On Sun, 03 Jul 2005 05:54:09, Frank Wajer wrote; > BTW. I will never ever touch an A, F or FA zoom lens, that must be > a nightmare.
Hi, I am no lens expert and not even sure if it would be appropriate to inject this remark in, but this thread reminded me of the lens axis discrepancy that was discussed in one of the Japanese lists which I summarize here. I also remember that the shimming of the glasses was discussed in this list several months ago. In assembling lenses, there are two different axis to be matched. These are optical and mechanical. Optical axis is inherent to the glasses when they were made. Mechanical axis would be the one concerning the assembling process. Ideally those two should match perfectly but they usually do not, and some tolerances are specified. These are different among the mfrs but I understand that Nikon and Pentax specify ±30μｍ while others (including Canon) specify ±50μｍ. Manufacturing tolerance of Glass OD and barrel ID have to precisely match. Also, each glass element has to be held precisely perpendicular to the barrel axis. In the past (i.e., good old days when cameras were crafted, not manufactured :-) when skilled craftsmen (women? :-) were doing this, they rotated each glass to find the best matching point but now it is automated. But the shimming is very important. Glass axis checking could only be made possible by a spot light source and laser light is used to check this. I was told that it was possible to see the mismatched axis by putting a spot light source. Incandescent lamp could simulate this but the best way is to use the laser pointer and check the reflection at each element which should at least look lined up. Even the glasses by reputable mfrs have some out of tolerance samples. Some people make it a habit to bring a laser pointer when they buy new lenses. I really do not know how effective it is to find defective lenses by this quick & dirty method but they say they sometimes find crooked pieces. If you find your photos consistently with certain corner (s) out of focus etc, you may have to doubt this tolerance (particularly after dropping etc). They also say that certain Russian lenses are terrible in this regard, and the misalignment is so visible. I am attaching a sample pic of light axis aligned up acceptably. http://ca.geocities.com/[EMAIL PROTECTED]/LensAlignment.html Not a good photo, but you get an idea :-). Cheers, Ken
