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Thread: Depth of Field, Depth of Focus, and Film Flatness

  1. #11
    tim atherton's Avatar
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    Depth of Field, Depth of Focus, and Film Flatness

    It all comes down to circles of confusion. This is a gradual in and out kind of thing.

    ahh - like making love to a beautiful woman

    now it's starting to make sense
    You'd be amazed how small the demand is for pictures of trees... - Fred Astaire to Audrey Hepburn

    www.photo-muse.blogspot.com blog

  2. #12

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    Depth of Field, Depth of Focus, and Film Flatness

    "However, if the subject area completely fills the depth of field area the depth of focus becomes so small that there is no tolerance for any variation in the film’s position"

    That would include a planar subject as stated previously.
    It would also a include a subject focussed at infinity such as the horizon, something which many photographers find difficult to get sharp without using tilts, which was also stated previously.

    --

    "There is not an absolute answer to this as it depends on the reproduction ratio. With aerial, assuming you are several hundred feet in the air (or more) the reproduction ratio will be quite small and depth of field will be pretty good"

    Aerial will fall into both planar and infinity(unless at very low level) and since the plane (pun intended) will be moving, a wide aperture to minimise exposure time is highly probable thereby making depth of focus critical. No wonder the Linhof vacuum back was used for this application.

    --

    The articles on LFPF here give examples of several people who have had problems with bad film holders causing uneven
    focus.

    --

    I think the light is dawning. Let me know when the sun's up.

  3. #13

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    Depth of Field, Depth of Focus, and Film Flatness

    Unless you have a measurement of all these distances this is an impossible question to answer.

    Okay, say I have a lens to subject distance of 500 m, a focal length of 135 mm (and a lens to film plane distance of 135mm,) and I want a COC of 0.1 mm on the negative.

  4. #14

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    Depth of Field, Depth of Focus, and Film Flatness

    How did you decide on .1mm as the size of your CofC?

    steve simmons

  5. #15

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    Depth of Field, Depth of Focus, and Film Flatness

    Steve's discussion is a good general analysys of the relation between depth of field and depth of focus. What he says is substantially correct in most practical picture taking situations, although some of us may differ about its universal application.

    I have two minor comments.

    First, depth of focus could be important in the one situation where its definition applies unambiguously: when the subject is confined to a single plane or very close to one. It tells us when slight errors in flim placement, film flatness, and focusing may become innocuous if we stop down (but not so far that diffraction is an issue).

    Second, Steve may have given the impression that the only relevant parameters affecting depth of field are "the reproduction ratio (the relative size of the image on the groundglass/film plane and its size in reality) and the f-stop being used", as well as the circle of confusion he mentioned earlier. Depth of field depends on one addtional parameter when expressed this way. Thus, in addition to the three mentioned, one could choose any one of lens focal length, subject distance (to exact plane of focus), or image distance, all of which are related if magnification is constant. If the exact subject plane is fairly close to the lens, then coc, relative aperture and magnification suffice for all practical purposes. But for relatively distant subjects, the fourth parameter is essential. I mention this because it is a widely held misconception that depth of field is independent of focal length if magnification is kept constant. I've never understood why people hold this belief so firmly when it is patently false for distant subjects.

  6. #16

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    Depth of Field, Depth of Focus, and Film Flatness

    I would say that if you're trying to answer that question you need to grapple with the concept of depth of focus. My point is, people have different interests, causing them to need to understand and quantify different things. You can correct them saying 'that concept is not important' but the truth is that's an oversimplification.

    You go ahead and grapple. Whatever you do, don't make another picture until you are completely satisfied with an answer.

    Sheesh.

    It ain't Rocket Science (and, BTW, we have at least one Rocket Scientist here.)

  7. #17

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    Depth of Field, Depth of Focus, and Film Flatness

    You go ahead and grapple. Whatever you do, don't make another picture until you are completely satisfied with an answer.

    Oog. I should never have said anything. My sincere apologies to everyone. I won't make this mistake again.

  8. #18

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    Depth of Field, Depth of Focus, and Film Flatness

    Oog. I should never have said anything. My sincere apologies to everyone. I won't make this mistake again.

    No no! My apologies! (Sam, let's trip all over each other in escalating apologies!)

    I overstated it because, well, I used to be a perfectionist. (DQ here will tell you I still am!)

  9. #19
    Dave Karp
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    Depth of Field, Depth of Focus, and Film Flatness

    Tuan,

    I think that "good enough" is quite client specific. I used to work for a company that had pretty demanding requirements for their professional photographer. Although much of what he did for us was catalog work, it had to be very good, because our customers were very discerning. High quality catalog photographs were both attractive and informative. In addition, the architectural photography work he did for us had to be top notch.

    Pertinent to the technical discussion, I suppose that the size at which the images will be used is an issue. For most commercial applications, the enlargements are probably not too large, and that was often true of our work too. For us, a full page or two-page spread in a catalog was about as large as most photos would ever get. Similarly, all of the work that I have done using a Quickload and a 545 holder was enlarged to no more than would fit on an 8.5x11 inch page. That would, I suppose, make a big difference in the acceptable circle of confusion. If all of the photos would have had to be 16x20 or larger, then perhaps we would have run into a problem using a Quick or Ready Load in a Polaroid holder. (Even so, those transparencies always looked pretty darn sharp under the loupe, at least to my eyes. I would not have been worried about enlarging them to that size, or bigger, but I might be wrong about that.) At the old company, when we did want a large photo (for use at a trade show or the like) we tried to use LF shots to make sure we had the best looking image possible.

    At the other end of the scale, high volume catalog work and other similar professional photography does have to be just "good enough" and what "good enough" is is often just passable (and possibly quite below the capability of the photographer). To be fair to the photographers, acceptable quality is the client's call, and it is the professional's job to deliver what the client wants. Then, if the photographer delivers a higher quality photo that took longer and resulted in less production (volume), they are costing the client more $ than it needs or wants to pay, and doing the client a disservice. From what I have seen, some of the other things that go into a professional's success include personality, customer service, availability, flexibility, and all of the many things that go into making other non-photographer entrepreneurs successful.

  10. #20

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    Depth of Field, Depth of Focus, and Film Flatness

    I think that "good enough" is quite client specific. [...] From what I have seen, some of the other things that go into a professional's success include personality, customer service, availability, flexibility, and all of the many things that go into making other non-photographer entrepreneurs successful.


    I'm a Minnesotan. We invented Nice. Any out-of-focus (hereon OOF) problems we get, we can NICE right back into focus. It's our little niche in the universe. It works on everything but coffee. Don't drink our coffee.

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