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Thread: Aperture to depth of field normalisation across various formats

  1. #11
    Corran's Avatar
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    Re: Aperture to depth of field normalisation across various formats

    On 8x10 that Aero-Ektar would be like a 25mm lens and f/0.35, approximately, in terms of DOF. On 4x5 it's like a 50mm f/0.7. That's at least my calculations.
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  2. #12

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    Re: Aperture to depth of field normalisation across various formats

    Quote Originally Posted by Corran View Post
    On 8x10 that Aero-Ektar would be like a 25mm lens and f/0.35, approximately, in terms of DOF. On 4x5 it's like a 50mm f/0.7. That's at least my calculations.
    4x5 has linear dimensions that are half those of 8x10, therefore the equivalent focal length for the same angle of view would be half that of 8x10 - ie from 178 mm on 8x10 to 89 mm (or 90 mm if you wish) on 4x5. Similarly for DoF f/2.5 on 8x10 would be similar to about f/1.3 on 4x5, not f/0.7 - the f-number also gets halved.

    Best,
    Helen

  3. #13
    Corran's Avatar
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    Re: Aperture to depth of field normalisation across various formats

    I'm sorry, what I meant to say was, in terms of 35mm equivalents, those numbers apply. This was in response to the poster inquiring about getting the same DOF effects on small-format cameras.

    So,

    8x10 -> 178mm, f/2.5 =
    35mm -> 25mm, f/0.35

    or

    4x5 -> 178mm, f/2.5 =
    35mm -> 50mm, f/0.7
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  4. #14
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    Re: Aperture to depth of field normalisation across various formats

    For a given final print magnification, the only thing that affects DOF for any format is the size of the actual aperture in the lens used (not relative aperture). Of course, lenses don't list the actual aperture, so you have to either guess or calculate.

    It amuses me to read all the convoluted derivations photographers go through, all because of the stupid f/stop convention. It's not that they are wrong, it's just...wow.

    I wish the apertures had never been expressed in F-stops, and the actual aperture was listed instead. Then DOF would be easy to keep constant between formats, but exposure would change. Personally, it wouldn't bother me if exposure was different with different formats at the same aperture setting...I would expect that anyway. At least the actual image physics would be constant.
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  5. #15
    Corran's Avatar
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    Re: Aperture to depth of field normalisation across various formats

    The math makes it all equal out to the same thing so what does it matter? Having a constant f/stop scale across any lens on any format makes more sense (to me, anyway) than an aperture setting that is meaningless unless you calculate it out on that given lens/format.
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  6. #16

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    Re: Aperture to depth of field normalisation across various formats

    As usual, if you follow what Emmanuel tells you in these matters, you won't go wrong.

  7. #17

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    Re: Aperture to depth of field normalisation across various formats

    if you follow ... what Emmanuel tells you in these matters, you won't go wrong.

    .. and f/0.35 ..

    ... You won't go wrong, sure, Leonard (thanks for the appreciation) except if you push your DoF calculator beyond its reasonable physical limits
    Any aplanatic lens, i.e. all those we regularly use in photography, cannot open more than f/0.5
    However the parabolic reflector in your car's headlights might represent something like f/0.35 ... image quality, however with such a "lens" might be arguable.

    Nevertheless I'm sure that collectors will find here an incentive to buy one of those famous "Barry Lndon's" F/0.7 Carl Zeiss Planars to mimic in 35 mm photography what can be done with a 7" F/2.5 Aero-Ektar!

    Another absolute limit of DoF calculators is reached when you demand very small values for the circle of confusion : any reasonable value for this tiny circle cannot be smaller than a diffraction spot. Roughly speaking : N microns where N is the f-number.
    Hence the legend says that Saint Ansel never entered "c"-values smaller than 64 microns in His Holy Dof Calculator Made in Carmel, CA

  8. #18
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    Re: Aperture to depth of field normalisation across various formats

    Hey thanks for awesome link sergei
    through a glass darkly...

  9. #19

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    Re: Aperture to depth of field normalisation across various formats

    Thank you Emmanuel
    What an awesome answer
    particularly
    'But the rules of thumb are simple : multiply your f-number by the ratio of the focal lengths, and eventually check with a DoF calculator more precisely according to the actual operating conditions.'
    much appreciated
    Ric

  10. #20

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    Re: Aperture to depth of field normalisation across various formats

    Quote Originally Posted by Ricgal View Post
    Thank you Emmanuel
    What an awesome answer
    particularly
    'But the rules of thumb are simple : multiply your f-number by the ratio of the focal lengths, and eventually check with a DoF calculator more precisely according to the actual operating conditions.'
    much appreciated
    Ric
    There's a lesson for us - despite the method having been mentioned twice beforehand, it was Emmanuel's version that got through.

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