Scratched back element on 35/2. Implications?

I was looking closely at my new-to-me FD 35mm f/2 lens that I picked

up on eBay a while back. I got a good price on it, so I was not

expecting miracles. I noticed what I thought was a smudge on the

rear element, but looking closely it through a lupe turned out to be

fine scratches, where it looked like the convex element rubbed

against something. Any idea what implications this will have on

image quality? I haven't noticed anything so far, but I'm about to

go travelling in China/Vietnam, and I was hoping to use this as my

primary lens.

 

Thanks in advance for your answers.

I had a 24mm and a 20mm with scratches like you describe, and they did affect image quality - mostly when I was stopped down below f8 or so with hyperfocal distance set. In the slides there appeared an out-of-focus area of reduced contrast. For casual shooters wanting only 4x6 prints it may not have been an issue, but it shows up in anything much larger. I replaced the whole 24mm but was able to have only the rear element replaced on the 20.

 

On the other hand I have had a 50 and a 105mm with slight scratches/smudges that never seem to cause any problems. I suspect the longer the lens the less the scratches affect the image (?).

 

I would try some tests (especially at small apertures) and make sure you don't see any out-of-focus patches in your images before taking any important shots. Good luck.

Thanks, Stephen. I shot some transparencies last week using the lens. I'll get some scans or prints done from them, and see if I can detect any defects. That's interesting about problems showing up at f/8 or smaller.

To quote Robert Monaghan "The worse place for a lens flaw is at the rear of the lens, rather than at the front. A scratch at the rear will project contrast reducing light rays directly onto the film. A similar scratch at the front of the lens would have its impact reduced by the subsequent lens elements."

 

He goes on to say "A small scratch will increase tendency to flare and reduce contrast by scattering light to where it shouldn't be on the film. One simple solution advocated by some photographers is to carefully fill the scratch with dark black india ink. The idea is that it is better and less impact if you reduce the light scattered by the scratch, rather than just leave it."

 

Filling scratches with india ink sounds nice in theory but the application might be a bit tricky. I am sure I don't want to try it.