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.
Photo credits: Denis Dryashkin.
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