Actually there's no obscure laws of physics here. Most general purpose lenses do not have a truly flat field of focus. This doesn't matter much when you're working at normal distances where depth of field and the fact that most objects are three dimensional tend to obscure that fact. When you are working at macro distances depth of field becomes so narrow that the curvature in the plane of focus becomes an issue. You can possibly eliminate this problem by stopping down to f11 ~ f22. The macro is designed to give a much flatter plane of focus so you shouldn't see quite so pronounced an effect.
The only effect the tubes are having is to allow you to focus to a much closer distance than normal. At 10:16 PM 1/23/2002 -0800, you wrote: >Hello all... > >I've got a question regarding macro photography. First, some background. >I've been experimenting with three different techniques for increasing >magnification: using a M50/4 Macro, a set of Vivitar auto extension tubes, >and a Vivitar Macro Focusing 2x TC. I use a A50/1.4 with the tubes and TC. >My target magnification is 1/2x (1:2) and my subject is a flat document (a >10 DM note, to be precise). I've placed a sheet of float glass over the top >of the note to ensure flatness. Film varies with whatever happens to be in >my ZX-M at the moment. Aperture for these tests is always f8. The camera is >tripod mounted and I've done my best to make sure the front lens element is >parallel to the subject. > >Now for the question: I get good results with the M50/4 Macro and great >results with the Vivitar TC + A50/1.4, but something strange is happening >with the tubes. I'm using the 20mm tube with the A50/1.4 to get ~1/2x >(actually 0.4x) and I cannot get the subject entirely into focus! When the >center is in focus, the corners are soft and when the corners are in focus, >the center is soft. I can stop down to f22 and use the DOF preview and >everything looks good, but that really isn't a fair comparison with the >other setups, is it. I don't see the same problem using the other equipment. > >So what is it about the tubes? Are there some obscure laws of physics at >work here? > >t >- >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 . - 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 .
