Hallo Timothy! The focussing plane of a non-macro lens is not flat, it is spherical (if spherical glass elementsa are used). Therefore, if you keep the center at focus,the corner will go out of focus and if you keep the corner in focus, the center will go out of focus, in general. Our eyes can not see that happening because the radius of 'circle of confusion' which creates the image is much smaller than the resolution of our eye. However, Stopping down increases the depth of field and you got good results at f/8 with the TC on. Now, when you are using the extension tubes, the depth of field is affected more. How much?? Well, the answer to this question is a nomogram that I have scanned from a book on Macro photography which gives the depth of field when a certain length of extension is added to the 50 mm focal length lens.
I can not send that nomogram as an attachment file as most of the pdml members will object. Therefore, I shall send you only in a separate mail. Hope this helps. Cheers, Ayash. On Wed, 23 Jan 2002, Timothy Sherburne wrote: > Hello all... > 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 .
