Right hand Drive, you be the judge

[kirbinster]

You don't see too many Righthand drive cars in NJ. Well on closer inspection you can probably tell this one is not either. Look at the plates, they are backwards. I took this by shooting at my outside rearview mirror with my 105mm Micro lens. My question for you lens experts is why when the lens was about four inches from the mirror did it focus on infinity to take this shot?

[AnonymousKiwi1]

I am no expert, but it's been my experience that you have to focus at the distance that the subject is from the mirror, not on the surface of the mirror. Which makes sense.
If you look at yourself in a mirror, your eyes focus at the distance you are from the mirror. If you want to look at something on the surface of the glass - a smudge, a photo stuck in the frame, your eyes re-focus.

[zona5101]

it focuses by determining contrast not distance like a laser range finder would. that is also why infrared on flash units (like nikon) send out bars of red light...it's illuminating so the sensor can determine how to make the bars sharp.

[in2fx]

I guess it has nothing to focus on regarding the mirror surface itself, but if there were rain spots on the mirror then it would have seen those and focused closer.

I had that problem one day while riding my motorcycle with a video camera behind the fairing screen and it started raining, so the auto focus got very confused and kept jumping between long and short focusing.
When I stopped and clipped the video camera to my helmet instead, then it was happy again as the rain on the lens was too close to affect it and it went back to long focus again.

[JeffSmith]

My question for you lens experts is why when the lens was about four inches from the mirror did it focus on infinity to take this shot?

You're making the assumption that a camera works the same way as your brain. Your eyes are merely receivers of light and your brain, based on your life's experience, determines how to translate that light into "reality" (whatever that is).

A camera does not work in the same way. The lens focuses light onto the sensor and Zona5101 is correct... the camera's brain does not determine distance or perspective, it's not that clever. The auto-focus calculation is purely binary and is based only upon contrast, that's why it doesn't always focus on what you want it to focus upon

If a camera was able to tell the difference between a reflected image in a mirror and a "real" image, how clever/terrifying would that be? If only the auto-focus could read our mind!