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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

  4. #4
    Sirius Glass's Avatar
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    Quote Originally Posted by Emil View Post
    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."
    What it means is that a digisnapper that does not have a focal plane that covers 24mm x36mm [full frame] will find that the Depth-of-Field Scale might not be calibrated to their digisnaps.

    Steve
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  5. #5
    Sirius Glass's Avatar
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    Quote Originally Posted by Emil View Post
    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
    The circle of confusion varies with the format. If the enlargement factor which is what I interpret as "set factor" is the same, the the depth of field would be the same.

    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.

  6. #6

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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

  7. #7
    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.
    "Only dead fish follow the stream"

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  8. #8
    richard ide's Avatar
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    Check the DOF thread under Macro which has some useful information.
    Richard

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  9. #9

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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.

  10. #10

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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.

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