Jamie wrote:
>> I have read that although leaf shutters may be rated at
1/500th, it is almost impossible to get then to physically move that fast.
It leads to approx. 1/2 stop overexposure error - barely noticeable, but
still there and probably why you don't see a 1/1000th speed on a leaf
shutter.
<<
The interesting thing with leaf shutters is that because the leaf *travel time*
effectively gives
a variable aperture during exposure you need to measure the integrated light
exposure rather than
just use a simple thresholded light level start-stop timer to test the
lens/shutter correctly. In
other words, what counts is *not* the "shutter speed" but the integrated light
exposure at the
particular aperture set (the exposure value). Where the diaphragm axis and the
shutter axis are
close together this means the shutter speed might read say 1/500 at smallest
aperture but "only"
1/300 fully open. In fact, it is theoretically possible to make a mechanical
correction for this
by coupling the lens aperture setting to compensate for effective shutter speed
variation. So what
you set would not be quite what you get, but the exposure value would be
consistent with those
settings. My guess is this is not normally done because the shutter vendors
sell universal
modular shutters and adding an extra coupling is probably just too much
trouble. Also any
improvement would be only at say the two widest apertures.
If you build an electronic shutter tester using just a pure timer, changing the
light detector
threshold can make a significant difference to apparent measured shutter
"speeds" of leaf
shutters. So beware the test setup may be adding errors in measured speed.
Focal Plane shutters
are less sensitive to this test setup parameter, provided the area of the
sensor is small, and if
it is placed just behind the shutter. For FP shutters if the sensor has a large
area then it
becomes more important to set the threshold level correctly, just like the leaf
shutter case.
Modern shutter testers probably have a microprocessor deciding what threshold
to use after
digitizing the shutter signal continuously, but even this may be misleading
with a leaf shutter.
In the case of "program" leaf shutters like in some Oly rangefinders, the
shutter and aperture
blades are one and the same. With these it should be easier to correct the
overall EV to be
consistent, to account for shutter travel time, both when set manually or of
course in program
mode. To test how well this works you need to test integrated light exposures
at the film plane,
rather than just "shutter speed" for a range of settings. Without any
experience my guess is even
if tested more carefully and even if designed better, the EV accuracy will drop
quickly as the
springs and shutter age.
A trivia question: what happens to the bokeh of a view camera with a shutter
well behind the lens?
The lens has two apertures in series for a significant part of the exposure at
high speeds. Will
this give rise to a different (chromatic?) bokeh?
Regards,
Tim Hughes
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