I almost argued with John on this. But upon reflection, he is correct, the reason for that is simply that the shorter lens has a smaller diameter aperture. As I have mentioned several times on the list, when everything is factored out the only thing that affects DOF is magnification and aperture diameter. In this case if the final image is the same size the magnifications are the same and thus the only thing affecting DOF is the diameter of the aperture.
For those who wonder about this, many formulas simply call "aperture diameter" aperture and folks think that means f-stop. It does not! Aperture is the lens opening, f-stop is the focal length divided by the aperture. -- graywolf http://www.graywolfphoto.com http://webpages.charter.net/graywolf "Idiot Proof" <==> "Expert Proof" ----------------------------------- John Francis wrote: > On Sat, Sep 16, 2006 at 01:28:26PM -0400, Mark Roberts wrote: >> Ryan Brooks wrote: >> >>> Adam Maas wrote: >>>> At f2.4, the 70 is about perfect for me. Essentially the same length and >>>> speed as the legendary Nikon 105 f2.5, which is a superb portrait lens. >>>> >>> You are getting more DOF with the 70mm though. >> Not true. Not at the same subject magnification, anyway. (And that's >> what you'll be doing if you compare both lenses as portrait lenses.) > > Actually, Mark, Ryan is right. > > A portrait taken with a 105mm lens at f2.8 on a full-frame camera will > have a shallower depth of field than the same portrait (taken from the > same spot and enlarged to the same size) made with a 70mm lens at f2.8 > on a body with a 1.5x crop factor. > You'd need a 70mm/f2 to get a DOF that matches a full-frame 105mm/f2.8 > > -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
