Archived posting to the Leica Users Group, 2003/04/25
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Hi > Forget about digital, for a moment. The concept of "effective focal length" > is kinda bogus. The focal length of a 50mm lens is 50mm. Period (to be > repetitive). Take a picture with a 50mm lens on a regular 35mm camera with > a 24x36 image frame, and crop it down to the field of a digital sensor, > what you have is a cropped image from a 50mm lens. Cropping does not change > the focal length, effective or otherwise. Now, do not move the camera > (assume it is on a tripod). Remove the 50mm lens and put on the 80mm lens > (or use a zoom lens :-)). You get an image with the same composition, 1.6x > (or so) larger -- but with reduced depth of field, based on my memory and > experience. Huh?? if u do not move the camera but change the lens to a longer focal length, yes of course you do get reduced DOF, but how on earth do u "get the same composition"???? i repeat...my premise is to use a 50mm lens on a digital SLR to get the same field of view as a 80mm lens on a film body. I understand that the focal length of a 50mm is 50mm. That's why i use the term effective focal length..... Perhaps Field of View is a better term than effective focal length. :) My argument has been to keep field of view and perspective constant for both film and digital shots, and i contend that the DOF would be the same for both shots. i'm not interested in comparing the same 50mm lens on both digital and film. > I have in the last few minutes tried an approximation to this experiment > using a Canon G3, and zooming instead of changing lenses. Guess what -- the > results validate the theory. Without moving the camera, the depth of field > at the longer focal length was dramatically less then that at the shorter > length, after adjusting the images to the same apparent size. Gee -- why am > I surprised? I was a mathematician! > > Try it yourself using your own camera and lenses. You will be convinced. but dun u think ur test is moot, since u did not move the camera? i mean, u're changing only one variable, focal length, of course the DOF changes! i don't really understand the objective of a test where the camera is not moved and u just zoom in and out to prove that DOF changes with focal length :) U mentioned "adjusting the images to the same apparent size" - i presume u're using Photoshop techniques. This is NOT the same as the premise i'm working on. boon hwee - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html