I'm not a techie, but have wondered about an issue. I've seen a little smudge on the mirror of one of my cameras. I'm not sure how AF works, but doesn't light go through tiny spots where the mirror is not mirrored? Everyone seems to be terrified to clean mirrors because it seems possible to clean the mirror off of the glass? I've cleaned sensors before. That's child's play. But mirrors?
So I briefly looked online, and basically people could not even agree what the mirror is mirrored with. Silver? Aluminum? Other?
For cleaning the sensor I used specially made lint free swabs and quick to evaporate methanol. I highly recommend cleaning you sensor (following the right procedures, and if it is dirty. You can test for that).
Here's the thing. If the guy at the canon shop can clean the mirror, why can't I? I just need to know the procedure and materials. Surely, somebody must know?!
It's too risky don't do it, the mirror box has too many features like AF sensors that can be easily damaged, I wouldn't do it, nothing will appear in the final photos.
My thought was if the center point on the mirror was smudged it would slow down the focus, because I usually just use the middle focus point anyways. So it would be one swipe on a plastic paddle with a lint free Peck Pad with methanol, or whatever is recommended. I don't care if there is a little dirt toward the edges.
But I won't do it unless someone says that an ordinary human can do it, and not only a skilled camera technician!
Quote:I'm not a techie, but have wondered about an issue. I've seen a little smudge on the mirror of one of my cameras. I'm not sure how AF works, but doesn't light go through tiny spots where the mirror is not mirrored? Everyone seems to be terrified to clean mirrors because it seems possible to clean the mirror off of the glass? I've cleaned sensors before. That's child's play. But mirrors?
So I briefly looked online, and basically people could not even agree what the mirror is mirrored with. Silver? Aluminum? Other?
For cleaning the sensor I used specially made lint free swabs and quick to evaporate methanol. I highly recommend cleaning you sensor (following the right procedures, and if it is dirty. You can test for that).
Here's the thing. If the guy at the canon shop can clean the mirror, why can't I? I just need to know the procedure and materials. Surely, somebody must know?!
No, the mirror does not have "tiny spots where the mirror is not mirrored". It is a little bit translucent a mirror, instead. And behind the main mirror is a secondary mirror, which directs the light that passes through the main mirror down towards the AF module on the bottom of the mirror box.
You can have dirt specs on the mirror, remove these gently with a very soft lens brush. If you have a real "smudge", then you have touched the mirror with a finger, which is not something to be doing.
To remove such a smudge, use a good sensor swab. These are soft and use a "right" kind of alcohol. Apply almost no pressure when wiping the mirror.
A smudgy mirror will not slow down AF, but it may make AF less reliable (must be a very big smudge). So.. stop putting fingers on the mirror 
A poster on the Dpreview stated that while at a service center, he witnessed a technician cleaning a DSLR mirror with a sensor paddle and a "fluid" .. to all intents and purposes he was also an "actual human being" at least outwardly! 
As to what metal is used is difficult to find out, some say aluminum some silver, early mirrors were coated with an amalgam of tin and mercury, it could well be a more modern amalgam to make it more resistant to corrosive air attack, the mirrored side being exposed to the air.
The danger is the semi-mirrored area which is thin to the point of being translucent.......but......
.... wait, Yay!.. I have a dead donor Samyang GX10 which is no use to me.......... I will clean it's mirror (gently) and let you know the result.....
.....however, I'm not sure if the mirror.......... is species sensitive!
The point with the mirror in a DSLR is, it has an uncoated/unsealed surface.
The mirror in your bathroom has glass in front of the reflective surface ...
the DSLR-mirror has absolutely nothing in front.
Which metal is used does not matter much ... the reflective layer is very thin,
and the metalsurface is not hardened in any way ... even relatively soft materials
can scratch into it.
Rainer
Hi Arthur,
I promised to try cleaning the mirror of my now dead donor Samsung GX10 (equiv. to Pentax K10) .......
.......... I've just now dragged it out in it's half open box from under the settee, I have no further Pentax gear left including it's front camera cap.......thus the mirror was well and truly dirty.......it's also in the permanent up position...it's main board was plundered for another GX10.
So it's mirror was just made for this test, it was covered in "bits" and was grimy....an ideal candidate.....I used a "Reidl" wet clean kit which uses the usual paddles and refined alcohol.........I had to hold the mirror in the down position with a cocktail stick....paddle dosed, I started cleaning gently .....the bits were well stuck on and needed additional scrubbing to unstick them, after ten or so passes I examined the result through a decent magnifying glass.......all looked well, no visible scratches!.
With the mirror in the half down position the secondary mirror folds out...it's possible to see through the primary mirror holding up to a bright window and visa verser looking through the main one.
I then decided to be less gentle......and "scrubbed" back and forth fairly brutally to see if anything changed....after about twenty scrubs I looked again....no sign of any difference...except cleaner, the semi translucent center part of the main mirror has a rectangle which is quite a significant portion of the whole and all looked much the same. The view through the finder looked as it always did, a semi brutal test!
So, the "call is yours" Arthur.........IMHO a quick pass with a clean proper paddle with the appropriate alcohol would do little harm.....who knows what "smear" you have on your mirror? It's finger-prints with their acid content that risks the most on an unprotected metalized surface.
If it was me I would just do it....but then most here would have guessed that! 
PS. It may be worth a quick clean with a rocket blower to the under-mirror to remove any dust lurking there and also on the AF prism........(I don't know Canon's system)
Go well!
Thanks, to all! But especially Dave's whose adventures with camera and lens parts are the stuff of legends. So far I have heard two people who believe cleaning the mirror does not involve magic, simply a blower, a padded paddle, and the correct solvent. In my mind the trickiest part will be supporting the mirror, and being very gentle, as it looks very thin.
I'm still just assuming that the sensor cleaner is what could be used on the mirror because it evaporates so quickly, and presumably is very pure.
My experience cleaning the sensor on one of my Canons was anti-climatic. It actually has a filter over the sensor which is becoming less universal the higher the sensor resolution is. And the results were dramatic! You could actually see artifacts in images that were taken at high apertures before, and nothing after. Ah, it appears I have some Eclipse sensor cleaning fluid on hand. I won't be proceeding for a couple of days at least, because, err, I still need my camera to work for a few days or more. But when the time comes I will probably go for it, unless the smudge magically disappears.
Interestingly the Canon 5D Mark II seems to focus the best in live view. But I think that is true of all cameras where your subject is not moving much. Not necessarily the fastest, but the more accurate focus. But I digress. I only mention it because the AF speed is not as fast as expected...in difficult lighting situations...I don't expect miracles...
Brightcolors: It's always interesting reading your comments for technical details, like about the mirror. Maybe it would be accuracy effected, and not speed, because diffraction could be occurring when hitting a translucent substance. If it were bad enough, maybe it would block some of the light going through causing low light performance? All guesswork, but it's always nice when a problem matches up with what you believe to be the cause. Anyway, I will hopefully learn a bit more during this planning phase!
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