Archived posting to the Leica Users Group, 2015/11/03

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Subject: [Leica] The SL
From: wildlightphoto at (Doug Herr)
Date: Tue, 3 Nov 2015 01:14:25 -0800 (GMT-08:00)

Richard Man wrote:

>You can get autoexposure only if the camera knows what the open aperture
>and the taking aperture are. If the lens does not generate those info, then
>some kind of manual system is needed

Autoexposure with TTL metering in apertured-preferred mode is simple with no 
auto-diaphragm, no data other than light, ISO and shutter speed is needed.

With TTL metering and an automatic diaphragm the camera needs to know the 
difference between open aperture and working aperture.  It does not need to 
know what the actual open aperture is or the actual working aperture, only 
the difference.  Any other data is extra and can be used for the user's 
convenience, but is not required for autoexposure function.

Doug Herr
Birdman of Sacramento