autofocus points .. closest object

How does the camera judge distance?

I believe with very complicated algorithmic software and the AF sensor. Maybe infrared beams and sensors in some cameras. Funny though....in EXIF data it tells you exactly how far your subject is from your sensor.

A simplistic explanation:

The camera focuses the subject using phase detection(or contrast detection) by turning the motorized focus ring. The lens(some, not all) "knows" how far the focus ring has been rotated and comunicates this information back to the camera via the electronic contacts at the lens-camera interface.

nate42nd wrote in post #14617995
in EXIF data it tells you exactly how far your subject is from your sensor.

Er, I wouldn't call it exact! The exif reckons the subject in this image was 41 cm away!

hollis_f wrote in post #14619885
Er, I wouldn't call it exact! The exif reckons the subject in this image was 41 cm away!

If you're using multiple focal points then the AF system will look at the pahse-difference info for each point. That will let it know which of the points is over the closest subject, so it uses that info to tell the lens how far to move.

So you didn't notice the stargate 41cms in front of you then . Pity, it connected to a world where 1DXs were delivered on time!

hollis_f wrote in post #14619885
Er, I wouldn't call it exact! The exif reckons the subject in this image was 41 cm away!

You've got some ultra ultra wide angle lens there!