If you have the convenience package on the S4 it has auto-dimming mirrors.  What this handy feature does is tint all the rearview mirrors to keep headlights behind the car from blinding you.  Sure, it's a simple enough to reach up and flick the little day/night tab to protect your vision but this is a decadent feature I've become very accustomed to.  When I drive a car without this feature it really bugs me (I'm such a weenie).  The time from normal to full dim cycle is usually around 11 seconds and appears to slow slightly as ambient temperatures drop.  Of course, if you're using the automatic climate control this is nearly a moot point.  I'm not privy to just how the dimming of the glass works... perhaps some day I'll stumble across this info.

There are 2 sensors that decide when to dim the mirrors.  One on the back of the mirror (facing the windshield) senses if it's night or day and one that faces rearward which looks for headlights.  

I've heard some people complain about the mirrors.  If you don't like the auto-dimming you can simply put a small piece of black electrical tape across the headlight sensor and it will never dim again.  Downside, of course, is then there is no way to deal with headlights from behind.  

I have tinted windows on my S4 (35% legal tint).  The tint does not impair the dimming function of the mirrors--they still work as expected. 

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