I have heard claims that there is no problem with the OM-4(T/Ti) etc. from
stray light in the viewfinder, owing to the location of the metering cells in
the baottom of the mirror compartment.
Today I tried out my OM-2SP (sorry to bore you!) with most of my lenses, and
I was walking back towards the house and it gave a little "beep!" - just one.
I thought "Hmmmmm...". I had my back to the sun, so did the camera. I then
deliberately pointed the camera so that the sun would hit the viewfinder. I
got several more 'beeps' for "overexposure". Setting the lens to smallest
iris aperture (f/16 or f/22 I can't remember) and re-trying, it didn't beep.
Setting the aperture back to f/8 or a lower f-number, it beeped again.
The OM-4Ti also beeped when the viewfinder was presented towards the sun. So
thet is 1/2000 at f/8 worth of light getting to the silicon cells, EV 17.
Presumably the light gets in through the pentaprism and past the
semi-silvered mirror into the photo cell. Of course, it isn't all black down
there because the shutter curtain might reflect a bit of this stray light to
the cell, depending on the taking angle of the little dewdrop-shaped lens in
front of it, or it might get to the lens on the photocell directly through
the mirror and get refracted by the dewdrop lens into the cell.
I used to suspect that this was possible, but I don't think I have ever
encountered it before.
Of course, this wouldn't be true for the OM-2 or OM-2n because of its fully
silvered mirror! So correct exposures with the old OM-2(n) look a bit more
secure in Auto mode.
Dave Bellamy.
http://hometown.aol.com/sitesearch/
Well! I just returned from going outside to disprove this ridiculous
assertion. -You are absolutely right.
I thought that, surely, your meter was picking up a brighter patch
of the ground while tilting the viewfinder toward the sun. Using my
OM4T I repeated your tests and obtained the same findings. As a
further experiment I put the cap on the lens and obtain an exposure
reading through the viewfinder. Moving my head to the side a bit gave
more exposure to the reading. Angling everything and holding the
camera far enough away from the eye so that I could still see the
display when the sun fell across the viewfinder, the display went
into overexposure and beeped, with the lens capped!
So manual exposures look a bit chancy due to viewfinder light
regardless of camera. However, automatic exposures, the way I
understand it are secure because the semi-silvered area of the mirror
is covered by the secondary mirror during exposure and the camera
sets the exposure based on the light coming through the lens.
Does anyone know the relationship between what you set on the spot
meter and what the camera does? Does the camera merely average the
settings in the viewfinder and expose accordingly regardless of what
is coming through the lens like a manual exposure setting? Or does
it somehow relate the spot exposures to the difference between the
weighted meter data detected before and during exposure?
--
Winsor Crosby
Long Beach, California
mailto:wincros@xxxxxxxxxxx
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