> If from the same position you take two lenses of differing focal > lengths, at the same aperture and arrange to have the image printed at > the same magnification, the DOF will be identical. You are... wrong. See my post on the math (and a web reference). Further, I've done the tests. My observations agree with the math (and with the physics of optics). Regards, Bob. From: "Steve Hodges" <shodges@wantree.com.au> > Bob Blakely wrote: > > > > Another reason for changing focal length is to control depth of field (DOF). > > But it doesn't do that either :-) > > The only things that affect DOF are aperture and magnification. > > If from the same position you take two lenses of differing focal > lengths, at the same aperture and arrang to have the image printed at > the same magnification, the DOF will be identical. > > Again, this effect is simply caused by relative croping of the frame. > > I've done the maths, I've seen the images, I know it's true. > > The reason we change lenses is to change field of view or to change > magnification (they go hand in hand). The latter allows us to change > out position and retain an absolute size of the image of an object in > the viewfinder. It eases the task of changing perspective without the > image of the object growing or shrinking too much.
