On Wednesday, June 12, 2002 at 17:05, Winsor Crosby
<olympus@xxxxxxxxxxxxxxx>
wrote re "Re: [OM] OM 4 (Auto) - Shutter speeds" saying:
> >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.
Yep.
> 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.
I'm sure it starts then, and when enough light is seen, the 2nd
curtain starts closing. Since the film starts becoming visible and
continues increasing in visible proportion throughout the measurement
period, 1/60 would have 3/4 curtain and 1/4 film reflectivity
measured. 1/30 would be half and half.
> 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.
In the table I sent previously, most slide films were bang on, while
negative films underexposed by up to 38%.
Thank goodness for film latitude.
Tom
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