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Message: <blockquote data-quote="yauman" data-source="post: 364745" data-attributes="member: 15418">I have not seen this in a Nikon yet but we have a couple of old Canons with that problem.  In the shop we have a Canon 30D with that problem so we cannot sell it and I use it for testing use lenses and for show and tell in my DSLR class.There is a very thin piece of glass over the sensor to protect it (so when you clean, you are really cleaning that piece of glass and not the sensor!)  The glass is glued on with optical glue (optical glues are glue that's optically clear and with known index of reflection - all lens with multi-elements are assembled with this glue.)  What happens is that if the glue is defective or if the glue job left air bubble there will be a gap.  It's usually harmless but if moisture gets in and if you live in an environment where fungus can grow, a small spot will develop. After some time, that spot become bigger, usually not round, more like spidery.  No amount of cleaning can get rid of it as it's not on the upper surface.  The only cure is to replace the sensor assembly.  (For that use 30D it was not worth it - occasionally we screw up and bought a bad used camera which cannot be re-solded!!! )My understanding is that in some higher end cameras, that piece of glass which is also the anti-alias filter in some designs are not glued on but rather screwed on or spring-load latched on and thus can be removed for cleaning. (eg Canon 7D) The D7100 do not have an anti-alias filter so that glass over the sensor is just plain high quality optically clear glass and it's glued on.Like I said, I have not seen a Nikon with that problem yet, so I'm just speculating.  If I were you, I'll send it back to Nikon to have them clean it.  The process is very simple and safe.  Go to their support website, fill out the form; they'll send you a pdf file to print out the shipping label and that's it.  Takes about 1 week to 10 days turnaround.Hope this helps.</blockquote>

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