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Thread: Using 3D print for Waterhouse stops

  1. #11
    Drew Bedo's Avatar
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    Re: Using 3D print for Waterhouse stops

    Full disclosure: I have never done anything like this before. I am just now working through a similar project in my head at this point. Trying to make a set of drop-in stops for an old brass lens.

    Ron: I do not believe that it is as simple as that. I think that the AREA of the hole must be half of the next larger aperture to make it one full stop smaller. Having solved for half or twice the area from one stop to the next, you would use the diameter of that size hole. Whatever the measured diameter of the aperture is, AS VIEWED FROM THE FRONT OF THE LENS, that number is used to calculate the focal ratio (the f number). The front element magnifies the appearance of the drilled hole. Yet the actual physical size (area) of the hole as drilled is what must be compared to the physical size of the next larger or smaller stop for drilling.

    That seems to be a cumbersome statement to me. I will try again. To calculate the f number of an aperature installed in a lens, it myst be measured by placing a ruler across the front eleent and visually obserbing the magnifgied image of the aperture. This number is used to calculate the focal ratio. However to get the physical size of the hole right, it must be compared to the area of next stop in area, and that diameter used in drilling the hole.

    The physical hole and the dimension measured at the front element will not be the same. Both are important to know.

    We may be saying the same thing, but I think I have it right.

    I welcome remarks by anyone who can explain this problem more clearly.
    Last edited by Drew Bedo; 13-Jun-2020 at 04:02.
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  2. #12
    Ron (Netherlands)'s Avatar
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    Re: Using 3D print for Waterhouse stops

    Quote Originally Posted by Drew Bedo View Post
    Full disclosure: I have never done anything like this before. I am just now working through a similar project in my head at this point. Trying to make a set of drop-in stops for an old brass lens.

    Ron: I do not believe that it is as simple as that. I think that the AREA of the hole must be half of the next larger aperture to make it one full stop smaller. Having solved for half or twice the area from one stop to the next, you would use the diameter of that size hole. Whatever the measured diameter of the aperture is, AS VIEWED FROM THE FRONT OF THE LENS, that number is used to calculate the focal ratio (the f number). The front element magnifies the appearance of the drilled hole. Yet the actual physical size (area) of the hole as drilled is what must be compared to the physical size of the next larger or smaller stop for drilling.

    That seems to be a cumbersome statement to me. I will try again. To calculate the f number of an aperature installed in a lens, it myst be measured by placing a ruler across the front eleent and visually obserbing the magnifgied image of the aperture. This number is used to calculate the focal ratio. However to get the physical size of the hole right, it must be compared to the area of next stop in area, and that diameter used in drilling the hole.

    The physical hole and the dimension measured at the front element will not be the same. Both are important to know.

    We may be saying the same thing, but I think I have it right.

    I welcome remarks by anyone who can explain this problem more clearly.
    Thank you Drew, I get your point and you are correct: I just took a test with my f 6.3 250mm Tessar: measuring the front lens provides an opening that corresponds with the division 250/6.3 ≈ 39,7mm whereas the measurement of the hole itself (taking off the front element) is ≈ 34mm. So the division of focal length / f-stop does not provide a measure for the actual f-stop hole, but gives a measure of the so-called entrance pupil.
    Perhaps there is a (general) mathematical way (e.g. a ratio) to deduct the second measure (34mm) from the first one (39,7mm) - or perhaps there isn't because each type of lens dictates its own proportions ..
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