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  1. #21

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    Once again, DoF calculations are derived from considerations concerning hyperfocal distance.
    Then yes, you get the impression that there is a double dependency on focal length.
    But it's plain wrong to confuse hyperfocal distance with DoF.

    What depends on focal length (besides image scale) is the degree of increase of blur. The DoF range however is the same at same image scale, no matter what lens.

  2. #22
    Lee L's Avatar
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    Quote Originally Posted by Sirius Glass View Post
    When magnification [same size image] is used then it is true, because that is the algebraic substitution that was made. By changing it to one quarter of the focal length the derivation does not apply.
    The cited web page does use the same image size [magnification of an object on the film] for both the short and long focal length lenses. The change is not to make a given object in the image one quarter of each focal length, the statement is that the 'rule' you've stated only holds approximately when the shorter lens is focused closer that 1/4 of the hyperfocal distance for that lens at a given aperture. At longer subject distances the 'rule' you've stated doesn't even hold as an approximation. Both long and short focal lengths are used at a distance from the plane of best focus in the image that sets that plane at the same magnification.

    Lee

  3. #23

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    Quote Originally Posted by Lee L View Post
    [...] At longer subject distances the 'rule' you've stated doesn't even hold as an approximation. [...]
    Only when you begin to mix in that pesky thing called infinity, i.e. start thinking in terms of hyperfocal again.
    What's the scale of an image of something 'at infinity'? Yet that image has a size???

    Etcetera.

    I think it's time again to remind ourselves that DoF is in most part a fictional entity, and that how we perceive it depends on way too many variables. No mathematical description will be true much anyway.

  4. #24

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    Quote Originally Posted by Q.G. View Post
    Only when you begin to mix in that pesky thing called infinity, i.e. start thinking in terms of hyperfocal again.
    What's the scale of an image of something 'at infinity'? Yet that image has a size???
    Infinitesimal?
    Some might say I have a bad attitude! Too bad.

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