Archived posting to the Leica Users Group, 2004/02/16
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Jonathan: >Correct. For example suppose your 250mm lens when focussed slightly >short of a bright 1mm ball, casts a "circle of confusion" on the film of >roughly 3mm (i.e. it is slightly out of focus). Regardless of the size >of the film, the circle will be 3mm. But that's the circle of confusion on the negative (or sensor). If you contact printed the negative or had a 1:1 reproduction from the sensor, the COC will be 3mm on the final print. If, on the other hand, you need to enlarge it by 1.5x, the circles of confusion will be enlarged by the same amount. Usually, when you enlarge a print, the viewing distance increases. So even though the circles of confusion are larger, the increased viewing distance means that the apparent depth of field stays the same. But if you're enlarging just a portion of the negative, then the apparent depth of field decreases because viewing distance stays the same. Here's a more concrete example. If I use a 35mm lens at f2, I'll get a certain amount of DOF. If I use a 90mm lens at f2, I'll get a much shallower DOF. If I take the central part of a negative shot with a 35mm lens and blow it up so that it is the same size as a print made from a negative shot with the 90mm lens, the DOF of both will appear to be the same. The circles of confusion with the 35mm are smaller than those with the 90mm lens. But if I enlarge the negative just enough so that the prints are the same, I'll get "identical" prints in terms of perspective and DOF. (Not quite identical, but close enough for practical purposes.) It's camera-to-subject distance, f-stop, final magnification, and viewing distance that determine perspective and DOF. Eric - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html