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Thread: Symmar-S300mm lens

  1. #1

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    Symmar-S300mm lens

    I have the elements for a Symmar-S 360mm, 6.8 lens. Beautiful. Recently have been looking for a Copal 3 shutter so I can use them with my 8x10. The shutters are so expensive. So, I bought a Symmar-S 300, MC in a Copal 3 tonight,($330) thinking I can use the 300 with my 4x5, and switch over to the 360 6.8, for use with my 8x10. What is the best way to judge the aperture difference. Will 5.6, wide open for the 300, be 6.8, wide open on the 360? I thank you in advance for information on this. Robert

  2. #2
    Accordionist's Avatar
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    Re: Symmar-S300mm lens

    Hello, best way to recalibrate f-stops is to measure the diameter of hole in the aperture iris. f = focus lenght / hole diameter. So, calculate the diameters for every fstop and then u could make a new scale.

  3. #3

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    Re: Symmar-S300mm lens

    Quote Originally Posted by Accordionist View Post
    Hello, best way to recalibrate f-stops is to measure the diameter of hole in the aperture iris. f = focus lenght / hole diameter. So, calculate the diameters for every fstop and then u could make a new scale.
    As mentioned often before, the f-stop isn't the mechanical opening of the diaphragm but the diameter of the image of this opening in the entrance-pupil.

    As described here, place a small flashlight in the focal plane and illuminate the lens backward, place a piece of (translucent) paper in front, close to the entrance lens element, you'll be able to measure the diameter of the entrance pupil easily, it is the diameter of the illuminated circle you can see through the paper.

    Also more detailed here.
    Last edited by Peter K; 27-May-2010 at 01:09. Reason: link added

  4. #4

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    Re: Symmar-S300mm lens

    Excellent. Thank you.

  5. #5

    Join Date
    Sep 2003
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    Re: Symmar-S300mm lens

    Since the difference in focal length isn't all that much between the 300 and the 360mm all you probably need to do is just open up the aperture a third or so of a stop when you use the 360mm cells, both lenses will more than cover 8x10 either way.

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