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Thread: Aperture to depth of field normalisation across various formats

  1. #1

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    Aperture to depth of field normalisation across various formats

    I am setting a very precise portrait brief for 5x4 and would like to open it up to other formats whilst retaining the same dof.
    Is there a chart or formula somewhere to calculat what aperture would correlate 150mm and f16 on 5x4 with 50mm on 35mm format? I am guessing f8 ish.
    I would need to do the same with MF and dx/apsc formats-- 10x8 too come to think of it
    Thank you for your help
    regards
    Ric

  2. #2

    Re: Aperture to depth of field normalisation across various formats

    The Rodenstock DOF/Scheimpflug pocket calculator calculates both for all forats from 35mm to 8x10. It will also indicate exposure corrections, if needed.

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    Re: Aperture to depth of field normalisation across various formats

    Quote Originally Posted by Ricgal View Post
    I am setting a very precise portrait brief for 5x4 and would like to open it up to other formats whilst retaining the same dof.
    Is there a chart or formula somewhere to calculat what aperture would correlate 150mm and f16 on 5x4 with 50mm on 35mm format? I am guessing f8 ish.
    I would need to do the same with MF and dx/apsc formats-- 10x8 too come to think of it
    Thank you for your help
    regards
    Ric
    https://www.google.com/search?q=dof+...ient=firefox-a

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    Doesn't equivalent dof for equivalent angles of view just factor in the ratio of the film diagonal? That is to say, if you had a short framed with a 90mm lens on 6x7 shot at f/11 then to get roughly the same dof with a 150mm lens on 4x5 you'd shoot at f/22 (double the diagonal, so 11x2 = 22). Not sure I've explained that right but thats the rule of thumb I tend to use.

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    Of course in that example the ratio is actually about 1.7 not 2, so more like f/18 if were being picky,but you get my drift.

  6. #6
    Jim Jones's Avatar
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    Re: Aperture to depth of field normalisation across various formats

    Ric, f/4 on 35mm format has about the same DOF as f/16 on 4x5 where the angle of view is about the same and the print size, viewing distance, and acceptable Circle of Confusion are the same.

    When images captured on different formats are viewed at the same size and distance, the DOF is dependent only on acceptable CoF, subject distance, and entrance pupil diameter. This reduces the calculations to math we can manage in the field. It also makes DOF scales on view cameras work with any focal length lens for a standard print size and CoF. My math skills have deteriorated in the 60+ years since high school, so I'll leave it to one of you youngsters to present this subject in a more digestible form.

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    Drew Saunders drew.saunders's Avatar
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    Re: Aperture to depth of field normalisation across various formats

    This site has many versions of their calculator, an explanation of the CoC chosen for each format, and you can pick your own CoC for the charts if you don't like the default for your format.

    http://www.dofmaster.com/
    Flickriver (to avoid Flickr's annoying new format): http://http://www.flickriver.com/photos/drew_saunders/

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    Re: Aperture to depth of field normalisation across various formats

    Quote Originally Posted by dave_whatever View Post
    Doesn't equivalent dof for equivalent angles of view just factor in the ratio of the film diagonal? That is to say, if you had a short framed with a 90mm lens on 6x7 shot at f/11 then to get roughly the same dof with a 150mm lens on 4x5 you'd shoot at f/22 (double the diagonal, so 11x2 = 22). Not sure I've explained that right but thats the rule of thumb I tend to use.
    It is that easy. You can apply the much maligned 'crop factor' to both the focal length and the f-number.

  9. #9

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    Re: Aperture to depth of field normalisation across various formats

    Dear all- thank you all for your well informed and balanced posts. Being less than a* at maths I will probably check out the DOF calc for iPhone on the site suggested.
    All the best
    R

  10. #10

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    Re: Aperture to depth of field normalisation across various formats

    Hi !

    An important point in your normalization process is that all final prints coming from various cameras, formats and lenses are enlarged to the same final size; and the public looking a them looks at them from the same distance. And that various lenses for the various cameras embrace approximately the same angular field. Those conditions might not be easy, nor really necessary in practice in order to make good portraits.

    But let's imagine that those conditions are roughly satisfied, hence the maths involved are very minimal: you can assume that the diameter of the circle of confusion is proportionnal to the film format or to the focal length of the lens in use.

    A general rule is that for far-distant objects the same depth of field will be provided if you have the same hyperfocal distance and the same subjet-to-lens distance.

    hyperfocal distance = H = f*f/(N*c) where f is the focal length, N the f-number and "c" the diameter of the circle of confusion.

    If you assume that you keep the same subject-to-lens distance, the same angular field, and increase the "c" factor in exact proportion of the format, then the (f/c) ratio is kept constant. Hence the same hyperfocal distance and the same DoF will be kept throughout all the situations when the factor (f/N) will be kept constant.

    Eventually this is so simple, that no maths are required, even no calculator is required: scale the f-number in the same proportion as the focal length.
    Divide the focal lenght by 2 => divide the f-number by 2 = 2 clicks on a classical f-stop scale and you'll keep the same DoF.

    Imagine that you start with a 90 mm lens in 35 mm format, this is roughly twice the diagonal of the 24x36 mm format.
    in 4x5 the approximate equivalent will be a 300 mm lens, if we do not enter into subtle considerations on different image aspect ratios (1.5 for the 35 mm film format = 24x36 mm format, 1.25 for the 4x5" format).

    Assume that you stop down to f/16 with the 300 mm in 4x5", the ratio of focal lengths is 300/90 = 10/3,
    the equivalent f-number for (f/16, 300 mm) with the 90 mm will be 16 x 3/10 = 4.8
    The conclusion is that, with the above mentioned conditions, the same depth of field as in LF can be achieved, 4.8 is a max f-stop that many 35 mm lenses do provide.

    But it might not be a good idea to keep the same subject-to-lens distance. Keeping this distance constant however ensures that perspective rendition is exactly the same throughout all formats and whichever the focal length might be.
    And even with the same working distance, various focal lenghts will correspond to various (image/object) magnfication ratios: in portrait work, we are not dealing with far distant objects, and we have to use general DoF formulae valid even for macro work. No simple rule, unfortunately can be given then.

    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.

    An interesting, and actually provocative, example of very shallow depth of field in LF portraits is given here in this series of portraits by Henri Gaud, format = 8x10", at 1:1 ratio with an aero ektar lens, focal length 7" = 178 mm, f-number = full aperture = f/2.5
    http ://trichromie.free.fr/trichromie/index.php?post/2010/02/20/Hug

    Note that a portrait taken at a working distance of 14" is usally considered as an absolute "DON'T" !! An absolute horror, according to classical portrait rules!! Fortunately, here the DoF is so shallow that we do not see too many horrible things, except the eyes of the model on which focusing has been done

    Trying to obtain similar results in 35 mm would be a true challenge; I do not know which focal length nor f-stop could deliver similar results, a DoF calculator could give an answer, but my guess would be something like f/1 or so !

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