Hello again! Thanks to Peter, Ayash, Kevin and Fred for their responses, which all came to basically the same conclusion: the A50/1.4 simply does not have a flat enough field of focus to work well for macro photography; the curvature of the lens elements are becoming more apparent as the depth of field decreases.
Okay, with that said, I believe the solution was to use a slower lens or stop down. Is it accurate to conclude that the slower the lens, the flatter focusing field? The 50/4 is flatter than the 50/2 is flatter than the 50/1.7, et cetera? t On 1/23/02 10:16 PM, Timothy Sherburne 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 .
