Finding focal length of old lens

Discussion in 'Large Format Cameras and Accessories' started by Marcus, Aug 22, 2009.

  1. Marcus

    Marcus Member

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    Hi,

    Hopefully somebody can help me with this.......

    I have a 1/2 plate camera called 'The International' made by Lancaster & Son of Birmingham, of about 1886 vintage. The brass lens is wonderful & has excellent optics!

    I would just like to know what the actual focal length is and/or how to work this out, e.g.- 6 inch??

    Written on the lens is - "J. Lancaster & son, Birmingham, 1/1 pl patent, Rectigraf". The F stops are F/1 - F32

    The lens is 2 inches diameter, by 3 inches in (total) length.

    Thanks,

    Marcus
     
    Last edited by a moderator: Aug 22, 2009
  2. Ian Grant

    Ian Grant Subscriber

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    Find the Sun :D Not easy in the UK :smile:

    Focus the sun on some paper, measure the distance from approX the middle of the lens (glass elements ) to the paper, that's the Focal length :smile:

    Ian
     
  3. Marcus

    Marcus Member

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    Thanks for the reply.
    The lens has lenses at both ends with aperture ring in middle/centre. Does this change the calculations? Or do you mean the actual centre of the length of the lens?

    Marcus
     
  4. KenFretz

    KenFretz Member

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    Take the camera outside and focus on a distant object (you are trying to focus it at infinity). The distance from the front of the lens board to the inside of the ground glass should be the focal length of the lens.

    Ken
     
  5. Marcus

    Marcus Member

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    So, when I have the camera open with full bellows extended (it being a draw-back camera), basically it is the length of the full bellows extention from lens board to ground glass?

    Marcus
     
  6. timbo10ca

    timbo10ca Member

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    No- focussed at infinity will require minimum bellows draw- then measure as Ken said. If you had the bellows fully extended you would be focussing at something near, i.e. close to if not at or beyond 1:1 (macro/close-up)
     
  7. Mahler_one

    Mahler_one Member

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    Go to JBrunner's site...he has a wonderful explication of how to find the focal length of any lens.
     
  8. John Kasaian

    John Kasaian Member

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    I'd take the lens in a darkened room with a window opposite a plain, preferrably white wall. Point the lens (make sure it is wide open) towards the window and when you can focus an image (upside down) of whatever is outside clearly on the wall, measure the distance between the lens and the wall. This will also give you an idea of the image circle the lens can cover.

    It works for me! :smile:
     
  9. Marcus

    Marcus Member

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    Ah, I get it now......!! Thanks. Will try it tomorrow when the big yellow disc in the sky returns!

    Marcus
     
  10. Marcus

    Marcus Member

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    What's the web address? Marcus
     
  11. bdial

    bdial Subscriber

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  12. Marcus

    Marcus Member

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    Many thanks for all your replies, you have been most kind & helpful!

    I have found the focal length, as far as I can ascertain. It is 11 1/2 " (possibly 12" at a push). Have just got to find some 1/2 plate film now. Any ideas anyone??

    Thanks again, Marcus