Archived posting to the Leica Users Group, 2003/05/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]All 90mm lenses, basically, produce the identical depth of field. If one 90mm lens has more contrast and higher resolution numbers, and the ability of recording fine detail better than the others, it will "appear" to have LESS DOF than the other lenses. This is because of the visual sharpness difference between the sharp and unsharp areas. There is an abrupt change between the very sharp and the less sharp areas. This is also why lens manufacturers say that to fully utilize the DOF scales on modern lenses, use it as if you were shooting two stops wider, ie; shoot at f/16, read the scale for f/8. The scales were computed back when lenses were not so sharp and film was not nearly as good as today's lenses and film. If your lens exhibits lower contrast, less fine detail recorded, etc, then the difference between the sharp and unsharp areas is less apparent thus giving the "impression" of greater DOF. It is a smoother transition between the sharp image parts and the less sharp image parts. This is a very well known fact and is why many people think that old lenses have greater DOF than new lenses. It's not true, it's simply the visual difference (sharpness contrast) between the sharp and unsharp areas. Jim PS... Depth Of Field is what you see in the photograph. Depth Of Focus is at the film plane and, even though (like depth of field) is dependent upon the aperture, is a different subject. At 09:06 AM 5/7/2003 -0700, Jerry Lehrer wrote: >Don > >In this regard, I wish that someone could explain why and HOW >different lenses with the same aperture and focal length can produce >different depths of focus. How can the laws of optics be ignored? > >Get as technical as you need to. I can handle it, but don't resort to >mysticism, please. > >Jerry - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html