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Photo Forum / Film Photography / Large Format / July 2004roll-film back: DOF question
When using a 6x7 rollfilm back on a 4x5 camera (say with a 90mm lens), my understanding is that you end up with a crop. What about DOF? Will it be the same as using a 90mm lens designed for 6x7 on a MF camera body? Or will it be shallower because the image circle is much bigger? DOF is related to - Lens Focal Length - Lens Aperture - Distance Film size does *not* enter into the equation(s). >DOF is related to > [quoted text clipped - 3 lines] > > - Distance I think your missing magnification. DOField on a contact print is different then say a 10 enlargment. Larry >>DOF is related to >> [quoted text clipped - 6 lines] > I think your missing magnification. DOField on a contact print is different > then say a 10 enlargment. Correct. > Larry > DOF is related to > [quoted text clipped - 5 lines] > > Film size does *not* enter into the equation(s). That is not quite right. The way film size enters into the "equation(s)" is that for the same size final print, you have to enlarge more for a smaller format, less for a larger format. Since you mention equations, let's look at one. The formula for hyperfocal distance is f^2/Nc where f is the focal length, N is the f-number and c is the diameter of the maximal acceptable circle of confusion in the film. For a small format, you would have to choose a smaller c because of the greater degree of enlargement. A smaller c in the denominator means a larger hyperfocal distance, which in turn means less depth of field if you focus exactly on a subject at the same distance. So if the three things you mention are the same, you get less depth of field in the smaller format than in the larger format. >DOF is related to > - Lens Focal Length > - Lens Aperture > - Distance >Film size does *not* enter into the equation(s). Wrong. A key factor in the equation is called the Circle of Confusion. This depends upon viewing distance, with 1/100" appropriate to a 10" viewing distance. This is a subjective determinant however, as debate within the lens manufacturers varies by up to 3:1 from 1/70" to 1/200" ! The official equation factors focal length, f/stop, and the hyperfocal distance at that f number. While circle of confusion is most often stated in terms of 35mm frame, the multiplication factor to the final print size also enters the computation of the circle of confusion. So film size DOES come into the equation when the final magnifacation is also factored into the circle of confusion calcuation! --Wilt It will be the same as using a 90mm lens on any camera at the same lens-to-subject distance and with the same aperture. What may, as a practical matter, sometimes increase depth of field using the roll film back as compared with 4x5 film is the possibility that with the roll film back you will set up farther from the subject than you would have if you were using it as a 4x5 lens since the angle of view will be so much narrower when the image area is only 6x7. . The greater the lens-to-subject distance the greater the depth of field, all other things affecting depth of field remaining the same. > When using a 6x7 rollfilm back on a 4x5 camera (say with a 90mm lens), my > understanding is that you end up with a crop. What about DOF? Will it be > the same as using a 90mm lens designed for 6x7 on a MF camera body? Or > will it be shallower because the image circle is much bigger? Ah. I see. Hence the reason digicams have such large DOF (with their tiny sensors) is that in order to fit everything into the frame at a reasonable distance, a very short focal length must be used. > It will be the same as using a 90mm lens on any camera at the same > lens-to-subject distance and with the same aperture. What may, as a [quoted text clipped - 10 lines] >> the same as using a 90mm lens designed for 6x7 on a MF camera body? Or >> will it be shallower because the image circle is much bigger? > Ah. I see. Hence the reason digicams have such large DOF (with their > tiny sensors) is that in order to fit everything into the frame at a > reasonable distance, a very short focal length must be used. The answer to your original question is that you would actually get less depth of field with the roll film back if you used the same lens, the same aperture, and the subject were at the same distance from the lens. The only way to understand this is through the formulas, but one way to help you understand it is as follows. 90 mm is a wide angle lens for 4 x 5 but is a normal lens for 6 x 7. Going from a wide angle lens to a normal lens, with aperture fixed, reduces depth of field for a subject at the same distance. If that were the only issue, it could explain why you get less depth of field. Unfortunately, you are also changing formats, going from 4 x 5 to 6 x 7. With everything else equal, that tends to increase depth of field. But now you have to worry about quantititative matters. The first effect is significantly more pronounced than the second effect, so the net result is that you end up with less depth of field. Similarly, you can't understand the reason why digicams have so much depth of field without thinking about it quantitatively. If you want a good explanation, see Bob Atkins article at www.photo.net, which is one of the few places I've seen without mistakes in explaining DOF. It is often enticing to use qualitative discussions to try to explain one particular thing you've found out to be true. But such explanations are often wrong when applied to some other related situation. The advantage of a scientific explanation is that it is clear when it works and when it doesn't and most important why it does or doesn't. >>It will be the same as using a 90mm lens on any camera at the same >>lens-to-subject distance and with the same aperture. What may, as a [quoted text clipped - 10 lines] >>>the same as using a 90mm lens designed for 6x7 on a MF camera body? Or >>>will it be shallower because the image circle is much bigger? > It will be the same as using a 90mm lens on any camera at the same > lens-to-subject distance and with the same aperture. This is wrong. See my other response. It depends on the format since a smaller format has to be enlarged more. In fact, the way the math works out, you get less depth of field with the 90 mm lens and the roll film back, assuming the subject distance is fixed and the final print is the same size. > What may, as a > practical matter, sometimes increase depth of field using the roll film back [quoted text clipped - 4 lines] > greater the depth of field, all other things affecting depth of field > remaining the same. Just what happened would depend on how the different quantitative factors compared. In the specific case you describe, it does in fact end up that moving further back so the image size in the final print is the same, you end up with more depth of field. That would be where there is a primary subject not too far from the camera as in portraiture. But had the equations been different, or the underlying assumptions different, that might have worked out differently. The thing to keep in mind is that different factors may affect DOF at different rates. So just knowing that one thing increases and another decreases doesn't tell you what the net effect will be. It depends on how fast each increases or decreases. >>When using a 6x7 rollfilm back on a 4x5 camera (say with a 90mm lens), my >>understanding is that you end up with a crop. What about DOF? Will it be >>the same as using a 90mm lens designed for 6x7 on a MF camera body? Or >>will it be shallower because the image circle is much bigger? Leonard Evens said: > This is wrong. See my other response. It depends on the format since a > smaller format has to be enlarged more. In fact, the way the math > works out, you get less depth of field with the 90 mm lens and the roll > film back, assuming the subject distance is fixed and the final print is > the same size. I don't think it is wrong. Depth of field relates to the size of the circles of confusion in the negative, which in turn is affected by only three things as I said before, lens focal length, aperture, and lens to subject distance. "Enlarging more" (i.e.image magnfication) is one of the factors relating to "acceptable sharpness" in the print, not to depth of field. If you wish to introduce image magnfiication into the discussion then you also should talk about the viewing distance from the print that you're assuming and explain what you consider to be an "acceptably sharp" print at any given magnification and any viewing distance. But those things shouldn't, IMHO, be confused with depth of field. "The factors affecting depth of field are governed by the following principles: (1) The depth of field doubles if the f number is doubled . . . (2) if you double the subject distance the depth of field increases by four times . . . (3) if you reduce the focal length by one half, the depth of field increases by four times. . . " Adams, "The Camera," p. 49. Note the absence of any mention here of film format or image magnification from this explanation of how the three factors affecting depth of field work. Adams then goes on to discuss image magnification and print viewing distance as two of the factors, along with depth of field, that relate to "acceptable sharpness" in the print (he doesn't mention personal standards of "acceptable sharpness" but obviously that is relevant also). So I think that if we're talking about depth of field we are talking about the size of circles of confusion in the negative and that is affected only by three factors of which image magnification isn't one. If you're talking about image magnficiation you're talking about a factor that doesn't affect depth of field but that rather affects acceptable sharpness of the print. > > It will be the same as using a 90mm lens on any camera at the same > > lens-to-subject distance and with the same aperture. [quoted text clipped - 30 lines] > >>the same as using a 90mm lens designed for 6x7 on a MF camera body? Or > >>will it be shallower because the image circle is much bigger? > Leonard Evens said: > [quoted text clipped - 7 lines] > of confusion in the negative, which in turn is affected by only three things > as I said before, lens focal length, aperture, and lens to subject distance. You can, if you wish, define depth of field in your own unique way, but that is not the way it is usually defined. The usual definition assumes a normal user who is looking at a certain size final print at a normal viewing distance. A typical standard for the print would be an 8 x 10 print viewed at 10 (250 mm) to 12 inches. It is the maximum acceptable circle of confusion in the print that is relevant. The choice of print coc depends of course on how discerning the viewer is. One plausible choice is 0.2 mm or thereabouts. But some people can see better than that and would choose a smaller value. Once you choose the coc in the print, then the coc in the film is obtained by dividing by the enlargement factor. For 4 x 5, that is about 2, so the coc in the film would be about 0.1 mm (or less for fussier viewers). For 6 x 7, the enlargement is about 3.6, so the coc in the film would be about 0.2 divided by that or about .05 mm. > "Enlarging more" (i.e.image magnfication) is one of the factors relating to > "acceptable sharpness" in the print, not to depth of field. If you wish to [quoted text clipped - 3 lines] > magnification and any viewing distance. But those things shouldn't, IMHO, be > confused with depth of field. See above. You choose a standard for print size and viewing distance. As I said, an 8 x 10 print viewed at 10-12 inches is a good choice. Most viewers are not comfortable viewing something at closer than 10 inches, and it is usually assumed that people will try to view a print at about the diagonal distance. For an 8 x 10 print, that is a little over 12 inches. If the print is larger, then people will generally get proportionately further back. For example, a 16 x 20 print might normallybe viewed at about 2 feet. If so, a coc of size 0.4 in such a print would be acceptable corresponding to a coc of 0.2 mm in a print half the size viewed at half the distance. Of course, there always will be people who will insist on getting closer to the larger print than the diagonal distance. For such people, a smaller print coc would be appropriate and hence a smaller coc in the film. I think you are making the assumption that depth of field is an absolute characteristic just of the lens. What you say would be a good way to proceed if we only viewed contact prints, but that is not the case in modern photography. In photography as practiced today, depth of field is not an absolute quantity but is relative to what is needed for the final image. > "The factors affecting depth of field are governed by the following > principles: (1) The depth of field doubles if the f number is doubled . . . I'm not sure what you mean by that. Depth of field depends on a variety of factors, one being the subject distance, so a simple statement like that doesn't make sense. One way to quantify such statments about how depth of field changes is to ask how much you have to change the f-number to obtain the same depth of field. From that perspective, your (1) is a tautology. > (2) if you double the subject distance the depth of field increases by four > times . . . Within certain ranges, that is approximately true, but it isn't generally true. For example, by doubling the distance, you could go from finite depth of field to infinite depth of field. In terms of f-number change, the statement is approximately true. (3) if you reduce the focal length by one half, the depth of > field increases by four times. . . " Adams, "The Camera," p. 49. Again, in terms of f-number change, the statement is literally true. > Note the absence of any mention here of film format or image magnification > from this explanation of how the three factors affecting depth of field > work. But those factors are implicit. In all these statements, Adams is assuming a fixed format. Remember that Adams is talking as a practicing photographer, not as an optical scientist. As such, his statements are relative to his typical way of working with his typical equipment. If you had questioned him further, he would of course have told you that with different equipment and with different aims, the rules would be different. By the way, Adams does make some rather obvious mistakes in places, so he isn't the best reference in some of these matters. > Adams then goes on to discuss image magnification and print viewing > distance as two of the factors, along with depth of field, that relate to [quoted text clipped - 3 lines] > circles of confusion in the negative and that is affected only by three > factors of which image magnification isn't one. You are confusing two things here. For any point in the scene which is not in the exact subject plane, the image of that point in the film plane will be a disc, called a circle of confusion. The closer the subject point is to the plane of exact focus, the smaller will be the size of the image disc or circle of confusion. But depth of field is calculated by specifying the maximal possible circle of confusion which can not be distinguished from a point. But that term is clearly subject to assumptions about who is doing the distinguishing and under what conditions. If you viewed a contact print at 10 inches you would choose one value for the maximum. If you were viewing a 2 x enlargement also at 10 inches, you would choose the same value for the enlargement, but necessarily half that value for the film. > If you're talking about > image magnficiation you're talking about a factor that doesn't affect depth > of field but that rather affects acceptable sharpness of the print. So how do you choose the maximal acceptable coc in the film? Do you use the same value for an 8 x 10 camera and a 35 mm camera? If you do that, you are going to get values very different from what you see in DOf tables. You are making a valid distinction, but I don't think you have really thought it all through. In particular you are ignoring the need to choose a maximum allowable coc for the negative and how that choice depends on a variety of assumptions. If you prefer, you can restrict the term 'depth of field' to refer only to depth of field of contact prints viewed at 10 inches, and distinguish that from "adequate sharpness" in enlargements, but that would be a rather unusal way to use the terms and would not be consistent with what most other people are doing. > [...] > Of course, there always will be people who will insist on getting closer > to the larger print than the diagonal distance. [...] I find this to be more and more the case lately and I have a tentative theory that it is due to experience of the same persons with digital imaging. When they can magnify, they will in order to explore deeper and deeper into an image. Maybe we will see the day when shows have a roped-off distance. :) Hi This 10 inch standard view distance has been around a long time. It controls what is perceved as a standard lens on any given camera. Depth of field tables are derived from it. The motion picture industry lives by it. Example, they take a half frame 35mm image and magnify it 200 times on a 40 ft wide screen and yet everything is sharp from a typical ticket holders view point. How could this be? Larry > > Of course, there always will be people who will insist on getting closer > > to the larger print than the diagonal distance. [...] And with a 20x loupe, no less.  SignatureNicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. Remove spaces etc. to reply: n o lindan at net com dot com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ > You can, if you wish, define depth of field in your own unique way, but > that is not the way it is usually defined. It isn't unique at all. It's the way Ansel Adams among others describes it in the quote I provided earlier and with all due respect, as between you and Ansel Adams I think I'll stick with Ansel. > The usual definition assumes a normal user who is looking at a certain > size final print at a normal viewing distance. A typical standard for [quoted text clipped - 3 lines] > the viewer is. One plausible choice is 0.2 mm or thereabouts. But > some people can see better than that and would choose a smaller value. This is a way of defining it if you're talking about depth of field in the print. It isn't the only way and depending as it does on four variables ("normal user," "certain size final print," "normal viewing distance," and "choice of print coc") it causes depth of field to vary widely depending on how these variables are defined and used. I have no problem talking about depth of field in the print and introducing these variables. But your original statement was that print enlargement size had to be taken into account in calculating depth of field. It doesn't have to be, it can be but if it is then the other variables have to be taken into account also. > > Leonard Evens said: > > [quoted text clipped - 131 lines] > use the terms and would not be consistent with what most other people > are doing. >>You can, if you wish, define depth of field in your own unique way, but >>that is not the way it is usually defined. > > It isn't unique at all. It's the way Ansel Adams among others describes it > in the quote I provided earlier and with all due respect, as between you and > Ansel Adams I think I'll stick with Ansel. It is not just me. I am just stating one of the standard explanations of depth of field, which are all equivalent. For example, read Bob Atkins's article on depth of field at www. photo.net. You shouldn't treat this as a personal disagreement between the two of us. Consider the following argument and ask yourself how you respond to it. One of the crucial parameters in discussing depth of field is the hyperfocal distance. There are two formulas for the hyperfocal distance which give slightly different answers in practical situations. The most commonly used formula is H = f^2/(Nc) where f is the focal length, N is the f-number and c is the maximal acceptable coc in the negative. Given the hyperfocal distance, for objects not in the close-up range, the formula for near depth of field is (with D the distance to the plane of exact focus) D^2/(H + D) and the formula for rear depth of field is D^2/(H - D) unless D is greater than or equal to H, in which case the answer is infinite. (Adams in Camera and Lens actually gives the formulas for the near and far limits from which these formulas, which aren't due to me, arise by simple algebra. He doesn't give the formula for hyperfocal distance, but refers you to another reference.) (Different formulas are used in the close-up range, and there are also formulas which work generally, but let me keep it simple.) These or the closely related formulas for near and far DOF limits are standard formulas which can be found in books on photographic optics. I didn't invent them. They have been used for many, many years to construct depth of field tables. Note that there are FOUR quantities that must be fed into the formulas. They are f, N, D, and c. Your discussion accounts for f, N, and D, but you don't mention c. c must be specified. So I ask you (again) how you specify it? Do you use the same value for every format? If so, what is that value, and on what basis did you choose it? If you use different values for different formats, what is your argument for making those different choices? >>The usual definition assumes a normal user who is looking at a certain >>size final print at a normal viewing distance. A typical standard for [quoted text clipped - 13 lines] > account in calculating depth of field. It doesn't have to be, it can be but > if it is then the other variables have to be taken into account also. Look at my discussion above. The formulas require FOUR variables, and they don't say anything specific about format. The variable c must be specified somehow, whether you want to talk about final prints and enlargements or not. The usual way to determine a proper value for c is based on making some assumptions about the final print. If you have a way which is independent of any such assumptions, please tell me what it is. |
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