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    Join Date
    Feb 2015
    Dallas, Texas

    Lens math? Image circle ⭕️ calculation

    Do you love optics math calculation problems? Here’s one!

    Givens: 150mm APO-Sironar-S at f/22
    Infinity focus
    4x5 film in typical plastic film holder
    Edit: no shifts!

    At what distance behind the lens would the image (on the 4x5 film) begin vignetted by a metal disc with an circular opening of 80mm diameter?

    Application: For example. If I use an 80mm I.D. tube to make an extension (with the lens mounted at one end), how long (maximum length) can the tube be before it vignettes the image?

    For extra points: the increments of length to build the tube are sections of he following lengths: 20mm, 30mm, and 50mm. Multiples of any length are allowed. What would be the optimal combination?

    Do you know the math? I’m thinking this is a slice of a cone at a certain distance problem, with the corners of the 5x4 inch (diagonal) the limit for the cone diameter. Anyone brushed-up on their calc?
    Last edited by pchaplo; 9-May-2018 at 14:11. Reason: No shifts
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