P3D Re: Mechanical shutters

[Oliver Dean <3d-image@xxxxxxxxxxxxxx>]

John Goodman wrote:
> To get a better idea of one variable, shutter speeds, I 
> recently used a Delta shutter tester with a few cameras, 
> and was surprised by the results. The Canadian-made 
> Delta SH-T1 (available from Micro-Tools) is a nice unit, 
> imho. It has its sensor on a cord, which nicely fits 35 mm 
> camera film rails but can also be used with MF or larger 
> formats, has a bias adjustment for light intensity, and 
> high/low millisecond scales.
> 
> While many measured shutter speeds were well within a 
> half stop of indicated and were reliable, what was 
> disquieting was the "scatter", the range of possible 
> variation at a given set speed with some cameras. For my 
> Realists (recently CLA'ed), some actual speeds were a full 
> stop off (slower than indicated), and the scatter was also 
> very high in some cases (about a stop). The faster shutter 
> speeds, the most common settings for me, sometimes 
> showed 50% variation. For slide films, good exposure 
> accuracy is a plus, if not a necessity, right? Hence my 
> dilemma.
> 
> I'm eager to learn how other list members, with far more 
> experience, deal with this problem.

Hi, John!

For the Stereo Realist in particular, there is another factor that can
add to the confusing array of variables -- shutter bounce.  Back in the
80's, my nice f/2.8 Realist was giving progressively darker exposure
toward the left side of the left view than on the left side of the right
view, but this was only happening occasionally. It was annoying,
however, in that the underexposure gave pronounced retinal rivalry on
the left side of the stereo image, especially in the shadows.  Shutter
tests were telling me that both shutters were giving matched exposure
times, and the f-stops were perfectly matched. The worst discrepancy on
shutter timing was 1/7 second for both shutters at the 1/5 second
setting, not an unacceptable variation. 

When I went to a PSA Convention in Denver, one of the local camera
stores was having a free shutter testing clinic.  What was special about
their shutter tester was that it displayed a transmission curve on an
oscilloscope rather than giving dial readings or digital readouts.  When
they tested the right shutter (the good one) on my Realist at wide open
aperture, it showed a normal nearly square wave representing the time
the shutter was open.  But for my left shutter, although the wave period
showed the same time for the shutter being open, the wave showed a dip
in the amplitude (brightness of the image) in the middle of the open
period.  This dip indicated that a shutter blade, after opening
completely, was "bouncing" and partially closing before opening again to
finish the exposure!  Since the Realist uses a two bladed shutter
located behind the lens instead of between the lens elements, the
bouncing blade was progressively affecting only half of the partly
formed image it was intercepting, thereby resulting in the underexposure
I was getting on only half of the left image.

This would happen when I was using the fastest shutter speeds, which,
for normal exposures, would be complemented by a lens aperture setting
at a wider opening.  Accordingly, if I closed down the aperture for a
longer exposure, the smaller opening would mask most of the partial
closing of the shutter. Also, the longer exposure would make the shutter
bounce a much smaller percentage of the overall exposure time and would
therefore make it less noticeable.  After receiving the test results, I
avoided the problem by not using 1/200 sec. exposures, which wasn't too
much of a sacrifice since I am in the habit of using a tripod,
polarizers, and longer exposures so as to maximize my depth of field
with smaller apertures. Someday I guess I'll have Ron Zakowski overhaul
my shutters!

I wish more stereo camera repair places had shutter testers like the one
I saw in Denver!

Other than the above, I haven't noticed any problem with inconsistency
of exposure so far as my results are concerned, but I DO bracket when
there is a "must have" shot that is static enough to get repeat
exposures. However, I haven't done regularly the sophisticated kind of
shutter testing you have done. Instead, when the camera has been on the
shelf in my cave for a while, after scraping off the moss, cobwebs, and
bat guano (well, I'm exaggerating -- I don't usually get cobwebs), I
operate the shutter a few dozen times at each shutter speed to make sure
there is no hesitation at any speed.  

After the repeat operations, if there is even a slight hesitation at
only the slower speeds, I still assume that it is time to get the
shutter serviced.  If there is hesitation at the beginning of the repeat
operations but it disappears during the repeat operations, I try the
shutter again the next day.  If the hesitation is still gone, I may take
a chance and go ahead with using the camera.  If the hesitation
reappears, I figure it is servicing time.  I don't recommend this for
anyone -- if your shutter shows signs of hesitation or other
inconsistency at any time, don't do as I do - do as I recommend :>},
which is to get your camera serviced!

Cordially,
Oliver Dean