Aberrations


posted on 21st of november, 2012

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.

Comments (5)

Posted by Debramillet on November 24, 2012
Great article, learned alot, Thanks!
Posted by Leakeem on November 22, 2012
Thanks for sharing this info! You really had some nerves to clean the lens by yourself! :)

It's a prime lens so I guess that's why it is easy. I wouldn't dare do that again on zoom and AF lenses.
Posted by Leakeem on November 22, 2012
Thanks for sharing, good to know. Off-topic: there is an urban legend about some lenses which have inside a $1 silver coin. Did you find one?

Nope. No $1 coin. but I did find a disturbingly large amount of dirt. It was hard to clean.
Posted by Thanatonautii on November 22, 2012
Thanks for sharing this info! You really had some nerves to clean the lens by yourself! :)
Posted by Alvera on November 22, 2012
Thanks for sharing, good to know. Off-topic: there is an urban legend about some lenses which have inside a $1 silver coin. Did you find one?



Comments (5)

This article has been read 459 times. 2 readers have found this article useful.
Photo credits: Denis Dryashkin.

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(Leakeem)
Mandaluyong City, PH

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