Archived posting to the Leica Users Group, 1999/11/16

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Subject: Re: [Leica] shutterspeeds
From: "Jacques Bilinski" <jbilin@axionet.com>
Date: Tue, 16 Nov 1999 20:42:03 -0800

> The lens diameter partly determines this efficiency
> and when the aperture is large the efficiency is lower: at an
> aperture of 1.4 the efficiency is about 70% and 90% when stopped down
> to 8.0. So underexposure when using wider apertures might be partly
> caused by the intrinsic characteristics of the shutter. A 1/3 stop
> under exposure at full aperture, where most people will underexposure
> anyway, will add to the total underexposure

Erwin, I am trying to understand why the above might be true but am unable
to do so. If one considers a (theoretical) perfectly adjusted focal plane
shutter at (for example) a shutter speed of 1/30th sec then any light ray
passing through the plane of the focal plane shutter will be be able to pass
through the plane of the shutter only for 1/30 sec. Would this not be true
for any light ray at any angle? Even if one takes into account that the
shutter is not at the film plane but slightly ahead of it each light ray
will still only pass through the shutter for only 1/30 sec. Is this an
effect caused by the fact that the 1st and 2nd curtains are not in the same
plane, but that one is slightly ahead of the other? Or is this effect caused
by the fact that the curtains are not completely thin but have a finite
thickness?  I understand why a leaf shutter would result in under exposure
at higher speeds - the shutter is acting like a diphragm during the time
period in which it is opening and closing, but I don't see why this is the
case for a focal plane shutter.