Two days ago, I bought a lens adapter so I can use old Nikon Film lenses with my camera. But the night before that, I cleaned a 50mm prime lens myself. I disassembled it and reassembled. Yeah. I know about the do-not-disassenble-lenses-or-else-you-risk-getting-lens-elements-out-of-alignment but I still did go about disassembling it. The huge screws on the metal mount of the lens was very tempting.
I bring along my camera and the 50mm f/1.4 attached to it every time I go out so I can do some test shots. There arose a problem. I couldn't get focus at infinity. I suspect the adapter caused it so I had this unusual idea of partly unscrewing the rear element so it will be closer to the sensor so it can get focus at infinity. I did get infinity focus.
Pictures were fine after I reassembled the lens but after the "unscrewing" I started noticing some problems. I see yellow or purple fringing on out of focus parts of some pictures I take. It is most prominent in high contrast areas. I heard and saw of purple and yellow fringing but I never saw it happen at the same time - turn the focus ring and its yellow, turn it back then its blue.
I thought I did something wrong. I thought I got the lens out of alignment. I reviewed the images before I partly unscrewed the rear element and found the same problem. I just didn't notice it that time. Then I thought I got something out of alignment when I disassembled the lens.
So I did some research on what will happen if a lens is out of alignment. Then I heard of spherochromatism. Apparently, nothing is out of alignment. What I was seeing was spherochromatism which is very apparent in lenses at very large apertures.
Then I saw this article about aberrations. The Seven Deadly Aberrations
The problems discussed there are often the reasons for image refusals here. At least that's true for me. I hope that article will give more insight on many of the image defects that we encounter. It talks about lens construction and how lens are designed to prevent those problems.