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Front tilt loses middle
Leonard Evens wrote: Tom Phillips wrote: One might point out that circles of confusion are those points of non-critical focus which lie either in front of or behind the film plane, i.e., object or image points which are not in critical focus. What is in focus on the actual film plane is a point, not a circle. I think this is misleading. The circles of confusion are in the film plane. They arise when images points come to exact focus either in front or in back of the film plane. Yes. I meant this. I perhaps should have said "circles of confusion "arise" from those points of non-critical focus which lie either in front of or behind the film plane." But I understood you to be saying from your eariler post that everything on the film plane was a circle of confusion, since you did not mention that what is in critical focus on the film plane is either not a circle, or is small enough to be considered a point. You then consider a cone with vertex at the point of focus and base the exit pupil of the lens. That cone intersects the film plane in a region. When that region is small enough, you can't distinguish it from a point. Deciding just when that happens is the basis of DOF calculations. Thus when one tilts or swings one is bringing into focus objects beyond the plane of critical focus. So, the reason one uses circle of confusion size in determining DOF is to determine the extent of non-critical focus one wants to _appear_ in focus at a given print size. You have the general idea, but I think you haven't quite visualized the geometry. After 30 years, I think I do this well enough... Also, circles of confusion are used in the analysis whether or not you tilt or swing. I never said otherwise. In fact, I attempting to point out that even though you can tilt or swing to bring objects (not in critical focus) into focus on the film plane you still need to calculate DOF... When the lens plane is parallel to the film plane, if you assume the exit pupil is a circle, the blurry regions in the film plane described above are circles. If the lens plane is tilted with respect to the film plane, they are ellipses whose exact shape and orientation depend of position in the field. But usually they may be approximated by circles, and that is good enough in almost all situations for practical large format photography. Well, we don't and never have called them "ellipses of confusion." Perhaps we should just recognize descriptive language and communication, even when sometimes related to scientifically inclined technical jargon, is an inexact science. Plus people make mistakes... |
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