Winsor,
Those thought processes are the exact reasoning behind my not owning a 4t. I
mean no disrespect toward you or to the dissertation you have just given on
shutter speeds, movement, etc. I am impressed. My problem is that I borrowed
my brother's
4t one time and every time I started to compose a shot, light flashes such
as crescents and spots would start forming in the light paths between my
eyes and my brain. Now I know what was causing this phenomenon. My brain was
overriding my physical tendency to trip the shutter due to a message that I
wasn't getting...."Switch to manual....Switch to manual...."
Richard
----- Original Message -----
From: "Winsor Crosby" <wincros@xxxxxxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Wednesday, April 24, 2002 10:06 PM
Subject: [OM] Auto Exposure on OM4T(Long)
I hope someone can clarify this for me. The way I understand OTF
autoexposure on the OM4T is that once the shutter button is tripped
and the mirror moves out of the way, the two part meter in the bottom
of the mirror box reads light reflected from 1st curtain which then
opens to uncover the film. At 1/60 sec. the entire frame is
uncovered and a center weighted light reading is taken from the film.
Depending on the light level and camera settings the second curtain
is released when the film receives enough light for an exposure. So,
apparently, at 1/60 of a second and slower a center weighted reading
can be taken from the film.
However, for faster speeds, the 2nd curtain has to be released before
the 1st curtain has uncovered the frame. The shutter speed is fixed
once the second curtain is released. Would the off-the-film metering
feature only apply to speeds at 1/60 sec. and slower? At faster
speeds the meter sensor would have to be building a center weight
reading reflection from both the patterned1st curtain and the film in
order to release the second curtain. At higher speeds, such a 1/1000
sec. over 900f the light reflected into the meter lenses would come
from the shutter curtain, not the film.
At 1/125 second the 2nd curtain has be released when the 1st curtain
is half open. I am not sure how long it takes for the chip to decide
that the light level and then release the 2nd curtain. The 1st
curtain may not even be open half way when the meter reading is made.
It may even be that all speeds higher than 1/60 sec. are determined
by the reflection from the shutter curtain.
If either case is true this has some practical considerations to
speculations that have come up in the past. One would not have to
worry especially about a bright area at the leading or trailing edge
of the frame closing the second curtain too soon since the reading is
center weighted and the lens over the meter cell at the bottom of the
mirror box cannot distinguish between reflectance from film or a
shutter curtain.
Second is the discussion some time ago about the effect of film
reflectance on the meter reading. In spite of wide ranges in
reflectance has anyone had problems with consistent exposure problems
with some films? If not I suspect it is because the reflectance from
the 1st curtain takes such a large part in the exposure.
Any thoughts?
--
Winsor Crosby
Long Beach, California
.
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