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So my friend asked to see my lens, and the idiot I am, I let him. Before he fully got his hands on it, he touched the rear part of the lens (the mount) he touched the glass and left a thumb print. I quickly got my pen from my camera bag and started cleaning it. I'm quite new to DSLRs and I'm a student so buying a DSLR left a huge dent in my wallet. I'm just really worried if any problems might occur due to my friend touching the rear side of the glass.

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    This lens rentals blog post about image artefacts introduced by some "minor scratches" on the front element might be an interesting read.
    – null
    Oct 3, 2016 at 21:14
  • @null I almost referred to that myself, but I suspect rear element damage would have a more significant effect.
    – Philip Kendall ♦
    Oct 3, 2016 at 21:22
  • @PhilipKendall I thought the same and left it only as a comment. It's still relevant to the underlying, more general question how much abuse gear can take and that image quality degradation from them can have unexpectedly low amounts.
    – null
    Oct 3, 2016 at 21:30
  • Just a note. People who know about cameras don't go asking people to open up their cameras to look at it, because that unnecessarily introduces risks. I'll never ask someone to remove their lens from their body just to look at it. I don't need to do that at all. I talk about the camera, how are the pictures? is it heavy? What do you like? etc. I don't go touching their stuff.
    – Nelson
    Oct 4, 2016 at 2:33
  • I stopped someone trying to stick their fingers into the sensors after I showed them the body with the lens removed. I have learned since then to simply not entertain these curiosities. They can go screw around with display models.
    – Nelson
    Oct 4, 2016 at 2:35

4 Answers 4

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Don't worry about it. No, really, don't worry about it, and certainly don't panic. All that's going to happen if somebody touches a lens's glass is that it will get a bit of oil and muck on it, and that can trivially be cleaned off as you've already done.

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  • So touching the rear part of the lens wont do any bad? If thats the case, thats a relief. Its a new lens so i'm paranoid as hell. What about touching the metal contacts,
    – Kabir Ahmad
    Oct 3, 2016 at 21:04
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    Touching the metal contacts should have no effect. You twist the lens as you put it on. Which effectively cleans the contacts.
    – DeMorcan
    Oct 3, 2016 at 21:28
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    @KabirAhmad Skin oils will corrode metal contacts over time, but can be easily cleaned off (just use a bit of microfiber cloth).
    – Crashworks
    Oct 4, 2016 at 8:39
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The smudge diminishes general picture quality (adds some fuzziness) until you clean it away.

If you get scratches, the same effect happens, just it is then permanent (as you can't clean scratches away).

Probably the worst that can happen is that you scratch the lens while trying to clean it; and that means microscratches, not big visible ones. Be sure to use the right kind of cleaning material (I think those microfiber clothes they give you with new lenses should be good but I don't know for sure). Clearly, no paper towel or such! They do scratch the surface; invisible to your eye, but quality-reducing nonetheless.

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  • Hey, I used the cleaning pen whoch has a brush on one side and some flat surface thing on the other. It cleaned all the smudges on the lens.
    – Kabir Ahmad
    Oct 3, 2016 at 21:52
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What damage could be caused by touching the rear part of a lens?

What could happen is that a bit of grit stuck to a grubby finger could scratch the rear element. But if you don't see any scratches after your recent experience, it's nothing to worry about.

Also, overly aggressive cleaning can wear away lens coatings over time, and the best way to avoid that is to avoid the need to clean the lens in the first place. So avoid touching the front or rear lens elements if you can, but don't freak out if/when it happens.

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  • Thank you for the comment :) I bought a UV filter after ur comment.
    – Kabir Ahmad
    Oct 3, 2016 at 21:49
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    Bad idea: that UV filter will almost certainly cause more quality degradation than the thumb print on the back of the lens would.
    – Philip Kendall ♦
    Oct 3, 2016 at 21:55
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    Oh no...its not a permanent quality supression. I guess Ill use it when im taking pictures at the beach or something where theres sand flying everywhere. Or more "robust" enviroments.
    – Kabir Ahmad
    Oct 3, 2016 at 21:58
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    @PhilipKendall <i>Citation Needed</i>
    – Carl Witthoft
    Oct 4, 2016 at 11:21
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The functions of lens coatings include reduction of glare and to address chromatic aberration. Anything that is not chemically inert (which is to say pretty much any liquid or solid that can touch the lens) interacts with those coatings on contact.

So, yes, cleaning the smudge promptly is essential to prevent it from becoming permanent in the coating. However, the best approach is to not let anything touch the coated glass at all.

Obviously the front of the lens is similarly coated, which is why it is good practice to keep a UV filter on it. When that filter gets scratched or scuffed, replace it with the spare you keep in back-stock for exactly that occasion rather than going without until you get a new one.

Updating to address concerns in comment on another question
Unless a lens filters out or completely corrects for UV light, chromatic aberration gives it a slightly different focal point than visible light for which the lens is corrected. If the film or sensor is reactive to it, the result is out-of-focus UV degrades the image. A UV Filter with high quality glass is helpful in those conditions.

However, given a lens in which UV is completely corrected or the film/sensor is not sensitive to it then a naked lens will produce the best possible image. I have generally not found consumer- or even most prosumer-grade systems capable of rendering with sufficient resolution that a UV filter degrades the image. Even then all the other factors such as hand-holding, long focal length and shutter speed generally eclipse any degradation from a good quality UV filter. This has especially been so for me when using small format film and almost all modern sensors but that's just one person's experience.

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    The biggest image quality hit from flat filters on the front of the lens are reflections and ghosting. How much they affect the image depends in large part upon the type of scene being photographed. Any scene with both very dark and very bright areas (such as a cityscape at night) are nearly impossible to shoot with a flat filter in place without getting reflections. For more, please see: photo.stackexchange.com/questions/35052/…
    – Michael C
    Oct 4, 2016 at 16:17
  • Excellent point. There's a lot of texture in the decision space and plenty of cases in which naked glass is better. I realize it's a bit of a religious debate and I'm in the camp that says in the majority of cases a quality disposable piece of glass in front helps more than it hurts so that's my baseline. By the time someone is into the technical shoots that can resolve the difference, or with that much contrast, they generally have the background to understand explanations of when to deviate from the baseline.
    – T.Rob
    Oct 4, 2016 at 16:40
  • I'm more in the camp that bare glass is best unless there's an awful compelling reason why it needs to be protected. Hoods are my first line of defense against both flare and impact damage. The problem I have with most disposable protection is that it is much easier to damage than the front element of most lenses. And in some cases a very high quality filter can cost more than a front element replacement.
    – Michael C
    Oct 4, 2016 at 23:08

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