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Thread: f-stop trivia

  1. #11
    Helen B's Avatar
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    Guess what, there's one on eBay right now. I thought the trick in the question was that the f-number is calculated from the diameter of the entrance pupil, not the diameter of the aperture.

    Best,
    Helen
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  2. #12

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    Helen!! Did you mean to 7 places or to 6 places. I mean there is a difference.
    Claire (Ms Anne Thrope is in the darkroom)

  3. #13
    Helen B's Avatar
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    Quote Originally Posted by Claire Senft
    Helen!! Did you mean to 7 places or to 6 places. I mean there is a difference.
    Now how can I be expected to know what I meant? But, ahem, maybe I wrote 'seven sig[nificant] fig[ure]s', not places.

    Best,
    Helen

  4. #14

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    Well maybe you did! Just a thought are these sig figs a good for an old man as prunes?
    Claire (Ms Anne Thrope is in the darkroom)

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    Quote Originally Posted by Helen B
    Guess what, there's one on eBay right now. I thought the trick in the question was that the f-number is calculated from the diameter of the entrance pupil, not the diameter of the aperture.

    Best,
    Helen
    The trick is knowning what the entrance pupil is and how large it is... (lets not get into pupil coma)

  6. #16

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    f 12.79 and be there!

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    Hi !
    IIRC from my long ago optic classes, the F stop diameter is got from the exit pupil diameter of the lens. the writing f:2.8 means that the focal lenght (here near 250 mm) is divided by 2.8 to get the exact pupil diameter.
    But, on a telephoto lens, the size of the hole delimited by the aperture blades is magnified by the lenses between it and the film. So it is difficult to calculate the size of the hole given the measured exit pupil size.
    Do I make myself clear ???
    If you own a large format lens, of telephoto design (or wide angle disgn , thins are reversed but same reasonning apply) you can using a ruler measure the size of the pupil, remove the rear lens element and measure the hole diameter... Be carefull, lenses and blades are easilly damaged.
    Have a nice day !

  8. #18

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    Oh, I forgot ...
    You said that the lens is engraved 250 mm focal length. This is not true.
    If you measure YOUR lens focal lenght, you'll see it is a bit more or a bit less than 250 mm. This is due in tolerance in calculation and in making the lens. Of course, this will impact the calculation of the F stop opening diameter...
    Have fun !

  9. #19

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    I've heard of EXIT pupils. They are to be considered
    when purchasing binoculars. Entrance pupils are a
    new one on me. Dan

  10. #20
    Ole
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    Quote Originally Posted by GeorgesGiralt
    Hi !
    IIRC from my long ago optic classes, the F stop diameter is got from the exit pupil diameter of the lens. the writing f:2.8 means that the focal lenght (here near 250 mm) is divided by 2.8 to get the exact pupil diameter....
    No, the F-stop is from the entrance pupil. When doing close-up work the exit pupil becomes important for calculating exposure compensation, but not for calculating the F-stop (or indeed DoF).

    The designation "telephoto" is often misused to mean any lens longer than "normal". They are not necessarily telephoto lenses even if they're long - my 135mm Hector is a "normal" lens in a long barrel. I don't know whether the Soligor is a long lens or a telephoto lens, though...
    -- Ole Tjugen, Luddite Elitist
    Norway

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