[Leica] The SL

[red735i at verizon.net (Frank Filippone)]

Yes, all true.....but you forget that DOF is, for example at F8, greater
than at F2.  So focusing is much less precise without auto stop
down..........

Frank Filippone
Red735i at verizon.net


Do I have it right that the desire for automatic diaphragm control is for
focus at maximum aperture then stopping down to shooting aperture at the
moment of exposure?

Doesn't the use of the EVF potentially permit the finder brightness to be
controlled? If the lens is stopped down (ie not operating with auto
diaphragm) of cause the image arriving at the sensor is what is used for
focussing. So the desire for auto diaphragm control comes from the OVF
systems?

M lenses of course are already being employed with the initial SL units in
the field.
>
>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.