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  1. #1

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    Wrong shutter for the lens?

    I am looking at at Caltar 75mm 4.5.

    The lens is mounted in a copal press shutter with 6.8-45 aperature scale listed on it. Am I getting something wrong? Although the lens is a 4.5 the shutter only opens to 6.8--so, the max aperature I will be able to use is 6.8.

    I hope this question makes sense--the seller is very confused and now so am I.

  2. #2

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    It's not the wrong shutter but it is the wrong aperture scale. You can send the lens to someone like SKGrimes to have the proper scale inscribed. You can also do it yourself if you have a caliper micrometer and another 4.5 lens properly inscribed.

  3. #3
    karavelov's Avatar
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    Quote Originally Posted by Ted Harris View Post
    ... You can also do it yourself if you have a caliper micrometer and another 4.5 lens properly inscribed.
    Also you could do it withoutanother lens, you can do the math yourself... the diameter of the aperture = 75 (the focal lenght)/f (the desired f stop - 4.5, 5.6, 8, etc.)

  4. #4

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    Yup, you've got the wrong scale. You could make a new scale, but that still leaves the question about who and how this lens' "shutter-transplant" was performed (since it is likely not the original shutter, which would have had the correct scale in the first place) Things like spacing is critical. Did the person who neglected to furninsh the correct scale neglect to be precise in maintaining the proper spacing etc..of the elements?
    I'd be concerned.
    SK Grinmes can check the spacing and make a new scale.
    If you're serious about this lens I'd see how much they'd charge and take that off the asking price.
    My 2-cents anyway.

  5. #5
    resummerfield's Avatar
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    Quote Originally Posted by John Kasaian View Post
    .....SK Grinmes can check the spacing and make a new scale.
    If you're serious about this lens I'd see how much they'd charge and take that off the asking price.......
    SK Grimes will do an excellent job, but it certainly will not be cheap. If you haven't already bought the lens, I would pass...
    —Eric

  6. #6
    Bandicoot's Avatar
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    Quote Originally Posted by karavelov View Post
    Also you could do it withoutanother lens, you can do the math yourself... the diameter of the aperture = 75 (the focal lenght)/f (the desired f stop - 4.5, 5.6, 8, etc.)
    This will get you there or nearly there most of the time, but for precision what this equation is giving you is the effective aperture, rather than the physical size of the opening. The effective aperture is the diameter of the lens' entrance pupil, which is the size that the (physical) aperture appears from the front of the lens after it has been magnified (or, rarely, diminished) by the effect of the lens elements in front of it. For some lens designs the difference is very slight, but for others it can be significant.

    When I've had to make a new aperture scale I've focused at infinity with a lens with a known accurate scale and used a Minolta Booster II on my meter to take readings off the ground glass with the frame filled by an (out of focus) grey card lit by a stable artificial source. Then I've put on the unknown lens, focused at infinity again and marked off the matching apertures by getting the same meter readings from the card. Then I've done the visual check by reading off the apparent size of the aperture as seen from the front of the lens as a 'sanity check' - if the sizes seem about what they should be then I've got it right, if they're way off I start over again.

    This is reliable, and not all that difficult or time consuming. A focal plane meter like a SinarSix would do the job equally well, and I expect you could bodge something up with a spotmeter if you didn't have a tool like the Minolta booster.


    Peter

  7. #7

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    Peter, I just recently acquired a couple of shutters, already scaled, so that I could use some lenses I bought in barrel and a set of cells I was given. The scales on the shutters are, of course, wrong for the lenses. My stupid simple easily-implemented workaround is to mount up a set of cells -- yes, the spacings are right -- and open the diaphragm until I just see its leaves through the front cell, then back it off a hair. That's wide open, and once I know where wide open is on the (wrong) scale, finding as many stops down as needed isn't hard.

    Cheers,

    Dan



 

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