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  1. #1
    EASmithV's Avatar
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    Determining Focal length

    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?
    www.EASmithV.com

    "The camera is an instrument that teaches people how to see without a camera."— Dorothea Lange
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  2. #2
    EASmithV's Avatar
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    Does it have to be focused to infinity to get the measurement?
    www.EASmithV.com

    "The camera is an instrument that teaches people how to see without a camera."— Dorothea Lange
    http://www.flickr.com/easmithv/
    RIP Kodachrome

  3. #3

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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.

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  5. #5

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    Ah....I was just about to furnish the link as well...

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

  6. #6
    EASmithV's Avatar
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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...
    www.EASmithV.com

    "The camera is an instrument that teaches people how to see without a camera."— Dorothea Lange
    http://www.flickr.com/easmithv/
    RIP Kodachrome

  7. #7
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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.



 

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