CarlMarin <CarlMarin@xxxxxxx> moved upon the face of the 'Net and spake thusly:
> In a message dated 98-01-14 01:50:01 EST,Chris write:
>
> >
> > The ACTUAL aperture is not known to the body, only that the lens is
> > stopped down N stops.
> >
> > T
>
> Hi Chris,
>
> Of course the whole scheme is an analog by gosh and by golly sort of
> arrangement. I never really thought about this part of it before, maybe its
> a trick question. How does the camera know whether you've got a f1.2 or an
> f3.5 screwed on so it can give you compensated wide open correct
> readings?
It doesn't care.
When you are focusing you are at wide open. The camera measures
what's coming through the lens.
If you stop the lens down one stop, IT STAYS WIDE OPEN, but the meter
moves by one stop. The aperture ring just tells the camera how many
stops it should fudge the reading by. Only when the shutter opens
does the lens actually stop down.
You can easily fool the system by stopping down and pressing the DOF
preview on the lens. The meter reading in the camera will
immediately drop by the number of stops the lens is closed down, as
the actual light reaching the meter cell drops.
> it's basically a case of the light coming thru the lens is the light coming
> through the lens and the body doesn't care, then why would something like my
> Tamron 500f8 mirror need an fixed aperture tab at all to get correct exposures
> out of the camera as it is shot wide open all the time, doesn't stop
> down?
It should be fixed at the "wide open" position, yes.
Even if you have a lens with no auto-diaphragm which stops down as
you adjust aperture, the lens simply needs a fixed aperture-reporting
lever and the TTL metering still works.
The OM aperture arrangement is very very clever; it's only drawback is
(near) impossibilty of in-viewfinder aperture reporting.
The lack of in-viewfinder display rarely bothers me, since I just
remember where I am. A quick spin down to f/1.8 and
count-the-clicks-back suffices to tell me the aperture.
The only time it does bother me is doing macro work where I sometimes
can't get my head around the front of the camera (most recently
because of a nest of rose thorns) to see the exposure settings on my
OM-1 when I want to record them.
Chris.
--
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--- Christopher Biggs - Stallion Technologies - chris@xxxxxxxxxxxxxx ---
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