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.
