Focussing on a mirror image

When I take photographs of a subject reflected by a surface (water, mirror, glass) I notice that the focussing distance is not the distance from my camera to the surface, but mostly it is close to infinity.

For example: From a distance of 1 meter I take a photo of a reflection in a small water puddle. Autofocus sets the focus to infinity and not to ~1 meter.

I recall from basic physics that mirror images are "virtual" images of the real object, but I am not sure anymore how that works. I would be glad if someone could explain why I have to focus on infinity and not on the reflective surface itself.

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An easy way of thinking of this is focusing on a distant object, and then placing a piece of glass between you and that object. You don't now focus on that piece of glass, which is closer, as the object you were focussed on would be blurred. All the reflective surface does is change the angle you are looking at the object, not it's distance, and with clear glass in front of you the angle is (largely) unchanged. – Dreamager Jun 27 '12 at 22:37

Simple: You are focusing on the reflected subject, not the reflective surface.

Ok, I'm not good at explaining this sort of stuff, I just understand how it works, but here's a drawing

You see, when you are focusing on a subject, it's a reflection on the reflective surface, but the subject it's not there, it's further away, to explain better, lets say the reflective surface is 1 meter away from the camera and the subject is 1 meter away from the reflective surface, the subject it's actually 2 meters away from the camera, and thats where you are focusing. I hope I explained myself.

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My question was not how to focus on a reflection, but why I have to focus on the reflected object and not on the surface. – Saaru Lindestøkke Jun 28 '12 at 8:09
because of this: i.imgur.com/7kSAf.jpg sorry, not great drawing techniques – philberndt Jun 28 '12 at 10:47
That's the sort of drawing I was looking for (of course a bit more, eh, with straight lines). Yes, it makes sense, if you would include this in your answer and elaborate on it a bit this would be my answer. – Saaru Lindestøkke Jun 28 '12 at 11:02

The focus distance is the distance to the object via the reflecting surface.

Try taking a photo of yourself in a mirror at various distances, the distance from the camera to you, via the mirror, is twice the distance to the mirror. Your camera will indicate the focus distance as that.

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As a intersting side note sortof related, when you look at youself in a mirror, the the image width is always twice the mirror width where you are, no matter what distance you are from the mirror. – Olin Lathrop Jun 27 '12 at 23:53

You are focusing on objects, which are reflected. So you are not focusing on reflected surface. You are interested in light rays, which go from object "through reflection" to your camera. Not only from reflective surface to camera.

You can try to shot for example puddle - try to focus on ground and you will se that reflection is blurred. Then try to focus on reflected objects and ground will be blurred.

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