About a week ago I posted a question, asking whether a lens + teleconverter yields the same depth of field as a "naked" lens of the equivalent focal length and aperture. I did not receive any satisfying answers, and was frustrated that I had not made myself clear, despite giving a specific example.
Well, I've located a forum in which this very matter is debated: http://www.robgalbraith.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=9&t=000076 In my judgment, the definitive posting is this one, from Don of DLC Photography; however wildlife photographer Moose Peterson took issue with it: OK, just received a reply from Tom Webster, who confirmed that as long as the image magnification and aperture are the same, then the dof will be the same regardless of how you got there. I gave him the example of using a 400/f2.8 lens with a 2x teleconverter relative to an 800/f5.6 lens (which isn't commercially available, but that doesn't effect the optics). With his permission, here is his reply to me: quote: Depth of field is dependent on image magnification and the f. stop used regardless of what method is used to obtain the image magnification and f. stop. An f. stop is the ratio between the focal length of the lens and the actual aperture (opening) in the lens diaphragm. A 400mm f. 2.8 lens has an aperture 142.9mm in diameter (400mm/2.8 = 142.9mm). An 800 f. 56 lens has an aperture of 142.9mm (800mm/5.6 = 142.9mm). When you attach a 2X teleconverter to a 400mm f. 2.8 lens the focal length is changed to 800mm but the aperture, the actual opening in the diaphragm, has not changed. The aperture is still 142.9mm which yields a final f. stop of f. 5.6 at 800mm (800mm/142.9mm = 5.6). Now, both lenses have the same image magnification and the same sized aperture and, by optical law, both lenses will yield the same depth of field at f. 56. ..As long as image magnification and the f. stop remain the same, depth of field will be the same. Add enough extension (!) to an 800mm lens to yield 1:1 on a piece of film and set that lens to f. 5.6 then rack out a 90mm macro lens to yield 1:1 and set the macro lens to f. 5.6, believe it or not, you will still end up with the same depth of field because the f. stop and image magnification are the same. [EMAIL PROTECTED] -------------------------------------------------------------------- mail2web - Check your email from the web at http://mail2web.com/ . - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
