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Thread: 26" lens

  1. #1

    Join Date
    Oct 2017
    Posts
    4

    26" lens

    Hi,
    Hope someone can help me out with this question and if it's a daft one, I apologise

    I have just bought a 26" Charles Beseler projection lens and wanted to know if it says 26" does that mean it's a 660mm lens. Also how will I figure out what aperture it is and the image circle.

    I have tried to google the answers I need but I can't find what I am looking for.

    I paid 120 for it and I'm hoping I didn't pay to much for it as I couldn't find any similar lens for sale at the time.

  2. #2

    Join Date
    Jan 2009
    Location
    Denmark
    Posts
    5,282

    Re: 26" lens

    Yes, nearly all projection lenses have the EFL written as you describe. Beseler made Petzvals early on, but went over to the Cooke triplet design with shorter barrels and only 3 lenses to manufacture!

    The early Petzval projection lenses always have a arrow pointing in the direct of the projector - or, these days, camera!

  3. #3
    Randy's Avatar
    Join Date
    Dec 2010
    Location
    Virginia, USA
    Posts
    1,108

    Re: 26" lens

    For determining the aperture, I think you just hold the lens with a ruler across the front and measure the actual illuminated area you see as you are looking thru the front of the lens. If it is 75mm, and your focal length is 660mm - 660/75 = 8.8, so you could say your lens aperture is f/8 (rounded down). If it measures 110mm, 660/110 = f/6. I think that is a "fairly" accurate way to determine fixed aperture. Not sure of an easy way to determine image circle.
    https://dl.dropboxusercontent.com/u/52893762/bigger4b.jpg

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