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Thread: Speed accuracy?

  1. #1

    Speed accuracy?

    Over the past couple days I have tested all the shutters I own using a photovoltaic cell and my digital storage scope. The scope is a very accurate time base and captures a time v.s. light intensity trace.

    I was shocked at the results!

    Even in my best, recently services shutter showed speed errors of up to 25% at the high speeds. Some shutters had errors up to 50% !!!!

    Of the 5 shutters I tested, all were consistent in their speed and varied less than 5% from one exposure to the next. Shutters with low speeds (slower than 1/5 Second) usually were the most accurate at slow speeds.

    I was surprised (and somewhat appauled) to find shutter speeds were so much different than the set speed, even on the recently serviced shutter. My faith in mechanical shutters has been shaken! I look forward to getting the Compur electronic to see if it is more accurate.

    Has anybody else tested a number of shutters? If so, what were the results?

    I did note the set speed v.s. the actual speed in my notebook for each shutter. Maybe I should put this info on a sticky label and attach it to each lens board.....

  2. #2

    Join Date
    Sep 2003
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    1,794

    Speed accuracy?

    Or do a film test? I've always thought some of the griping about film speeds really was related to equipment and not the film.

    OTOH 25% sounds like a lot but it takes 100% to be off one stop.

  3. #3

    Join Date
    Dec 1997
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    7,697

    Speed accuracy?

    I've tested mine several times using the Metrolux II darkroom controller. My experience was similar to yours, most speeds were off by a third to a half stop. Oddly enough, the Copal shutters on lenses I bought used were more accurate than the Copal shutters on lenses I bought new. I kept a list of the actual speeds with each shutter and used it for a while but eventually got tired of consulting it every time I made a photograph. A third to a half stop isn't a real problem with the b&w film I use. The most important thing is consistency I think.
    Brian Ellis
    Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
    a mile away and you'll have their shoes.

  4. #4

    Join Date
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    Speed accuracy?

    I think your results were fairly typical. Of course, if you had just purchased all your shutters new, you would be entitled to better results.

    One factor that is usually ignored because there isn't all that mmuch to do about it is shutter efficiency. At f32, you're getting a lot more effective exposure than at f5.6, because at the latter opening the shutter blades are partially covering the opening for a longer period of time. Similarly, at top speed, the shutter blades are only partially open essentially all the time, while at 1/2 second the blade movement time is trivial.

    As Nick pointed out, these problems are reduced because of doubling and halving from one stop to another. Your approach of keeping note of actual exposure time is a good one.

  5. #5

    Speed accuracy?

    From what I remember reading somewhere, shutter speeds vary with temperature so even if you write the difference on the lensboard it can change! I believe it was an article on the futility of trying to be overly precise with regard to Personal EI. I stress OVERLY as I realize you need to be centered somewhere close.

  6. #6
    Robert A. Zeichner's Avatar
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    Feb 1999
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    Speed accuracy?

    The speed accuracy of mechanical shutters is never perfect in the LF world. It's quite common to see speeds marked 1/400th sec actually measure at 1/165th sec for example. Those high speeds are almost never used when stopping down to f22 or even smaller apertures as we commonly do. The slow speeds on the other hand are ones we use a lot and need to be known and consistant. I usually measure all my shutters before a big trip. The procedure I use is to first exercise the shutter at the speed I am measuring 2 or 3 times. Then I make 3 measurements and average. Each measurement should not vary by much (+/- 5%). If the measurements are all over the place, you've got trouble. When you're done, make a little correction chart on a label and stick to the lens board. Note that on many shutters there is a crossover point below which slower speeds are a bit faster than they should be and above which faster speeds are slower. This is normal. When you take your measurements, do so at an aperture that is typical for the type of work you do. Particularly at faster speeds, a factor known as transit time enters into the picture. This is time it takes for the shutter blades to fully open, then close again. Obviously at faster speeds, that transit time, while pretty quick, can add into the total exposure time and slow it down more significantly than at slow speeds. I hope this is of help to you.

  7. #7

    Join Date
    Jun 2001
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    obx,nc
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    Speed accuracy?

    I put a piece of tape on my lens barrels and marked the speeds as 250 = +30 % (250 is overexposing by 30%). I only note differences of 25% or greater. On one lens 400 = 200. Thats the worst one. I also exercise my shutters several times when I'm testing them and before I make the actual exposure.

    dee

  8. #8

    Join Date
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    Tamworth, Staffordshire. U.K.
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    Speed accuracy?

    Metal expands and contracts with changes in temprature, and this will affect the accuracy of your shutters. Having read and enjoyed most of your earlier posts I'm amazed to read that you actually use shutters, does this mean that you have progressed above 6 ASA? ;-). Keep up the good work.

  9. #9

    Speed accuracy?

    Hi C.J.,

    for Copal #1 tolerance is +/- 30%

    for Rapax #1,2,3 tolerance is +/- 20%

    for Synchro-Compur tolerances are +/- 15% slow speeds ; +/- 20% fast speeds

    Those are from the factory service manuals. Absolute accuracy is not the worry, consistent repeatability is the worry.

    Good luck with the electric shutter but it has tolerances too.

  10. #10
    windpointphoto's Avatar
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    Aug 2004
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    Racine, WI
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    Speed accuracy?

    How often do you use the high speeds? If you don't use them does it matter? I don't think I've used a speed over 1/15 on the 4x5 much less the 8x10.

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