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Thread: DoF, effective focal lengths and related stuff

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    Senior Member pellicle's Avatar
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    DoF, effective focal lengths and related stuff

    Hi

    I just finished a discussion on another forum which led me to post the stuff I put together for that into a blog article.

    Heaps of people seem to get confused about the "crop" factor and the depth of field difference between (say) a 50mm lens on a 4/3 camera and a 100mm on a full frame.

    So I wrote this to show my findings.

    http://cjeastwd.blogspot.com/2010/03/43rds-dof.html

    there are some interesting give and take aspects to it too ... finally I was quite surprised on how well good old 35mm film did for comparison with the G1 (as I needed something to put the 100mm lens onto)

    :-)

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    Re: DoF, effective focal lengths and related stuff

    Nice test! Good to see that theory actually works =).

    This is one reason why I want to experiment with film again. I suspect what you're seeing in your photos is the difference in DR for film vs. the G1 - film still has higher DR. It's also analog, so in theory, and depending on the film grain you use, you should be able to see more detail on film. The digital photos are also some engineer's approximation of how to interpret the light signal perceived by the sensor, so that's a factor as well.

    Goes to prove film ain't completely dead yet.
    -Dragos
    Panasonic GH1/G1, Canon FTb(n)/F-1, Mamiya C330F/RB67 Pro SD, Chamonix 45N-2, Nikon F5 + Assorted Lenses

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    Member sangio's Avatar
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    Re: DoF, effective focal lengths and related stuff

    Good summary.

    I think you're right about folks not using scanners correctly, or more likely using poor scanners. I've never had much luck scanning 35 mm film, but that's because I'm using a flat bed scanner, and it does a terrible job, even though it's supposed to be scanning at 4800 dpi.

    regards
    Santo

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    Sr. Administrator Jack's Avatar
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    Re: DoF, effective focal lengths and related stuff

    Couple of things to remember to keep DoF from being confusing:

    1) DoF is determined by subject distance, the lens focal length, the lens f-number, and the format size/acceptable circle of confusion ONLY.

    2) If photographs with the same final-image size are taken in two different camera formats at the same subject distance with the same field of view and f-number, DoF is inversely proportional to format size. So 4/3rds being half of the FF 35mm frame, to render equivalent DoF and subject distance a FF 35mm image made with lens of "F" focal length at "f" aperture number, will require the 4/3rds image be made with an aperture number of f/2 and a lens of focal length F/2. For example, if the image with FF 35mm is taken with a 100mm lens at f11, the 4/3rds image needs to be taken with a 50mm lens at f5.6 from the same position.

    3) Since DoF is often fixed by the requirements of the desired image, the corollary to the above holds: meaning for a given DoF and field of view, the required f-number is directly proportional to the format size. So between FF 35mm and 4/3rds per above, if the 4/3 camera required f5.6, the FF 35mm camera would require f11 to give the same DOF because the format size is 2x.

    Cheers,
    Jack
    home: www.getdpi.com

    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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    Senior Member pellicle's Avatar
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    Re: DoF, effective focal lengths and related stuff

    Hi Jack

    or to simplify more or less 2 stops


    Quote Originally Posted by Jack Flesher View Post
    Couple of things to remember to keep DoF from being confusing:
    ...
    So 4/3rds being half of the FF 35mm frame, to render equivalent DoF and subject distance a FF 35mm image made with lens of "F" focal length at "f" aperture number, will require the 4/3rds image be made with an aperture number of f/2 and a lens of focal length F/2. For example, if the image with FF 35mm is taken with a 100mm lens at f11, the 4/3rds image needs to be taken with a 50mm lens at f5.6 from the same position.

    I took my reference material from this guy back when I was considering 4/3

    When any lens of a focal length F is mounted on a Four Thirds body and stepped down to aperture (F-number) A, its effective depth of field will be the same as that of a lens with focal length 2F stepped down to aperture 2A, and working on a film camera.
    of course its never simple when you go past the simplifications ;-)

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    Sr. Administrator Jack's Avatar
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    Re: DoF, effective focal lengths and related stuff

    The problem with DoF is the simlification does not hold at closer focusing distances. You learn this quickly if you shoot large format on tabletop/product, and why product shooters regularly use cameras with movements.
    Jack
    home: www.getdpi.com

    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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    Senior Member pellicle's Avatar
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    Re: DoF, effective focal lengths and related stuff

    Hi

    Quote Originally Posted by Jack Flesher View Post
    The problem with DoF is the simlification does not hold at closer focusing distances.
    yep .... I agree ... but the really nice thing is that m4/3 has no mirror so I don't have to remember to lock that up... a nice example of mirror slap on my SLR vs my G1



    working distance is not benefited either ...

    PS, don't you just love the scratches you get in your film free from the lab these days ...

    :-)

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