Mark
Decent focal plane shutters have complex curtain movements. For
instance, as the first curtain travels it will accelerate, so it'll be
moving faster near the end of it's travel. This will make the effective
exposure vary across the film plane. To compensate for this, the second
curtain's movement is such that the "slit" widens as it travels, to
compensate for the acceleration of the first curtain.
What does this mean? The measurements of the "speed" of the curtain and
the width of the "slit" will vary depending upon the point at which the
reading it taken.
The worst case is the old focal plane shutters in Speed Graphic press
cameras. They had single curtains that had fixed-width slits (just a
rectangle cut out of the material) that could produce very uneven
exposure across the huge film area. They weren't "self-capping" eihter,
so if you cocked the shutter without replacing the dark slide, you'd
expose the film to light again! No wonder the Rollei Twin lens cleaned
up the press market whan it appeared...
Vaughan
--
From: Mark Marr-Lyon <o9938156@xxxxxxxxxxxx> wrote
The exposure time can be measured by only using one sensor, which I've
already done. There is a definite deviation in exposure time between
the left and right sides of the shutter, which is why I want to measure
the curtain speeds. I can infer that the closing curtain is moving
faster than the opening curtain, since the exposure time is smaller
on the left (from the back) side of the shutter, but I can't tell what
the actual errors are in each curtain. Two sensors (at least) are
necessary to do this, and I was trying to figure out how far apart to
mount them to get the times specified in the repair manual.
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