invalidate the jrista's explanation see the pdf any other ideas?