[Leica] Metering on Canon DSLR with Leica R Lens

[douglas.sharp at gmx.de (Douglas Sharp)]

Lee,

This is what Canon says about it:

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.

Thanks again to Bob Palmieri who forwarded me the information.

Another issue is that light entering the camera through the eyepiece has 
a VERY strong effect on exposure metering if you are using slow speeds 
in AV mode (for example macro shots from a tripod) so use the little 
rubber cover thingy that's on your camera strap to cover it up.
The metering methods AV and M  function exactly as they should with 
Leica, Contax, Yashica, Tamron,Fujica,Pentax, Novoflex, 
Pentacon,Praktica, Beroflex, Sigma  and a couple of Russian offerings in 
all modes - Well those are the lens makes I've used so far, and, if 
you're shooting RAW it doesn't really matter all that much either.

It may be interesting to meter against a grey card using a hand meter, 
note the value  and then transfer it to manual mode on the 20D. After 
taking a shot, keep the aperture as it is and switch to AV-mode and note 
the shutter speed, if you run this through for a few aperture/f-stops 
you should be able to work out a deviation function to allow you to 
correct for the exposure (not metering) shift.

I hope this helps
Douglas