On 5/9/2012 9:16 AM, Chuck Norcutt wrote:
> Hey, I'm happy to be simply wrong if it gets me the correct exposure. :-)
I did explain why, you know - because of limitations in the method of reading
exposure off the viewscreen
> But why does it work for the Oly OTF-TTL exposure system which never
> knows the maximum aperture of anything?
As I said, " (Does not, of course apply to live view.) " Let me extend that to
"(Does not, of course, apply to live view
or TTL-OTF.)"
The non linearity of exposure is strictly an artifact of the means of measuring
the light coming through the lens. As
has often been discussed here over the years, focusing screens use microprisms
and other techniques to maximize
brightness and clarity. Witness the different screens Oly and others made
optimized for WA, normal, tele, super-tele
lenses. When you then measure brightness from these screens, especially from an
angle, as is necessary in combined
metering/viewing viewfinders systems, measured brightness is affected by lens
characteristics.
This is why cameras have menu settings for different focusing screens. It's
possible that exposure using a simple, fine
ground glass style screen would not exhibit non-linearity with lens max.
aperture, but it doesn't matter, as we would
hate the screens and the camera makers aren't going to provide them.
When measured as Oly does in TTL-OTF, as reflected off the film plane, or in
live view, as sensed at the film plane,
there is no non-linearity. Thus there is no need to know anything about the
lens, only about the light falling on the
film/sensor.
The Sony SLT series of xSLRs should also not suffer from the problem.
Plain O Moose
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What if the Hokey Pokey *IS* what it's all about?
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