Determining Focal length

Discussion in 'Miscellaneous Equipment' started by EASmithV, May 21, 2009.

  1. EASmithV

    EASmithV Member

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    Well, I put a magnifying glass lens onto a piece of black mat board and stuck it on my speed graphic. I like the DOF it gives, as well as the edge distortions. However I want to determine the F-stop.

    I know that F-stop is the focal length divided by the diameter of the lens. However, I am having some terminology difficulties.

    I know this is probably a dumb question, but here it goes...

    Is the focal length the distance from the front standard to the film plane when focused at infinity?
     
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    EASmithV

    EASmithV Member

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    Does it have to be focused to infinity to get the measurement?
     
  3. Mahler_one

    Mahler_one Member

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    Go to Jason Brunners site, and watch his wonderful video on how to determine the focal length of any lens. Also, as I recall, there is a good discussion of focal length in general. Jason is really unique, and is very generous with his instructional videos.
     
  4. Mahler_one

    Mahler_one Member

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    EASmithV

    EASmithV Member

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    Thanks. That clarifies a lot. So it does need to be focused to infinity.

    Actually, I regularly use one of his bellows factor calculator thingies... Bacon...
    LOL don't need the stopwatch...
     
  6. ic-racer

    ic-racer Member

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    Do you know the power of the magnifying glass? You can multiply the 'magnifying power' times 5 to approximate the diopters. So if it is a 2x magnifier, then it is about 10 diopters. Diopter is 1 over focal length in meters. So 10 diopters = 1/10 meter = 100mm focal length.