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  1. #1

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    DOF-scale changes with format?

    Weirdest thing. I passed the website dpreview.com, and saw a mention of a lens. Now, I don't remember the exact wording, but it said something like "This lens has a Depth-of-Field Scale, but it is calibrated for fullframe, so it will be of no use to most users."

    NB: the above was written by an editor in a review. This was not a random user comment. I am so sorry I can't link to it, but I don't remember which review it was.

    Now... I am not an optics expert, but I do belive that a given lens at a given aperture and focused at a given point, will have exactly the same depth of field no matter the type or size of photosensitive material behind it.

    What am I missing?

  2. #2
    keithwms's Avatar
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    The different formats will have correspondingly different enlargement factors and hence different circles of confusion.
    "Only dead fish follow the stream"

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  3. #3

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    Alright, I looked up "circles of confusion" on wikipedia.

    Does the difference in depth of field relate to enlargement?

    If one enlarges not to the same final size for two images of different formats, but by a set factor of for instance 15, would the depth of field then be the same of the two images?

    I'm sorry about asking so many questions, I am just really curious

    Thanks you,

    Emil

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    Steve,

    What then, if I enlarge the same negative to two different sizes? Will the depth of field be different between the two prints? And if that is the case, what image size is the dof-scale on my lenses calibrated for? This has now made me very curious!!!

    Emil

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    keithwms's Avatar
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    Quote Originally Posted by Emil View Post
    If one enlarges not to the same final size for two images of different formats, but by a set factor of for instance 15, would the depth of field then be the same of the two images?
    Yes, technically, the definition of DOF is very much related to much you will enlarge. More aesthetically, it's about how smoothly the in-focus/out-of-focus transitions will be rendered.

    A major peeve I had with the APS digitals, especially the lower resolution ones, was that the focus transitions were much too edgy. The annoying thing is that all manner of novices were saying, "gee look how sharp my APS digital image is, isn't this great, I have a lot more DOF. I can do macro..." Well duh, they were simply not resolving the transitions very well. If you don't resolve something at all then who cares whether it's in or out of focus!!! <end of ramble...>

    Anyway, overall, it is important to learn where the definitions come from: you'll see that these are based on some standard assumptions about what the typical eye will perceive at standard distances & enlargements. But much more important, aesthetically, is how (or whether) you will use focus transitions... i.e. whether you will aim for front-to-back sharpness or whether you will use the transitions, artistically.
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  6. #6
    richard ide's Avatar
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    Check the DOF thread under Macro which has some useful information.
    Richard

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  7. #7

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    Yes, depth of field depends on the format (i.e. film or sensor size).

    To be a clear about this, assume you have perfect lenses on two cameras.

    One is a large format camera and one is a small format camera.

    Assume the two systems have the same field of view. (The field of view assumption is not really necessary, but it makes it conceptually easier.)

    Assume the two lenses are set to the same f-number.

    What the heck, might as well assume we have perfect film as well.

    Assume that two identical shots are taken with the two cameras.

    Assume the two shots are enlarged to the same final size, say 8x10.

    The depth of field in the two shots will be different. The small format camera will have greater depth of field than the large format camera.

  8. #8

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    Quote Originally Posted by alanrockwood View Post
    ...The depth of field in the two shots will be different. The small format camera will have greater depth of field than the large format camera.
    By the way, it is not too hard to prove this with simple lens formulas and a little geometry, but I won't bore you with the details right now.

  9. #9

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    In basic photo, we [unfortunately] learn that D of F is directly affected by three things: 1) aperture, 2) distance from subject, and 3) focal length of lens. Additionally, we also learn sometimes that film format affects it. Actually, it is only 1) f stop, and 2) magnification. Distance from subject and focal length of the lens can affect magnification, and thus affect D of F. Given the same angle of view/composition, film format also affects it indirectly by affecting magnification. However, they do not do it directly, as we learn in the basic photo classes and books. Changing either distance or focal length or film format will have no effect on D of F *unless* doing so also changes the magnification. The higher the magnification, the less D of F there is.

    The idea that D of F is defined as the front-to-back area that "is acceptably sharp" in the print has never set well with me as a useful definition. This means that in a large enough blowup, there could be absolutely no D of F, since even the plane of critical focus can fail to be "sharp", due to "overenlargement". I personally define it as the area in the image that is apparently *just as* sharp as the plane of critical focus. Thus, I view it as a comparison of the plane of critical focus to the rest of the image, rather than a simple definition of what is sharp on the print. Viewing it this way, something can be unsharp, yet still fall within the D of F if it is apparently just as unsharp as the plane of critical focus. Thus, it again comes down to magnification, not just print size. Viewing distance and size of print simply affect magnification (only this time it is your eyes/brain, not a lens/film), which affects D of F.
    Last edited by 2F/2F; 07-08-2009 at 01:55 PM. Click to view previous post history.
    2F/2F

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  10. #10

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    Quote Originally Posted by 2F/2F View Post
    Distance from subject and focal length of the lens can affect magnification, [...]
    Distance and focal length combined are magnification.



    The most important thing about DOF, by far, is that it is a perceptual thing. Not an 'autonomous' entity with an absolute dimension. (Despite all the formulae and calculators people like to let loose on it).

    The best definition is that as "acceptable unsharpness".
    Very clear about that it is a judgement, a perception, and not a measurement or dimension.
    Still pretty vague, since it provides no clue about what sharpness is.

    You can increase DOF dramatically, not by changing magnification or f-stop, but by changing how 'sharp' the sharp bit is. A good film and good lens produce shallower DOF than a bad lens on bad film. And vice versa. Simply by changing the difference, the visual contrast between realy sharp and not so sharp.

    Another undefined thing is viewing distance.
    The final magnification counts, so DOF gets less if you blow a negative up more.
    But only if you do not increase viewing distance accordingly.
    So a giant print can appear to have greater DOF than a small print of the same negative, if only you view the giant print from relatively further away than the small print.
    Last edited by Q.G.; 07-08-2009 at 02:36 PM. Click to view previous post history.

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