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  1. #31

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    Quote Originally Posted by Roger Krueger View Post
    And I really don't see the point in arguing over the standard DoF definition.
    Very true.
    The problem with the 'standard' definition we so often encounter in discussions of DOF however is that it is not that of DOF, but of hyperfocal distance.

  2. #32

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    Quote Originally Posted by Q.G. View Post
    Now it is not true that DOF on film changes with field of view. When magnification and f-stop are constant, DOF will be too, no matter how large or small the frame.
    Sorry, that's clearly incorrect for circumstances where the wider lens is even slightly near (or beyond) hyperfocal. Try it on dofmaster...at f22, 30mm @ 4ft. gives you 36.7 ft. of DoF vs. 300mm @ 40 ft. gives you 7.23 ft. of DoF. But at f2.8 it's only .91 ft. vs .89 ft.

    Again, it is just f-stop and magnification, but it's magnification at the point whose focus is being considered, NOT magnification at the plane of best focus. Since you can't keep all of the magnifications the same for different FsoV you get different DoF results.

  3. #33

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    Quote Originally Posted by Roger Krueger View Post
    Again, it is just f-stop and magnification, but it's magnification at the point whose focus is being considered, NOT magnification at the plane of best focus. Since you can't keep all of the magnifications the same for different FsoV you get different DoF results.
    You really don't.

    But i'm so tired of DOF, that i can no longer find the will to explain for at least 6 months.
    My fault, i know. I shouldn't have let it tempt me today.

  4. #34
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    Quote Originally Posted by Q.G. View Post
    "Lots of choices. Pick one." "Define" (in the "pick one" sense). "[...] perceptible by a person"
    And still "an objective scale"?

    Doesn't work.

    The second approach, based on arbitrarily chosen ("let us say") dimensions, does no better.

    As you say: "The criteria may be arbitrary"
    And there the whole things come falling down. DOF is not an objective thing.


    A statement like "The second method is independent of factors such as magnification, viewing distance, photo size, etc." even cannot be a thing related in any way to DOF.
    DOF, in essence, is dependent on factors such as magnification, [etc.]
    Technically correct, but not useful as a practical matter. OTO CoC are useful as a practical matter (caveat below), as in for a given format we all know what we are talking about, and therefore can actually "pick one". If someone picks another for the same criteria, one can extrapolate with ease and the subject becomes somewhat objective, at least enough for practical discussion.

    I fully agree that none of the other things are related to DoF in the least, only to "apparent" DoF which is purely subjective.
    Field of View is a function of format if other factors are the same, and if those factors are the same the actual properties of DoF will be identical. The subjective effect might be quite different however, say between 100mm on minox vs 8x10 inches. The minox frame will offer the same properties within a severely cropped area and appear as a compressed telephoto lens. The 8x10 will be very wide and offer the perspective of and focus characteristic of a very wide focal length. This is because the perspective of a focal length is relative to the format. Folks tend to forget this. To offer the same FoV between formats one must change the lens or distance and focus, and you are no longer comparing anything meaningful concerning DoF, not that it's overly meaningful the other way either.

    The CoC for a given format ,focus, focal length, and stop allows one to calculate DoF in a practical way (there's that word again) I seldom calculate for stills because I can see it, but for motion picture where things are very fluid and judgments by eye can be very difficult I use a thing called a SamCine calculator. Basically a slide rule that allows you to calculate DoF for different format/focal/t-stop combinations using CoC as the objective criteria. Understanding the CoC implications for a chosen format is important to using the device. Where the CoC remains the same it will not have the same visual effect as a practical matter across formats, regardless of your subjectivity or objectivity, therefore CoC alone doesn't tell you anything. It's more than just about subjective/objective, it is about practical application, and there isn't some magic bullet formula that you can carry in your pocket that applies across the board. A only a working knowledge and experience can really inform. In other words, get out and shoot. A couple of exposures are worth more than all the words in this thread, mine included.
    Last edited by JBrunner; 07-21-2009 at 03:22 PM. Click to view previous post history.

  5. #35

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    That's exactly it: a practical matter.

    Forget all formulae and calculators, and use your eyes (is photography a visual medium?)

    Select a subject. Decide what your picture is about.
    Focus on that subject.

    Then use the aperture to set how it relates visually to the rest of what is in the frame, judging not by table or scale, but by looking through the viewfinder to see how the sharp vs unsharp balance changes.

    Yes, it is difficult when the groundglass gets darker and darker. But then, who promised it would be easy?

    And to all those who say their subject stretches from the front lens to infinity, my response would be that such images are boring, and perhaps need not be produced in the first place. Just my view
    But if you want to produce images without deciding what they are of anyway, just stop the lens down as far as it goes and be done with it.

    But whatever you do, do not even think about DOF as a quantifiable entity. As something that can be discussed and decided upon separately, apart from the particular image your lens is projecting on your screen.
    It quite simply is not.

    It will always be either too much, too little, or just right.
    But how much that is, is different every single time you create an image.
    So just use your eyes and look.
    Last edited by Q.G.; 07-21-2009 at 02:57 PM. Click to view previous post history.

  6. #36
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    Quote Originally Posted by Roger Krueger View Post
    Sorry, that's clearly incorrect for circumstances where the wider lens is even slightly near (or beyond) hyperfocal. Try it on dofmaster...at f22, 30mm @ 4ft. gives you 36.7 ft. of DoF vs. 300mm @ 40 ft. gives you 7.23 ft. of DoF. But at f2.8 it's only .91 ft. vs .89 ft.

    Again, it is just f-stop and magnification, but it's magnification at the point whose focus is being considered, NOT magnification at the plane of best focus. Since you can't keep all of the magnifications the same for different FsoV you get different DoF results.
    You are absolutely wrong. Unless you live in a different universe, physics says that you are wrong.

    I have posted in the past that while I was studying optics at Kodak, that I raised the question about why when I kept the image size and exposures settings the same, I could not get a better depth of field by changing lenses. The optical engineer started with the depth of field equation and made one substitution. Four lines later the focal length was in every term and then was divided out. The result is exactly as Q.G. stated.

    When magnification and f-stop are constant, DOF will be too, no matter how large or small the frame.
    Steve
    Warning!! Handling a Hasselblad can be harmful to your financial well being!

    Nothing beats a great piece of glass!

    I leave the digital work for the urologists and proctologists.

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