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Photo Forum / Digital Photography / DSLR Cameras / December 2006Acceptable Circle Of Confusion. (Dof related).
Often it is cited that dof only depends on focal length and apperature. But this is not true offcourse it also depends on focus distance and on the Acceptable Circle Of Confusion (ACOC) often cited as the Circle Of Confusion. The ACOC depends again on different things. The enlargement, the way the enlargement is viewed and how good are the eyes of the viewer and how critical is the viewer. A practical way for an ACOC is a fraction of the diagonal of the picture, for example 1/1500 of the diagonal. (Some people are more critical, other are less critical and offcourse it also depends on the viewing circumstances, but 1/1500 of the diagonal is fair and is fairly standard, more critical people would go to 1/1750 or even 1/2000 of the diagonal). If you define the ACOC as a fraction of the diagonal (of the endresult) then the format of the camera, for example full frame or APS crop is considered in the equation. Teaser : Comparing a full frame camera (24mm x 36 mm) with a APS sized (18mm x 24) mm and everything is scaled down with the same linear scaling and after processing the picture you enlarge the APS size by a factor of 1.5 more than the full frame, the two pictures will look exactly the same. But the condition is that everything has to be scaled down. Yes EVERYTHING. (It's not practical to scale everything down, it not even feasable and it's not possible, but this asside). Scaling down an angle gives the same angle. Scaling down a shape gives the same shape, but smaller. Scaling down the focal length (in mm) and the size of the aperature (size in mm for example), you get the same aperature number in both cases because the aperature number is a division of the focal length and the size of the aperature and both are scaled down exactly the same amount. If not everything is scaled down, in general a smaller sensor wil give more DOF in general, when used in similar circumstances. Ben Brugman > Often it is cited that dof only depends on focal length and apperature. But > this is not true offcourse it also depends on focus distance and on the [quoted text clipped - 32 lines] > If not everything is scaled down, in general a smaller sensor wil give more > DOF in general, when used in similar circumstances. But the smaller camera is smaller so gathers fewer photons and the image is more noisy, less dynamic range. >> But the condition is that everything has to be scaled down. Yes >> EVERYTHING. [quoted text clipped - 6 lines] > But the smaller camera is smaller so gathers fewer photons and the image > is more noisy, less dynamic range. But that's because you cannot scale the atomic structure of the sensor and you cannot scale the photons either. As said scaling EVERYTHING is just not possible. (And scaling the subjects in the realworld is not very practical in most situations). ben > "Paul Furman" <paul-@-edgehill.net> schreef in bericht > [quoted text clipped - 15 lines] > (And scaling the subjects in the realworld is not very practical in most > situations). LOL and the miniature people to look at the miniature prints. :-) >> "Paul Furman" <paul-@-edgehill.net> schreef in bericht >> [quoted text clipped - 17 lines] > > LOL and the miniature people to look at the miniature prints. :-) That's what the filming industrie was doing working on scaled down models, but not with a scaled down camera. Looking at the film you saw that it was totaly fake. One of the reasons for this was that everything was scaled down, but the camera not and therefore you got very limited dof which just did not look very realistic. From the middel of teh 70'ties this became better. (Example Star Wars the opening scene with the 'huge' space ship, there the DOF was very large considering the actual scale of the 'huge' space ship). But before that for example warfilms with warships the with the size of under a few feet, they just didn't look very realistic because of the Dof (and watermovement as wel), same goes for godzilla films, you just can see that it's a bad model, but also very limited dof. ben >>>"Paul Furman" <paul-@-edgehill.net> schreef in bericht >>> [quoted text clipped - 29 lines] > the Dof (and watermovement as wel), same goes for godzilla films, you just > can see that it's a bad model, but also very limited dof. Ever seen this: http://recedinghairline.co.uk/tutorials/fakemodel/ People fake shallow DOF to make a real landscape look like a toy model. Kind of an odd pursuit but it is amusing. >>>>"Paul Furman" <paul-@-edgehill.net> schreef in bericht >>>> [quoted text clipped - 34 lines] > People fake shallow DOF to make a real landscape look like a toy model. > Kind of an odd pursuit but it is amusing. Thanks, no I didn't know this one. I did the following myself. When I was still making slides, I sometimes made stereo slides. Mostly with one camera, but if you take two slides with a huge difference in distance (I have done 15 feet) of a large square in a town and you look at them in stereo, the houses look like toy houses as wel. I used to exagerate the distance between the two camera's to increase the stereo effect, but it made look everything small. With two 'fixed' camera's I had a distance of 1 to 1.5 feet, then the stereo effect is just enlarged, but going beyond that your actual scene starts to look small. (I wanted to do this the other way round, using a model town and make a very small distance between the two slides to get the opposite effect for that, I never managed to actually do that). But thanks for the above side, the effect is pretty amazing. Have to reproduce that one myself. (Must say I once say a picture of a valley taken with a technical camera with a similar result. The focal plane was turned is such a way to get a similar result). Thanks, Have a very Nice Christmas, (Happy Newyear as wel). Ben sh.t!! you mean Godzilla wasn't real :) regards Don >>> "Paul Furman" <paul-@-edgehill.net> schreef in bericht >>> [quoted text clipped - 31 lines] > > ben > >> "Paul Furman" <paul-@-edgehill.net> schreef in bericht > >> [quoted text clipped - 23 lines] > down, but the camera not and therefore you got very limited dof which just > did not look very realistic. As a side issue, here is more to this than that. Detail is lacking in scaled down filming. Ocean "waves" that hit a 12 foot long "ship" do not contain as much detail because 20 foot high real waves do not "scale" to 6" high waves. The only two movies that I can remember where they attempted to maintain detail to the limit of the film's capability were "2001" and "Starship Troopers" where the models (of the ships) had detail below that which the film could record. Additionally, motion effects (which differs from still photographic problems) do not scale well either. > Often it is cited that dof only depends on focal length and apperature. But > this is not true offcourse it also depends on focus distance and on the > Acceptable Circle Of Confusion (ACOC) often cited as the Circle Of > Confusion. ... and the magnification to print size from the (cropped or not) sensor/film to print. That is the thing people neglect ... specifically that the lens markings for DOF are referenced to a nominal print size, typically 8x10 inches. We just went through all this. Don't start again. Puh-leazzzzzze! Grok http://www.nikonlinks.com/unklbil/dof.htm and there is little more to add. Merry Christmas, Alan  Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. >> Often it is cited that dof only depends on focal length and apperature. >> But this is not true offcourse it also depends on focus distance and on [quoted text clipped - 5 lines] > that the lens markings for DOF are referenced to a nominal print size, > typically 8x10 inches. No not "... AND the magnification...", because this is allready contained in the ACOC, but yes the magnifiction etc. do determine what the ACOC is. > We just went through all this. Don't start again. Puh-leazzzzzze! If you do not want another thread, it's probably best to ignore the thread and not answer it at all, certainly not with going into the subject again. But thanks for your time and attention. ben > Grok http://www.nikonlinks.com/unklbil/dof.htm and there is little more to > add. > > Merry Christmas, > Alan > If you do not want another thread, it's probably best to ignore the thread > and not answer it at all, certainly not with going into the subject again. > But thanks for your time and attention. As it has all been discussed and disected to death very recently here, it begs the question 'why' are you trying to bring it up all over again? Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. I thought "MI5Victim" was chairman of the "circle of confusion club" ?? mikey >> If you do not want another thread, it's probably best to ignore the >> thread and not answer it at all, certainly not with going into the [quoted text clipped - 3 lines] > it begs the question 'why' are you trying to bring it up all over > again? >Often it is cited that dof only depends on focal length and apperature. But >this is not true offcourse it also depends on focus distance and on the [quoted text clipped - 29 lines] >aperature number is a division of the focal length and the size of the >aperature and both are scaled down exactly the same amount. For u >> f, then D = 2NCu^2/f^2 If you scale down to half size, the F-number N stays the same (it's a ratio, as you correctly said); C is halved and u is halved and F is halved. This nets out to a halving of D. The halving of D is "used up" in the extra enlargement to get to the final viewed image. However, the real world comparison most people are interested in is the case where u stays the same - IOW, you take a picture from the same place. Then D is doubled (C and F being halved as before, and the reduction of C is again used to increase the enlargement). The corollary is that, as you say, the image size on the sensor is much smaller and the quality is lower. This is, BTW, the same factor as is familiar to users of larger format film equipment over many years. Larger format = lower DoF for a given aperture, but a much greater quality in the final image. >If not everything is scaled down, in general a smaller sensor wil give more >DOF in general, when used in similar circumstances. Yes. However, note that these approximations only hold for u >> F. Where u ~ 2f (i.e. in photomacrography) then D = 2CN(1+m)/m^2, where m is the magnification, the ratio object size/image size on sensor or film (note - not on final image). This does not involve f at all, IOW a macro shot at m = 1.0 has the same DoF whatever size lens you use. Here the only effect of using a smaller sensor is to get a smaller angle of view (assuming the enlargement factor to get to the final image is the same, i.e. the value of C is the same for both). BTW, the word for the little hole thing in the lens is "aperture", not aperature. I normally try to resist spelling issues except for technical terms, but you may get into trouble looking in text for the wrong thing. Merry Christmas all. David SignatureDavid Littlewood > BTW, the word for the little hole thing in the lens is "aperture", not > aperature. I normally try to resist spelling issues except for technical > terms, but you may get into trouble looking in text for the wrong thing. Thank you for the correction, English not being my first language I not always immediatly see my mistakes. My apologies for that. On the other issues, where I said, scaling Everything, I also meant scaling the focus distance, and also scaling the subject. This is not realistic, because in most cases we can not scale the subject and therefore not we can not scale the focus. So if the subject size and focus stays the same. For the APS format it's like the subject is 1.5 times farther away than for the full frame which results in a langer dof. (Even in a relative larger dof). Some people think that if scaling 'everything' down, the lens, the sensor and everything 'in' the camera, that they should get the same picture, so that a smaller size camera should not matter. But the real world is not scaled down and therefore there reasoning does not work. (The smaller camera say the aps sized sensor has the hyperfocal also 1.5 times closer to the camera, so compared to to the larger format the subject is moving farther towards infinity with a smaller format. If it overtakes the hyperfocal distance Dof can go from limited to infinity.) If everything is scaled (not realistic), the subject and focus distance is the same. If you make a drawing of the situation, with a COC from the subject you just have to enlarge the drawing with a 1.5 or 1/1.5 to go from one situation to the other. For an APS sized sensor the situation is always 'less' macro than for the full frame hence the more dof. > For u >> f, then D = 2NCu^2/f^2 Here I do not know were all the letters stand for. So I can not apply the scaling to that. If the formula is exact (and not an approximation) the scaling should work exactly on the formula as wel. ben >>Often it is cited that dof only depends on focal length and apperature. >>But [quoted text clipped - 70 lines] > enlargement factor to get to the final image is the same, i.e. the value > of C is the same for both). > Merry Christmas all. > > David >> BTW, the word for the little hole thing in the lens is "aperture", not >> aperature. I normally try to resist spelling issues except for technical >> terms, but you may get into trouble looking in text for the wrong thing. >> >Thank you for the correction, English not being my first language I not >always immediatly see my mistakes. My apologies for that. It is a credit to your second language skills that I did not detect this. >On the other issues, where I said, scaling Everything, I also meant scaling >the focus distance, and also scaling the subject. This is not realistic, >because in most cases we can not scale the subject and therefore not we can >not scale the focus. So if the subject size and focus stays the same. For >the APS format it's like the subject is 1.5 times farther away than for the >full frame which results in a langer dof. (Even in a relative larger dof). I think that's more or less what I was trying to say. >Some people think that if scaling 'everything' down, the lens, the sensor >and everything 'in' the camera, that they should get the same picture, so [quoted text clipped - 17 lines] >scaling to that. If the formula is exact (and not an approximation) the >scaling should work exactly on the formula as wel. u = object distance; f = focal length; N = f-number; C = circle of confusion size. The formula is not exact; it is a simplification of a much more complex expression*, and is only valid for u much greater than f. As I said, where u is approximately the same as f, a different simplification is more appropriate, D = 2CN(1+m)/m^2. *For the terminally curious, D = 2f^2*u^2*N*C/(f^4 - N^2*C^2*u^2); for u >> f, the term N^2*C^2*u^2 can be ignored, giving the simpler form. David SignatureDavid Littlewood > *For the terminally curious, D = 2f^2*u^2*N*C/(f^4 - N^2*C^2*u^2); for u > >> f, the term N^2*C^2*u^2 can be ignored, giving the simpler form. Tried this formula on f = 50 u= 10000 coc = 0.03 (Aperature size is 8.92857, so the aperature number is 50/8.92857 = 5.6, because both the size and the f are scale the 5.6 is 'constant') N = 5.6 And used the factor .6666666, 1, 1.5 and 2 on f,u,coc for scaling. And Caculated the D for all those scales. And indeed D does scale correct for those tested values and and scales. I expect that other values and other scales work as well. Allthough I must say that for certain values the formula does not work correctly because they result in negative values (but they do scale). factor 2/3 16337.96242 (This is indeed 2/3 times factor 1) factor 1 24506.94363 factor 1.5 36760.41545 (This is indeed 1.5 times factor 1) factor 2 49013.88727 (This is indeed 2 times factor 1) My conclusion, is that this formula does correctly scale, and shows that; 'scaling everything, will result in exactly the same picture'. Ben > David > > *For the terminally curious, D = 2f^2*u^2*N*C/(f^4 - N^2*C^2*u^2); for u > > >> f, the term N^2*C^2*u^2 can be ignored, giving the simpler form. [quoted text clipped - 23 lines] > > Ben Ben, since these formulae are derived from ray optics (ie you could simply draw ray diagrams, play with them and end up with these), then, as you said, they should scale (simply take the piece of paper on which you drew the diagram, and shrink it in both directions). If the formulae don't scale, it means that they are approximations to the correct ones (which you can find in various places on the internet). eg look here http://en.wikipedia.org/wiki/Depth_of_field#Derivation_of_the_DOF_formulae If you scale the setup down to 1/2 the size, f is halved, s is halved, N does not change, and c is halved. So the DOF is halved. Now, this is only valid as long as geometrical optics works (I don't know what happens if the lens is not symmetric, but, as long as we can use this geometrical approach, it should still scale). It ignores the finite wavelength of light (ie diffraction effects), which we can't scale down. It also ignores the effect of this scaling on the noise (ie it ignores the fact that light comes in discrete packets). It probably ignores various other things I am missing. > Ben, > since these formulae are derived from ray optics (ie you could simply [quoted text clipped - 12 lines] > finite wavelength of light (ie diffraction effects), which we can't > scale down. From the original message: " But the condition is that everything has to be scaled down. Yes EVERYTHING. (It's not practical to scale everything down, it not even feasable and it's not possible, but this asside). " > It also ignores the effect of this scaling on the noise (ie > it ignores the fact that light comes in discrete packets). Which we scale down as wel. "...and it's not possible, but this asside)." > It probably > ignores various other things I am missing. Allthough EVERYTING has to be scaled down, I wouldn't know how time, lightspeed, the energy in the photons has to be scaled down. But as said it not feasable and not possible. But in reality the camera is scaled down, not the scene to be captured. Because of the scaling down, the hyperfocal distance is 'scaled' down. If the scene remains as it is and the hyperfocal distance comes closer to the camera, the dof for a certain focal distance increases. This is one way of me to show then when a camera is scaled down (smaller sensor, smaller focal length), the dof does increase. And yes making an absurdly accurate drawing of the situation and than enlarging or shrinking that drawing is an exelent way of demonstrating that you can scale this type of things. A lens which is not symmetric should scale down as well according to these rules. Ben > > Ben, > > since these formulae are derived from ray optics (ie you could simply [quoted text clipped - 41 lines] > you can scale this type of things. A lens which is not symmetric should > scale down as well according to these rules. Hi. To avoid any possible misunderstandings, I wasn't disagreeing with you, just typing thoughts. I was pointing out that all our discussions here are based on geometrical optics, which neglect the fact that light is made up of waves (and particles, but in this context, that's one level down from waves). Imagining an absurdly accurate drawing and scaling the paper is indeed an excellent way of demonstrating what you say (which I hadn't thought of), but that's not what I meant. I meant that all the things you, other people and myself are talking about here (DOF, focal length etc) are based on geometrical optics, ie ray tracing (the drawing I had in mind wasn't accurate at all, just labelled with various quantities:). As for scaling EVERYTHING down, the subject is interesting (if off-topic). Different physical laws hold for different length scales. To take an extreme example, look at water: its behaviour on the scale of centimetres is well described by hydrodynamics, but if I measure it very accurately (ie at much shorter scales), I need quantum mechanics (and more complicated stuff). But again, I this depends on the ratio of the scale I am measuring to eg the interatomic distances (which do not change). So not everything scales there. Or think of ocean waves (which I suppose are gravity waves) versus waves in a bath (which depend on the depth, surface tension etc). They're not really scaled versions of each other, as you pointed out. But again in this case, not everything has been scaled (eg the surface tension of water would be the same in a salty bath as in the ocean, so it didn't scale). If we really do scale everything, do things change or not? Since scaling everything is merely a change of units, we must actually rephrase the question. And things begin to get interesting. But this is completely off-topic, and soon I'll be told off for showing off by a certain omniscient entity frequenting this newsgroup, so I'll just restrain myself :) |
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