10

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.

7

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 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.

  • 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
  • 2
    because of this: i.imgur.com/7kSAf.jpg sorry, not great drawing techniques – philberndt Jun 28 '12 at 10:47
  • 1
    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
  • +1, just for the drawing! – Pete Becker Jul 10 '18 at 14:24
12

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.

5

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.

  • 1
    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
0

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.

0

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.