[Arthur Macmillan]

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!