[LargeFormat] Shutter speed lengthened by very small apertures

[Richard Knoppow]

----- Original Message ----- 
From: "Jim Hemenway" <Jim@hemenway.com>
To: <largeformat@f32.net>
Sent: Tuesday, December 09, 2003 2:22 PM
Subject: Re: [LargeFormat] Shutter speed lengthened by very
small apertures


> Hi Ed:
>
> As a followup, one of the 11x14s came out perfectly
exposed, the other
> is a little too dark as I had bracketed the second shot at
about a half
> of the way down from f45 towards f64.  In any case,
neither of them is
> too light as I would have expected based on your posting
about longer
> shutter speeds due to very small apertures on large leaf
shutters.
>
> When I bought my Rollei 6008i I was worried that the leaf
shutters would
> give me those longer than expected shutter speeds, but I
haven't seen it
> there yet either.
>
> I've had the big chromes in hand since the day before
Thanksgiving but
> hadn't posted because I haven't scanned them yet.  The
problem being
> that the best one was mangled a bit in the processing
machine at the lab
> that I use, which makes it unscanable unless I crop it
with a razor
> blade and straightedge.
>
> When I can make some room on my light table, I'm going to
try shooting
> them both with the CoolPix and then show you.
>
> Jim - http://www.hemenway.com
>
  The total opening and closing time of Compur shutters with
top speeds of 1/400th or 1/500th second is around 1/1000th
second. That is, it takes about 1/2000th for the shutter to
come open and another 1/2000th to shut again. Up the
penultimate speed this does not change the marked exposure
time significantly. At the top speed it amounts to around
20% (at least at 1/500th). So, if one measures the total
open time at a small area at the center of the shutter at
1/500th marked speed it will be a little less than 1/400th.
The center value for Compur shutters is around 1/380th
second. The marked speed is for the effective speed for the
full aperture of the _shutter_, which may be larger than the
maximum stop of the lens. Since film, even color film, has a
much larger latitude for overexposure than underexposure
this is a sensible way to mark the speed. However, a 20%
error is not very great and probably not detectable even
when shooting reversal material, which is much more critical
of exposure than negatives.
  Larger shutters have longer blade travel time but also
slower top speeds so the error from this cause is no larger.
Some large shutters, and I am thinking specifically about
Ilex Universal shutters, are apt to have larger errors
because their speed accuracy depends on a balance between
the main spring and retarder spring. These shutters are
often off by quite a bit. The top speed of the No.5 Ilex
Universal, which is marked 1/50th to 1/150th, depending on
the age of the shutter, is actually about 1/30th if the
shutter is set up right. The only other shutter of this size
is the largest Compound shutter. Compounds are much more
likely to be accurate at all speeds than Ilex Universals.
The top speed of the largest Compound is also 1/50th and
again is actually about 1/30th for the center of the
aperture. As nearly as I can tell large Wollensak shutters
are also better than the Ilex. Because all the springs in
Ilex shutters are hair springs they are easy to make of
spring wire (sometimes called piano wire) so its not too
hard to get one so that the speeds are reasonably close to
those marked. Note that these shutters have very
considerable mechanical hysterisis so that they should be
tripped two or three times before making the actual
exposure.  If that's done the speeds will be considerably
more repeatable.
  Simple shutters like the Pacakard are actually quite
repeatable provided you squeeze the bulb about the same each
time. I've measured several. The speed varies from around
1/30th for the smallest to around 1/20th for large ones. A
firm squeeze gets very nearly the same speed each time.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com