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  1. #1
    CollinB's Avatar
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    looking for the formula

    When I look on any lens or camera there is a focus scale that is logarithmic.
    How is that scale scaled -- how is it arrived at for each focal length?
    What I am wanting to do is consolidate all of my LF formulas into a single application --
    lens f/l + focus distance + exposure compensation.
    It will make a useful Android app.

  2. #2

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    We discussed this a few days ago. Exposure compensation Δf for a lens extended beyond its infinity position can be reckoned in terms of focal length f and lens-to-subject distance s.

    http://www.apug.org/forums/forum44/9...r-formula.html

  3. #3

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    If you’re interested in the distance the lens must be extended Δ forward its infinity position to focus at a lens-to-subject distance s,

    Δ = sf/(s - f) – f

    Example: f = 90mm, s = 3 meters = 3000mm

    Δ = 3000*90/(3000 – 90) – 90 = 2.78mm

    You’d extend the lens 2.78mm forward of its infinity position to focus lens-to-subject distance s = 3 meters.

  4. #4
    CollinB's Avatar
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    Thanks!

  5. #5
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    Keep in mind that that formula applies only to lenses that are not telephoto or retrofocus, in which cases different extensions are required to achieve a given focus. Fine for most LF lenses though.



 

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