At 15:38 6/12/02, Tom Trottier wrote:
Different films also cause different auto vs manual exposures on the slower
speeds since they reflect differently. See
http://www.markcassino.com/essays/ttlflash.htm and
An interesting article. It cites a TTL-OTF metering problem I've
*never* encountered with an OM-2S and OM-4, both with and without a
T-20, T-32, and Metz 40MZ-3i (with SCA 321), all in TTL-Auto when
using:
-snip
I won't make claim there's no difference in emulsion reflectivity as
I haven't measured it, but all botched exposures have *clearly* been
my doing, not the TTL metering system's. Some of these films, such
as Kodachrome, are not very forgiving of exposure error. I make no
compensation for normal lighting conditions! Both the OM-2S and
OM-4 seem to be very robust in center-weighted TTL-OTF unless
there's very strong back-lighting over a fairly large area of the
frame with a shaded/shadowed subject. Must be something wrong with
my OM-2S and OM-4 TTL metering.
:-)
-- John
My success rate with flash is not so great and I prefer to blame it
on the emulsion reflectivity. :-)
Seriously, I have been thinking about this. Given the reflectivity
variance of the films many photographers using one of the out of
average films would have noticed a one stop exposure error, or even
a half stop, depending on what speed was selected for the shutter.
Since there has been no hue and cry the film reflectivity must not
play as large a part as the marketing folks would have you believe.
At any speed above 1/60 sec. the second curtain has to start closing
before the first curtain has transited the frame. In order to start
the second curtain, the exposure has to be already calculated. That
means that if you believe the reading is taken off the film, at
1/2000 sec. before 1/32 of the frame is uncovered, the camera reads
a 1mm strip of film, makes the calculation and instantaneously sets
the second curtain in motion. What is obvious is that the reading is
taken off the curtain and the film only incidentally. In all cases
above 1/60 sec. the reflection from the curtain makes most of the
light registered by the cell.
I would not be surprised if all exposure determination started
between the time that the shutter button is pushed and the point that
shutter curtain opens. It would explain the shutter lag with the
OM2s and OM4s compared with the manual cameras. That would explain
why no one notices auto exposure errors at 1/60 sec. with a film with
out of parameter reflectivity. So I don't think there is any need to
worry.
That would leave only TTL flash with truly OTF exposure and with
flash there so many things that influence exposure it might be hard
to pin blown shots on film reflectivity. People who are good at
it(Not me!) probably have already worked out their personal
adjustments with flash.
Winsor
--
Winsor Crosby
Long Beach, California
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