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John Francis writes:
Peter Hirons wrote: John Francis wrote: There is a lot more to the optics of a lens than just an arbitrarily chosen area to focus it onto. Depth of field being the main difference - it will not be the same if you put a 300mm on an F5 and a 200mm on a D100 (same apertures of course). Are you sure about that? Absolutely! Search the web for "depth of field calculation" and do some reading. Well, when you do find the time, I suggest that you might try to explain it - to yourself, for a start. Don't forget to take the multiplication factor during printing into account - that affects the size of the circle of confusion. you can't use the same size circle on the two cameras. That is correct - but it doesn't give the result you seem to think it does. Todd Walker has a very good illustrations of what really hapens on this page: http://www.toddwalker.net/doftest/ It shows two near identical shots taken with the same FOV and same aperture with a Canon G1 (4.8c crop) and a Canon 10D (1.6x crop). It is pretty obvious that the DOF in the tow images is not the same, but that the FOV are. Check your calculations, and be prepared for a surprise. Been there, done that (my calculations are based on the DOF formula in Allen R. Greenleaf: Photographic Optics, MacMillan, New York, 1950). The results are plotted in two graphs on this page: http://heim.ifi.uio.no/~gisle/photo/dof.html Graph 1 just confirms what is illustrated by Todd Walkers pair of images. This is actually a well known fact - and one of the reason people that want shallow DOF don't like "cropped" sensors. Now, graph 2 on the same page show that if we keep the focal length constant, DOF actually decreases when we move to smaller sensor diameters. That is caused by the effect of the magification factor you are talking about - but it has less effect than you think. -- - gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ] ================================================== ====================== When you say you live in the real world, which one are you referring to? |
#23
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John Francis writes:
Peter Hirons wrote: John Francis wrote: There is a lot more to the optics of a lens than just an arbitrarily chosen area to focus it onto. Depth of field being the main difference - it will not be the same if you put a 300mm on an F5 and a 200mm on a D100 (same apertures of course). Are you sure about that? Absolutely! Search the web for "depth of field calculation" and do some reading. Well, when you do find the time, I suggest that you might try to explain it - to yourself, for a start. Don't forget to take the multiplication factor during printing into account - that affects the size of the circle of confusion. you can't use the same size circle on the two cameras. That is correct - but it doesn't give the result you seem to think it does. Todd Walker has a very good illustrations of what really happens on this page: http://www.toddwalker.net/doftest/ It shows two near identical shots taken with the same FOV and same aperture with a Canon G1 (4.8x crop) and a Canon 10D (1.6x crop). It is pretty obvious that the DOF in the two images is not the same, but that the FOV is. Check your calculations, and be prepared for a surprise. Been there, done that. The results are plotted in two graphs on this webpage: http://heim.ifi.uio.no/~gisle/photo/dof.html Graph 1 just confirms what is illustrated by Todd Walkers pair of images. (This is actually a well known fact - and one of the reason people that want a shallow DOF for focus control don't like "cropped" sensors.) Now, graph 2 on the same page show that if we keep the focal length constant, DOF decreases when we move to smaller sensor diameters. That is caused by the effect of the magification factor you are talking about - but it has less effect than you think. -- - gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ] ================================================== ====================== When you say you live in the real world, which one are you referring to? |
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