Mirror, Mirror, WTF?

Didn't quite know where to put this one. So here it is.

So, here is a pic of the wife getting a tattoo:

IMAGE LINK: http://www.flickr.com/​photos/tehgipster/5210​440975/

In this case it's a coincidence. Looks like the image in the mirror is the same distance from the camera (including the trip through the mirror) as her face or the guy's hands, whichever you focused on.

But in general, you can't easily focus on the surface of a mirror. It doesn't matter whether the light rays from an object travel straight to your camera lens or get bounced back through a mirror - the total distance is what matters.

If you want to autofocus on a mirror surface, put the focus point on the frame - otherwise there's nothing at the right distance that the AF sensor can identify.

-js

Go out to your car and focus on the mirror... now focus on what the mirror is reflecting. Your focus point in the shot just so happens to fit the focus distance of the object in the mirror.

If you look at yourself in the mirror, where does your reflection appear to be? Where the mirror happens to be standing/hanging, or twice the distance between yourself and the mirror?

Should give you your answer.

Gipetto wrote in post #11354100
...I guess I always assumed that the plane of the glass on a mirror is the focus point, not the actual object that its reflecting.

You assumed totally wrong. THink about it. If the plane of the mirror was the point of focus how could a SLR even work as both the viewfinder and the autofocus sensor use a mirror to bend the light.

Weird. Its also weird that this is the first time I've noticed this in 16 years of photography.
But, then, nobody has ever accused me of being observant

Well, this is how people get self-portraits from a mirror -- the camera focuses on the object in the mirror, not on the glass surface!

You can also get some interesting results by shooting through a window with reflections -- in this cas a broken window, the camera focusing on the interior but the reflections being pretty close to the same distance, and the glass surface lost because of a wide f/1.4 aperture:

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