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

  1. #21

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    Depth of Field, Depth of Focus, and Film Flatness

    jj,

    Thanks, no offense taken if none was intended.

    Steve,

    Only because I thought .1mm was the conventional standard for 4x5. Maybe .05mm would be better.

  2. #22

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    Depth of Field, Depth of Focus, and Film Flatness

    Steve,

    I thank you very much for this. I am a new LFer and have a question for you regarding the reproduction ratio. I have a tool named Rodenstock Depth of Field Calculator. There is a setting called Scale of reproduction. There are 1:1; 1:2; 1:5;1:10; 1:20; 1:infinite. Could you teach me how to set up the Scale of reproduciton?

    Thanks

  3. #23
    Dave Karp
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    Depth of Field, Depth of Focus, and Film Flatness

    Nice won't get an out of focus photo back into focus. Of course, that was not what was said or close to the the point of anything mentioned in the post, which was responsive to Tuan's comment regarding the quality of some professional photographer's work.

    Given two photographers equally capable of delivering in focus photographs, most marketing professionals I have known would rather work with the one with the easy to work with personality, rather than the jerk. And if the quality between the two was close enough, even with the less talented photographer who was easy to work with instead of the jerk. And if they had to work with the jerk for a while, they would certainly be looking for someone to replace the jerk ASAP.

  4. #24

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    Depth of Field, Depth of Focus, and Film Flatness

    Dave, you have hit the nail squarely on the head with your last paragraph. In the freelance side of my life, I've made a good living from clients that were tired of babysitting their previous photographers. Nicely stated.

  5. #25

    Depth of Field, Depth of Focus, and Film Flatness

    Leonard wrote, "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."

    This is technically true, but I like Steve's way of looking at it better. It is true that when you increase focal length while holding f-number and magnification constant (and focus on the same subject), you get a little more depth of field in front (of the plane of sharp focus) and less depth of field behind.

    But this only occurs because we are accustomed to the traditional definition of depth of field. If we used Harold Merklinger's object-field definition instead, which I generally prefer anyway, then Steve's statement is exactly correct (Of course, I am ignoring diffraction and assuming a perfect film and lens).

    For example, from p. 37 of Merklinger's "INs and OUTs of Focus": "If we do change lenses (or zoom) so as to maintain the same image magnification as the camera-to-subject image changes and keep the lens (or lenses) set to the same f-number, the depth of the zone of acceptable sharpness does not change."

  6. #26

    Depth of Field, Depth of Focus, and Film Flatness

    A couple of questions... Would it be accurate to simplify the terminology by saying depth-of-field defines the area of (apparent) focus in front of the lens, while depth-of-focus is the same phenomenon behind the lens?

    Second question: When making an enlargement, is the depth-of-focus up where the negative is and the depth-of-field down where the photo paper is, or is it the other way around? (BTW, considering how out-of-alignment many enlargers are and how much some negatives curl in the carrier, this could be every bit as important as when out shooting.)

  7. #27

    Depth of Field, Depth of Focus, and Film Flatness

    Mark, I will attempt some answers to your two questions.

    I have nothing against your way as stated in the first question. For example, in 1-to-1 photography without movements, the size of the depth of field and depth of focus are equal.

    You can imagine the light going in either direction. Therefore, there is no fundamental geometric distinction between depth of field and depth of focus. You can say that the paper lies in either depth, for the mathematics does not distinguish between the two.

    When Steve wrote, "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, " he maybe made a little misstatement or simplification of language. Instead of no sharpness loss, he really meant to refer to acceptable sharpness loss. The circle of confusion determines both your depth of focus and your depth of field, so depth of focus is a concept based on acceptable sharpness loss rather than no sharpness loss.

  8. #28

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    Depth of Field, Depth of Focus, and Film Flatness

    I think Mark just made two excellent points. I think his first one really brings the whole issue into focus. (Sorry for the pun.)

    For every photograph that has some image area that is out of focus, the photographer has just had a bad encounter with insufficient depth of focus. You may want to completely blame depth of field, but you can't. Depth of field and depth of focus are really the same principle being applied to opposite sides of a lens. And while it is appearant from these recent threads that a majority of photographers do not know or think about depth of focus (based on book references), it most certainly is an optical principle that leaves it mark on every photograph.

    The issue of having insufficient depth of focus is not going to be seen if the camera/lens system is properly adjusted and the alignment of the subject, lens, and film planes are sufficiently close for a given f/stop / circle of confusion / enlargement. But when one of them is poorly adjusted, then we often times will just attribute the focusing errors to "bad focussing". Well, it's just as much insufficent depth of field as it is depth of focus.

    If the depth of focus was always infinitely large (or even sufficiently large), then we would never have to worry about the position of the ground glass or the flatness of the film. But, unfortunately it's not infinitely large. If depth of focus was always zero (infinitely thin) then we would probably never get any image to appear in focus. I guess we should feel lucky that it is somewhere inbetween, and large enough that it does not make itself very appearant. (It's kind of neat that it is proportional to the depth of field.)

    But when we have a camera that does not have the ground glass in the same position as the film, then our depth of focus issue may be readily appearant. If the error is grossly off, then it will be quickly obvious to the photographer that something is wrong, and hopefully the error will be corrected. But what if the error is marginally off? A lot of wasted time and frustration may be the result of this. This is all a result of depth of focus. It's kind of silly to say it's a non-issue and can be completely ignored.

    Mark's second issue brings up a lot of situations that I'm sure we've all encountered with while printing - negative bow, negative stage misalignment, lens stage misalignment. How many people here use glass negative carriers? Why - because you couldn't reliable get all of the image into focus? Especially with big enlargements? Well, this is completely related to depth of focus. Popped negs, misalignment - both decrease the amount of depth of focus that we have available to make an in-focus print. Again, to say it is a non-issue and completely ignored is a kind of silly point of view in my opinion.

    Mark, I too like to think of the depth of focus at the film when enlarging, and depth of field at the paper. I don't know if there is an actual convention to the nomenclature for it.

    Kirk - www.keyesphoto.com

  9. #29

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    Depth of Field, Depth of Focus, and Film Flatness

    Jerry wrote:

    When Steve wrote, "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, " he maybe made a little misstatement or simplification of language. Instead of no sharpness loss, he really meant to refer to acceptable sharpness loss. The circle of confusion determines both your depth of focus and your depth of field, so depth of focus is a concept based on acceptable sharpness loss rather than no sharpness loss.

    I've certainly seen this come into play in the real world. While trying to get a shallow depth of field in a photograph, the area of subject that I had focussed for, using the "find near and far subject position and focus in the middle with the appropriate f/stop / circle of confusion" technique, I saw that after I processed the film the near and far focus we shifted slightly from where I was expecting them to be.

    Was it a problem with a misalignment of the ground glass? I've never seen issues that would suggest a misaligned gg. Was this a problem with focussing? I assume not as I have a mm scale that I use for focussing. But a slight misfocus due to any reason can push one side of the image out of the acceptable circle of confusion on the negative.

    Whichever issue you want to attribute the slight difference in expected focus range from actual focus range, it is most certainly was an issue with depth of focus.

    Kirk - www.keyesphoto.com

  10. #30
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    Depth of Field, Depth of Focus, and Film Flatness

    Umm, just a gentle reminder to Steve that he published in the March/April 1996 issue of his own magazine, an article entitled "A Battle of the Bulge: Sheet Film Holders", in which author Cervin Robinson outlined a scenario in which depth of focus matters (although he didn't use the term), and proceeded to report the result of tests of several different types of film holders for accuracy and precision of positioning of film relative to the ground glass. The author's conclusion, BTW, was that the slop in the positioning of film by different holders was of sufficient magnitude to be worth worrying about.

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