I'm still trying to determine whether my problems with my new f/4.5 75 mm Grandagon-M are just a matter of learning how to use the lens or whether there is something wrong with it. My tests so far indicate that there may be some significant field curvature. But I have a couple of questions.
I've tried to locate information about the resolution I should expect from this lens. I've found some information at www.hevanet.com/cperez/testing.html#65mm_thru_125mm for the f/6.8 version of this lens. I don't know how applicable that would be. In addition, I wonder if one of the entries for that lens is a typo. The edge resolutions at f/11, 16, and 22 are given as 42, 24, and 38. I wonder if the 24 is actually supposed to be 42.
Is there any other source of information about lens tests for this lens?
I've also unscrewed the rear group to see if I could see anything. Badger had screwed it in finger tight, which is the way it should be. As I unscrewed I noticed some slight creaking as if it needed some oil on the thread. There was a small place on the last outer thread where it looked as if the paint was chipped off. But it was not hard to unscrew or screw, and there was no evidence of stripped thread. Is that normal? (Presumably one doesn't want to lubricate the threads in order to avoid buildup on the lens elements.)
Finally, several people had suggested there might be a mounting problem. That presumably wouldn't mean the mounting of the lens in the lens board. I suppose there could be a problem with crossed thread in the mounting of the front group in the shutter. Will there be any problem unscrewing the front group from the shutter to check?
I've lots of experience with disassembing and reassembling various things without problems, but I haven't fooled with my large format lenses yet. One thing I did manage to do successfully was to disassemble an old 180 mm Mamiya C3 lens to clean some of the internal surfaces, something I would never attempt with one of my large format lenses. But I do consider myself competent to do routine things of this nature. Still any additional advice would be helpful.
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