So I was taking a photo of the reflection of an exit sign. The sign was about 20ft away. I was shooting the screen of a phone which was about 1 foot away. In order for the sign in the reflection to be in focus, I had to set the focus to about 20ft even though the reflection I was shooting a phone screen that was a foot away. The phone and table it was sitting on were very out of focus including the screen, but the reflection of the sign was crystal clear. Why?
In: Physics
Think about taking a photo through a window. You don’t focus on the window glass; you focus on the things beyond the window. A mirror is essentially the same thing except it bounces the light. You’re essentially looking “through” a mirror to an image that’s “behind” it, and you this have to focus your camera on the distance “behind.”
[This](https://i.sstatic.net/9twPx.png) diagram shows the path of two light beams as they bounce off of a mirror. As you can see, the angle that they reflect makes an image which appears to be a distance behind the mirror (the dotted lines).
Because it’s geometry, the visual of the diagram is going to be far more helpful than a wall of text.
The path that light has to travel from the object to your camera was 21 feet long, 20 ft from object to mirror then 1 ft from mirror to camera. You’re trying to focus on the object, not the mirror itself, so it’s just like any other time you’re trying to focus on an object 21 feet away. The fact that there’s a mirror changing the angle of this path partway through doesn’t matter or affect anything.
Physically, there is no difference between a perfect mirror, reflecting an object behind you, and a transparent pane of glass through which you see an object in front of you. In both cases, the light had to travel the same distance, and arrives at the same angle, so the optical equations are exactly the same.
As a result, even though the *mirror surface* is only a foot away, the *light* from the object you are examining moves and behaves exactly as though it is as far away from the phone as though it were “really” on the other side of the mirror.
This fact, that you can pretend that reflections are actually mirror-side real things, is used in math and physics to simplify the solution to complex differential equations it is called the “method of images.” Quite useful.
Oh, ok. Dang, I was thinking only about the two objects and not the “photo” in “photography”. I’m not taking a picture of the sign but rather the light reflected off of it, so the distance to me from the sign is how far its light travels regardless of whether Im shooting it directly or through the reflection. I think I understand. Please correct me if I’m wrong. Thanks for the explanations, everyone!
An interesting comparison can be made with rear view mirrors in cars. When glancing at your mirrors, the focus point is roughly the same as that through the windscreen, so your eyes do not have to re-focus. However, a number of modern cars now have camera based mirrors. When looking at those, the eye is required to focus on the actual screen itself, requiring the eye to re-focus, which takes a small amount of time.
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