Cleaning the mirror?

Edit: Got the answer. Sending it in.

The mirror is very sensitive. Sadly, fingerprint oils maybe be able damage them over time (less so with aluminum mirrors).

There is a critical section for your mirror - the partially transparent section used for the metering/focus mechanism. If your smudge is there, it could be impacting the camera electronics.

However, for what its worth, I have successfully jerry-rigged a mirror on my 5Dc in the field with a piece of double sided tape when it FELL OUT. I used a dry cloth to polish off any oils from handling (mainly the edges). Everything worked fine for the rest of the day. After the shoot, sent it back to Canon who fixed it for free (years out of warranty, though there was a recell for this very issue likely made it more of slam dunk).

The mirror does flip up when an exposure is made, so any marks on it will NOT show up on you photos. The mirror in an SLR/DSLR is usually front-surface silvered, meaning that you can very easily damage/scratch it if you just wipe it down. Perhaps a gentle wet cleaning, as is done when cleaning the sensor would be safe, but be aware of the potential for damaging it.

Is this something that Canon will "fix" if I send it in?

Yeah, but they'll likely charge you.

I think your best option is to send it to a professional camera tech for a strip down and clean. As others have said, the mirror is very sensitive and easily scratched. It's all very delicate and cramped in there too - if you have no experience and inappropriate tools, you'll probably end up making it worse...

Mistabernie wrote in post #14108501
Yeah, but they'll likely charge you.

A lot or what? I have no problems paying a small fee now to prevent further damage which may result in the replacement of the mirror.