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

  1. #1

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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 U.

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  2. #2
    Tin Can's Avatar
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    Re: Focal length, distance, and depth-of-field: clarification?

    Where is the aperture data?

    Aperture does make the difference
    sin eater

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

    Quote Originally Posted by Tin Can View Post
    Where is the aperture data?

    Aperture does make the difference
    He states f16. But magnification is a major factor. Unless you only make contact prints.

  4. #4

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

    Yes, I indicated f/16 as constant aperture; I am assuming same-size prints, because this is about the straight figures from a DOF calculator, which set whatever CoC it uses as default, and because my question has to do with whether or not DOF is equalized when the same FOV is maintained between lenses of different focal length. The calculator indicates a close match in one case, not in another, of two examples given.
    Philip U.

    Sine scientia ars nihil est. (Without science/knowledge, art is nothing.)
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    Re: Focal length, distance, and depth-of-field: clarification?

    Here's an online tool which actually simulates the blur as you change the variables. You can see for yourself, is it were.

    https://dofsimulator.net/en/



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    Tin Can's Avatar
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    Re: Focal length, distance, and depth-of-field: clarification?

    I forgot about DOF Simulator

    Love how it prioritizes LF formats at top of list

    Thanks Ken Lee
    sin eater

  7. #7

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

    Given a fixed choice of film size and perspective, if we want greater depth of field we need smaller f/stops.

    This means some combination of faster film, longer exposures and brighter lighting.

  8. #8

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

    Yes, I understand that the perspective (size relationships, for those who object to the term) changes as the lens moves toward or away from. I've seen the simulator before. Will take another look.
    Philip U.

    Sine scientia ars nihil est. (Without science/knowledge, art is nothing.)
    https://www.flickr.com/photos/156933346@N07/

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    Drew Bedo's Avatar
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    Re: Focal length, distance, and depth-of-field: clarification?

    Umm . . . .

    I understand that some calculations have been done.

    Why not actually set up the gear with a target (a bar code panel wil work) and ruler and see what actually happens when the variables of subject distance and aperture are changed? Actual exposures and film usage is not necessary. Documentation can be made on the GG with a digital camera or even a smart phone.

    A tangent puss to this approacvh is that you get to play with your gear without spending money.
    Drew Bedo
    www.quietlightphoto.com
    http://www.artsyhome.com/author/drew-bedo




    There are only three types of mounting flanges; too big, too small and wrong thread!

  10. #10

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

    Quote Originally Posted by Ulophot View Post
    Yes, I understand that the perspective (size relationships, for those who object to the term) changes as the lens moves toward or away from. I've seen the simulator before. Will take another look.
    This tool also simulates the blur, IE the depth of field or absence thereof.

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