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

  1. #1

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    Depth of Field, Depth of Focus, and Film Flatness

    Depth of Field, Depth of Focus, and Film Flatness

    Ok, I think we can all agree that depth of field is an important concept in theory and in reality when using a large format camera.

    Now, in both theory and reality there is one plane of sharp focus in any photograph. The location and orientation of this plane depends on the distance between the optical center of the lens and the film plane and the orientation of the subject plane, the film plane and that of the lens board (the Scheimpflug Principle). What appears to be in sharp focus in front of and behind the plane of sharp focus is really a function of the size of the circles of confusion. If these are small enough then those parts of the image will appear to be sharp when the photograph is viewed from a normal viewing distance. This area of apparent sharpness in front of and behind the plane of sharp focus is considered the depth of field. Depth of field is dependent upon 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 the reproduction ratio approaches 1:1 (closeup work) the depth of field will decrease. As the f-stop number becomes larger (meaning the size of the aperture gets smaller) the depth of field will increase.

    What is depth of focus? Depth of focus is the space behind the lens where the film plane position can vary slightly and still not cause any loss of sharpness. Sometimes, people have felt that this space can makeup for a slight variation in film position and lack of film flatness. Depth of focus does increase as the lens is stopped down. To some extent, the increase in depth of focus matches the increase in depth of field as the lens is stopped down. Depth of focus also increases as the camera and lens get closer to the subject: i.e. as the reproduction ratio begins to approach 1:1. Depth of focus is not affected by focal length of the lens. 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, thus depth of focus can not really be used to make up for an incorrectly aligned plane of focus (mis-aligned film and lens board planes) or a poorly positioned piece of film.

    So, what does this all mean. Well depth of field is an important concept in theory and reality. I think we can all agree on this opinion. However, depth of focus really becomes an academic concept that is not of much use to use when we are making a photograph IMHO. In most situations the depth of focus is so small as not to be an effective aid to us. One thought about the importance of depth of focus is that of the four primary books on large format technique, only one of the books even mentions depth of focus.

    So, how does this relate to film flatness in various film holders. Well in my 30 years of using large format and using every type of holder I could find, and in my knowledge of hundreds or working large format photographers doing the same, an unsharp image due to poor film flatness in a holder is an extremely rare commodity. Many studies have been done looking at the effective film flatness of various holders, whether or not they have a pressure plate, etc. and there are differences certainly. But the key question is do these minute variations make any difference to a photographer making a photograph. Rarely and only in the most extreme conditions is the answer. We can debate these minute differences, we can continue to do studies, etc., etc. but in reality they all work except in an occasional damaged holder situation or damaged camera back scenario.. Camera and film holder mfg's have worked with this tolerance requirement for years and years and years and have effectivley solved this probelm so we generally do not have to worry about it. Relax and photograph

    Steve simmons (www.viewcamera.com)

  2. #2

    Depth of Field, Depth of Focus, and Film Flatness

    It is a big issue now more than ever because of the non-standard backs that you get with ULF film sizes, and the resurgence of ULF camera use. Many people are cobbling together backs from different cameras.

  3. #3
    Eric Biggerstaff
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    Depth of Field, Depth of Focus, and Film Flatness

    Thanks Steve,

    Interesting post.
    Eric Biggerstaff

    www.ericbiggerstaff.com

  4. #4

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    Depth of Field, Depth of Focus, and Film Flatness

    Actually, here is a disclaimer. Above 11x14 the AINSI standards for film holders were never fully implemented so there may be some variation in these larger sizes.

    steve simmons

  5. #5

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    Depth of Field, Depth of Focus, and Film Flatness

    So - here's a question. We large format photographers usually shoot at f16 or smaller apertures, and I have seen that film flatness is not an issue for sheet film holders at those apertures. However, for some applications (aerial is the obvious one) it might be helpful to use much larger apertures. So at what f stop does 4x5 film flatness start to be an issue?

    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.



    By the way, I really would like an answer to that particular question...


  6. #6
    Michael E. Gordon
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    Depth of Field, Depth of Focus, and Film Flatness

    Relax and photograph

    Hear, hear! I realize that photographers come from all sorts of backgrounds, yet I'm continually amazed at the level of minutiae delved into and argued about on this forum. Photography - even with a view camera - doesn't have to be as complex or difficult as many of the posts on this forum might lead one to believe.

  7. #7
    Founder QT Luong's Avatar
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    Depth of Field, Depth of Focus, and Film Flatness

    "Unsharp" is a relative term. I will give an example from the 35mm world. Prior to the relatively recent introduction of full-frame 35mm digital sensors, complaints about Canon wide angle zooms being unsharp were relatively rare. However all of a sudden, everybody can easily look at the pixels, and there is almost a consensus that they are lacking. If high-quality scanning becomes affordable in LF, the same could happen.

    Working professionals are not necessarily concerned, or even aware, of image quality issues, because technical image quality (beyond a certain threshold of "good enough", which in my experience, is quite low) is way down in the list of priorities that makes one professionally succesful.

  8. #8

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    Depth of Field, Depth of Focus, and Film Flatness

    ""So - here's a question. We large format photographers usually shoot at f16 or smaller apertures, and I have seen that film flatness is not an issue for sheet film holders at those apertures. However, for some applications (aerial is the obvious one) it might be helpful to use much larger apertures. So at what f stop does 4x5 film flatness start to be an issue? ""

    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 (I can't give you the math becasue i do not know the film plane to lens and the lens to subject distance). It will also depend on how much you enlarge the image and how closely you look.

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

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

    steve simmons

  9. #9

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    Depth of Field, Depth of Focus, and Film Flatness

    Do these formulas help?

    http://www.matter.org.uk/tem/depth_of_field.htm

  10. #10

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    Depth of Field, Depth of Focus, and Film Flatness

    Thank you, Steve, for an informative post. Connecting all these concepts may be a second nature to you and other experienced photographers (in any format), but the well-thought explanation was very helpful for me.

    There are many things that could cause problems in the field (or elsewhere). For example, I always wonder "How in the world can this thing stay in good focus?" every time I insert a holder and, particularly, pull the tab of a Quickload packet. Although I've shot only 200 or so LF films, the focus came out OK at least by my standard (which may change in the future) except for some obvious human (me) errors.

    Relax and photograph---That's what I seek and actually do with LF. Mysterious light leaks, wrong meter ISO setting, missed light because I wasn't quick enough to set up... Yes, it's all maddening, but it ruins the whole purpose of shooting LF for me if I get too concerned. I (try to) shrug and say "What the f&%#." Alcohol helps a bit, too.

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