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  1. #1
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    Changing Speeeds On Cocked Shutter

    There has always been confusion as to the wisdom of changing shutter speeds on a cocked shutter. After spending a good many hours of re-working and studying an actual shutter, I can unequivocally say that it is not at all recommended. To clear up confusion as to the "why" of it is to first dispel worry of stripping a gear (in this case the Graphex shutter). There is no danger of instant destruction of that sort. You won't "break off"anything.
    However, you will soon ruin any accuracy the shutter has, or had. The cocking of the shutter puts pressure upon the speed cam, which is made of a fairly soft brass. In this case the drive gear segment has a peg that rides on the speed setting cam. Turning the cam (changing speeds) causes wear in a short period of time, which increases the travel of the drive gear segment, which in turn causes speeds to grow slower and slower. The time frame for this wear is surprisingly short--on the order of mere weeks for an old-time newspaper photog with his Speed Graphic. Months at most.
    When changing speeds with the shutter uncocked, the peg does not even touch the cam at all, so no wear. All this said, these iris shutters have such an infinitesimally small adjustment range that mere weather, hot or cold alone, can cause expansion or contraction issues as much as 3/4 of a stop. A shutter serviceman who slightly over-tightens the faceplate after a service job can add to the problem by another 1/2 stop. Additively, I can easily see that errors of a whole stop or more can occur when factoring in wear from changing speeds with the shutter cocked.
    One f/stop is a lot. Anyone who spends half his darkroom time just making test strips can relate to the annoyance.
    Don't change speeds on a cocked shutter.

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    hi tom

    i always knew this to be true on dial shutters but is this true with shutters like (older) betax where there are variable speeds inbetween the speeds?
    thanks
    john

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    L Gebhardt's Avatar
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    Thanks. I've wondered about this and assumed it wasn't a safe operation, but it's nice to have it confirmed.

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    Quote Originally Posted by jnanian View Post
    i always knew this to be true on dial shutters but is this true with shutters like (older) betax where there are variable speeds inbetween the speeds?
    True for dial-set Compurs, as you say. The recco to not change speeds on a cocked shutter was related to the high speed helper spring more than microscopic wear on the speed ring. Interesting point: Zeiss lenses on Hasselblad are ALWAYS speed changed on cocked shutter. Are they stupid or misleading us with erroneous engineering so we wear them out fast and have to buy replacements every 6 months?

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    Tom1956's Avatar
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    Quote Originally Posted by BrianShaw View Post
    True for dial-set Compurs, as you say. The recco to not change speeds on a cocked shutter was related to the high speed helper spring more than microscopic wear on the speed ring. Interesting point: Zeiss lenses on Hasselblad are ALWAYS speed changed on cocked shutter. Are they stupid or misleading us with erroneous engineering so we wear them out fast and have to buy replacements every 6 months?
    This thread is in the large format section. I was talking about the rim set shutter such as Wollensak. You do what you want.

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    Dial set Compurs were used for large format cameras too.

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    The Compur and Prontor shutters of Hasselblad follow the same design as shutters for large format cameras.

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    Quote Originally Posted by Tom1956 View Post
    This thread is in the large format section. I was talking about the rim set shutter such as Wollensak. You do what you want.
    I will, since no problems have ever arisen from such actions... and I will also defer to the knowledge of the manufacturer.

    From the Graphex rim-set (AKA Wollensak) shutter operation instructions (part of the Graphic camera manual): "The speed settings can be changed before or after cocking the shutter, but is more easily accomplished before cocking."

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    pstake's Avatar
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    Quote Originally Posted by Tom1956 View Post
    There has always been confusion as to the wisdom of changing shutter speeds on a cocked shutter. After spending a good many hours of re-working and studying an actual shutter, I can unequivocally say that it is not at all recommended. To clear up confusion as to the "why" of it is to first dispel worry of stripping a gear (in this case the Graphex shutter). There is no danger of instant destruction of that sort. You won't "break off"anything.
    However, you will soon ruin any accuracy the shutter has, or had. The cocking of the shutter puts pressure upon the speed cam, which is made of a fairly soft brass. In this case the drive gear segment has a peg that rides on the speed setting cam. Turning the cam (changing speeds) causes wear in a short period of time, which increases the travel of the drive gear segment, which in turn causes speeds to grow slower and slower. The time frame for this wear is surprisingly short--on the order of mere weeks for an old-time newspaper photog with his Speed Graphic. Months at most.
    When changing speeds with the shutter uncocked, the peg does not even touch the cam at all, so no wear. All this said, these iris shutters have such an infinitesimally small adjustment range that mere weather, hot or cold alone, can cause expansion or contraction issues as much as 3/4 of a stop. A shutter serviceman who slightly over-tightens the faceplate after a service job can add to the problem by another 1/2 stop. Additively, I can easily see that errors of a whole stop or more can occur when factoring in wear from changing speeds with the shutter cocked.
    One f/stop is a lot. Anyone who spends half his darkroom time just making test strips can relate to the annoyance.
    Don't change speeds on a cocked shutter.

    Is it off topic to ask if this holds true for TLRs? I'm talking specifically about mine (of course), which has a Compur shutter.

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    Quote Originally Posted by BrianShaw View Post
    The Compur and Prontor shutters of Hasselblad follow the same design as shutters for large format cameras.
    When I advance the film on my Hassy (503 CX), it cocks the shutter - without the film advanced, and shutter cocked the viewfinder is blank and I am not able to take a meter reading, which means that by necessity, the speed is always changed on a cocked shutter - I can't believe that they would design things this way if it was bad for the shutter. What the OP is saying implies that there is some fundamental difference between my LF shutters and the ones in my MF lenses - which just doesn't make sense to me.....

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