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Bryan

Scratch on lens rear element

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Mortified to discover a small scratch on the rear element of my Rokinon 12mm lens. I always try to keep the lens caps on when its not in use, but the rear cap is a slack fit and occasionally falls off. This, possibly combined with a forgotten coin in my pocket could have done the damage.

 

I need to check the performance of the lens thoroughly, but wonder if anyone has experience of this kind of damage and whether it is destructive to image quality?

 

Further, assuming that the IQ is impaired, is it worth trying to polish the defect out, and, if so, how?

Edited by Bryan

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Bummer, but I've never noticed that scratches on the front or rear elements make any difference to image IQ.

 

Forgive me if you've seen this article before. I posted it a few years back.

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Several of my lenses, particularly the older ones, are scuffed and have other defects. I haven't noticed any degradation in image quality except maybe when I shoot directly into the light.

Years ago I felt  there was some degradation with a Sigma lens that developed internal dust. I replaced it with a Nikon and felt the images were significantly sharper.

 

Since lenses are coated I'd take it into the shop, see what they can do and get an estimate rather than attempt to do anything myself.

There are various pens that are supposed to help but there's no guarantee.

 

Here's what Petapixel has to say about it.

 

https://petapixel.com/2015/07/02/how-much-does-a-scratch-affect-the-quality-of-a-lens/

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The back is better than the front, presumably. I've heard tell that if it does have an effect then black paint will minimise it. You've had it for resale value, so nothing to lose. Enamel and a tiny brush and a steady hand would be the order of the day.

But I would certainly be doing a few worst-case flare tests. It may not do very much.

 

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My Fuji XF18-55 focussed the finger marks on the rear glass (not actually a lens element). When I mentioned it as a warning on the forum David Kilpatrick cited instances he had experienced of marks on front or rear elements being within the depth of field (or is it focus) and visible on the image. As I recall, and would expect, it was particulartly an issue with wide angles. So sadly Bryan, a scratch on a 12mm might be visible in some cases especially with small apertures and some distances (close up?). Or it might just show up as flare in some circumstances.

 

That said in my early impecunious days I used a 35mm slr with a 50mm f1.8 lens with a chip (~1mm in diameter) out of the front element with no real problems. I remember, many years ago, some (video camera?) lenses could focus on an insect on the front element.

 

I guess it is a case of try it and see, perhaps you will be lucky or be able to work around the occasional issue. It may depend on the particular construction of the lens.

 

Good luck.

Edited by Martin P Wilson

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In my past career as a camera assistant, on a documentary I quickly mounted a 12mm distagon lens, the rear element stood proud and I just nicked it on the camera mount, it caused a series of cracks not scratches, we used a black sharpie and drew on the cracks, no one could tell when watching the rushes. the repair cost £2000.00, and that was in the late 80's. I learn't my lesson, never rush no matter who gives you the hurry up. ( by the way that not the reason why I'm not a camera assistant anymore) 

Edited by Kelv
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5 hours ago, Bryan said:

Mortified to discover a small scratch on the rear element of my Rokinon 12mm lens. I always try to keep the lens caps on when its not in use, but the rear cap is a slack fit and occasionally falls off. This, possibly combined with a forgotten coin in my pocket could have done the damage.

 

I need to check the performance of the lens thoroughly, but wonder if anyone has experience of this kind of damage and whether it is destructive to image quality?

 

Further, assuming that the IQ is impaired, is it worth trying to polish the defect out, and, if so, how?

It depends on how big the scratch and where it is on the element. The closer to the center, and the bigger the imperfection, the more degradation to the image. As someone who works on cameras and lenses for a living, I can tell you that putting anything on the rear element, as seems to be recommended by a few people posting here, will not help the situation in any way, and in most cases make it worse. The reason a person would put something like black or varnish on a scratch on a lens element, is to reduce flare, and this is way more important on a front element than on a rear element. However, a scratch, or anything on a rear element of a wide-angle lens will degrade sharpness much more than a scratch of the same size on the front element would.

 

As far as polishing the scratch out, no. Do some tests on the lens. If it seems sharp enough to your satisfaction, don't worry about it and keep on shooting. You should also test for flare if you regularly shoot into light or the sun. If it seems too soft or the flare is degrading the image, either replace the lens or have a specialist replace the rear element.

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10 hours ago, Martin B said:

 I can tell you that putting anything on the rear element, as seems to be recommended by a few people posting here, will not help the situation in any way, and in most cases make it worse. The reason a person would put something like black or varnish on a scratch on a lens element, is to reduce flare, and this is way more important on a front element than on a rear element. However, a scratch, or anything on a rear element of a wide-angle lens will degrade sharpness much more than a scratch of the same size on the front element would.

 

 

Hi Martin, I agree with you, the reason we put the black pen over the cracks was to reduce flare, we had no alternative we were in Petra and no lens hire houses around, and it worked in this instance, but I would not suggest it as a long term method.

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Thanks for your thoughts folks, I'll do some thorough testing and, if necessary, get a quote from a qualified technician.

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