Archived posting to the Leica Users Group, 2000/11/11
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]M.E.Berube jotted down the following: > How would Printing to the same (or different) size effect DOF at all? DOF is perceived sharpness. The key word is perceived. There is only a single plane which is sharp in a picture and that is the plane which the lens was focussed on. Everything else is more or less unsharp. To calculate DOF, we allow for a certain size of the circles of confusion (CoC), i.e., those portions of the image where a single point is rendered as a slight blur (or circle), which is everything outside the plane of focus. The CoC is chosen so that in a "normal" enlargement, we cannot distinguish between those very small blurry circles, and points which are pin sharp. When you enlarge a negative, you are enlarging everything on that negative, including the CoCs. We may consider CoCs of 0.03mm on a 35mm negative to be sharp when enlarged to 8x10", but if we blow that same negative up to 20x24", we may see that some areas we considered sharp before (because the enlarged CoCs when *printed* where indistinguisable from sharp points in focus) are now infact slightly blurred. We consider that blurred area to be outside the DOF (since it is blurry, right?) Note that when your talking about the CoCs, it's the CoCs in the *negative* that are interesting. But since these are enlarged along with everything else in the negative when the negative is projected down onto paper, print size does affect which portions of the shot we consider to be large. Hence, enlargment affects DOF. In most discussions on DOF, the size of the CoCs and the size of the print are assumed be standardized and typically don't enter into the discussion. But if you start decomposing the DOF concept, you realize that it is just a calculated property: there is nothing in the world that determines DOF as an independent, objective entity. You can change the DOF for any lens without without touching it, simply by using a larger or smaller CoC value in your calculations. You can change the zone of perceived sharpness in any negative by greater or smaller enlargement. The DOF markings on your lens just reflect the values the manufacturer plugged into the equations, nothing more, nothing less. M. - -- Martin Howard | Harrisburg '79 Visiting Scholar, CSEL, OSU | Chernobyl '86 email: howard.390@osu.edu | Windows '98 www: http://mvhoward.i.am/ +---------------------------------------