[LargeFormat] Shutter speed lengthened by very small apertures

[Jim Hemenway]

Thank you Richard, I'm still digesting this.

Jim



Richard Knoppow wrote:

> ----- 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
>