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  1. #11

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    I think you need to measure everything, the focal length and the aperture. See Jay Bruner's video on the subject:

    http://www.youtube.com/watch?v=CIFNdfjem18&feature=user
    Frank Schifano

  2. #12

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    frank is spot on ...

    btw its a 210/370 ...

    ( have fun )
    john
    Last edited by jnanian; 05-05-2010 at 12:13 PM. Click to view previous post history.

  3. #13

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    You need to measure the entrance pupil, the apparent size of the aperture through the front element, I hold the lens at arms length and measure with plastic calipers.

    The f.l. is easy to measure, focus at infinity, mark the bed/rail. Focus at 1:1 (use a ruler and measure the image on the g.g.), mark the rail/bed. Subtract the second number from the first and you have your f.l.

    Aperture is simply f.l. divided by entrance pupil diameter (expressed in the same units, of course)

  4. #14

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    Quote Originally Posted by BetterSense View Post
    ...I get 29mm fully open--that's 210mm/29mmm =7.25! That's quite a bit smaller than f/5.6 eh?
    Not that much really. The difference is approximately 2/3 stop. Don't use a dial caliper. A steel ruler marked of in millimeters is good enough and easier to get right. Measure the apparent diameter of the aperture through the entrance pupil. If the hole isn't circular measure from corner to corner and side to side and average the two. It's close enough.
    Frank Schifano

  5. #15

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    So measure apparent rather than actual diameter?
    Heavily sedated for your protection.

  6. #16

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    That's what you do.
    Frank Schifano

  7. #17

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    Quote Originally Posted by fschifano View Post
    [...] Measure the apparent diameter of the aperture through the entrance pupil. If the hole isn't circular measure from corner to corner and side to side and average the two. It's close enough.
    means that you shouldn't

    Quote Originally Posted by fschifano View Post
    Oh yeah, measure the aperture from the front, not the back. If you can remove the front cells to measure it, even better. [...]

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