Originally posted by philippe The trouble with disassembling lenses is the reassembling!
In order to align the different elements, one does need a collimator.
When not perfectly aligned, the focus point will be not equal over the whole image field resulting in partial unsharpness, if not over the whole image, and/or the image might be deformed!
The smaller the lens the more delicate it is.
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Then take care to orient the elements in the right direction on the optical axis...
Keep everything as clean as possible and apply oil and grease thriftily!
Good luck!
You are very right to point out the problems with collimation. In practice, though there are several things, which come to our help. Firstly, lens groups in old lenses are often marked with pencil strokes, to indicate the correct rotational position of one lens to the other. It is important to preserve and use these marks, when they are present. If there are no marks, simply make your own with a soft pencil. (Pencil is used, because it will withstand a lot of typical cleaning fluids (but no rubbing of course) and because it won't damage the coatings or glass surfaces as it does not contain hazardous chemicals (solvents etc.)
Secondly, camera lenses are all small (comparatively) and held in place quite well by centering rings and retainers, which avoid a gross assembly mistake. I find it much harder to be sure of the correct direction of the lens surfaces, than get a workable centering. Also, critical lens groups are often cemented and fungus usually doesn't spread through the glue. If it does a repair is only economical with very expensive lenses or if one simply wants to investigate how to re-cement a lens after cleaning. It is possible, but surely much more ambitious, than working with the complete groups.
And thirdly, collimation is less critical in older lenses with typically all spherical surfaces and fewer lenses. Modern lenses with sometimes several aspherical elements and loads of tiny corrector elements pose more of a problem.
I have taken apart, cleaned off fungus and reassembled successfully a few lenses and all were in perfect working order after the process. I cannot rule out, that I lost the last bit of collimation, but at least I could avoid serious and visible mistakes. It just needs time, care, a clutter free workspace and some application of common sense.
Ben