Why is the focus distance for a mirror image further than the surface of the mirror itself?

Question

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.

Answer 1

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.

Answer 2

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

You can shoot a puddle, for example - focus on the ground and you will see that reflection is blurred. Then focus on reflected objects and the ground will be blurred.

Answer 3

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.

Answer 4

Yes, you either get the lake (or mirror, with its beautiful carved wood frame !)) in focus, or the reflected clouds at infinity. If you want the ducks on the lake and the beautiful reflection of the clouds in focus together you need to do a hyperfocal length calculation and stop right down ie f/16 or similar. Its the different angle the light rays take when they travel from the image or the reflective surface to the retina.

Answer 5

Take out your shiny smartphone and hold it at arm's length. Stare at the screen. Now allow your eyes to focus on your reflected image. Then focus back on the screen. Your eyes are performing the same job as your auto-focus on your camera. But it's more obvious when you do it yourself. As others have pointed out, the reflected image is double the distance from the reflective surface, whether that's a phone, a mirror, or water, assuming you are staring at your own reflection. Though do be aware of the pitfalls of doing that for too long (Narcissus pined away, for example!). If your subject is further away in the reflection, the camera will still (probably) attempt to focus on them, which will be the distance from the lens to the surface, and then to the subject. If you switch to manual, you can then focus on the reflective surface instead. All depends what you are going for as an image, I suppose.

Answer 6

Your camera doesn't know there is a mirror(*), it focuses on the "virtual image" created by the reflection. And for nearby objects this virtual image is at a significantly greater distance than the object itself.

(*) unless the mirror is dirty, in which case it focuses on the mirror stains and your object is out of focus.