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Thread: Focal length, distance, and depth-of-field: clarification?

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    Focal length, distance, and depth-of-field: clarification?

    Another thread prompted this query, whic for me is just a matter of curiosity. The possibility of substituting a shorter focal length lens and moving closer was suggested as a way to increase DOF. I was sure of having read that the trade-off doesn't work that way, and that years ago I had confirmed it with my 35mm and MF lenses' DOF scales . Checking, I found that Steve Simmons is relatively clear, though his exact language leaves wiggle room, and I thought I had seen it from Adams and others, but couldn't find it easily.

    Turning to an online calculator (https://www.pointsinfocus.com/tools/...ens-calculator), I ran two examples for my two lenses, 135 and 210. Starting with the 210, I set a subject distance, selected a constant aperture (f/16), noted horizontal field of view (HFOV) and total DOF. Substituting the 135, I got as close as possible to the HFOV, and noted distance and total DOF.

    My examples are copied below. I used portrait sorts of distances, because that's my primary interest.

    (To avoid confusion, please note differences between decimals in feet and inches, e.g., 14.9 feet, 5 feet, 2.5 inches.)

    There is clearly one or more other variables operating, and I am curious about it, IF it can be described without presenting optical formulae.

    Thanks.
    210

    HFOV 9'
    Distance 15.6'
    TDOF 5'8.9"


    135

    HFOV 9'
    Distance 10'
    TDO 5'11.8"


    Example 2:

    210

    HFOV 2'7.3"
    Distance 5'
    TDOF 6.19"


    135

    HFOV 2'7.4"
    Distance 3.22'
    TDOF 6.24"
    Last edited by Ulophot; 26-Aug-2020 at 18:16. Reason: Error in Example 1, 210 mm
    Philip Ulanowsky

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