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  1. #1
    EASmithV's Avatar
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    Calculating aperture of Petzval lenses?

    I have a magic lantern Petzval I want to use with slide film, and to do so, i'm going to need a more accurate aperture measurement than what i've been working with.

    With a multi-element lens with the elements having plenty of airspace between them, where do I measure from?

    Here is what I did, I racked the lens back so it was sitting about half and half in the lensboard. Then, I measured from the ground glass for the lensboard (middle of the lens-ish) and took the measurement (5") then I took a measurment of the front diameter of the lens (1.6").

    5" = 127mm
    1.6" = 40.64mm

    127 / 40.64 = f 3.12

    Did I do this correctly? Are there better methods to calculate the aperature of multi-element lenses such as this? Can anyone offer improvments to what I did?
    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

  2. #2

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    Rather than calculate, why not measure?

    How about using the same camera that you are going to mount the lens on - but mount a 'normal' lens of similar focal length. Shoot a test scene or test card at 1/2 stop increments (or less if you can) - mount the Petzval lens, shoot the same scene - compare the negs and see which is closest.

    This way you take into account lens transmission as well as optical geometry. More information could be gained by comparing a colour chart - as an uncoated petsval may have a very different colour rendition to a modern coated optic.
    Steve

  3. #3
    EASmithV's Avatar
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    That it does. On a very saturated film it renders a very nice desaturate color palate.
    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

  4. #4

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    you are pretty much doing it right ...
    this video might help
    http://www.youtube.com/watch?v=CIFNdfjem18&feature=user

    thanks jason !

    have fun
    john



 

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