The Canon EOS and Photography Forums

Hi Guys,

My first post here.
Sorry to be a bit vague, but hopefully someone can give me an answer.

How does Auto Focus actually work?
Is it done inside the lens or camera body?
ie. Where are the brains?

I am aware that the drive is inside the lens, but where are the decisions made - and how?

Any help would be greatly appreciated.

Steve.

Here is a good video by Nikon:
http://www.nikon.com/about/technology/core/software/caf/

Short answers:
The "brains" are in the camera which drives the focusing motor in the lens in a feedback loop.  It does not use the sensor (After all Autofocus pre-dates digital cameras!)  If you have any expereince with feed back loops - ever tried to ride the thermostat to get a certain temperature?  ever driven a slow boat with a big "dead spot" in the center of the steering?  You will know that feedback loops are tricky to design and every component is a critical part of the system.  One of the known tradeoffs is speed vs accuracy.  The lens it self is a critical component of the loop which is why you can buy systems to help you tune *your* lens for *your* camera.

SLRS use phase detection which gives a vector error measurement, so they can directly drive to the correct focus.  A P&S has to move the focus once to see if the focus point is closer or farther before it "knows" which direction to drive the motor.

Cool article, Keith.  Now I am wondering if Canon IS works the same way as Nikon VR?

VR and IS (and OS and VC) all work in the same way. Gyro sensors detect angular rotation of the lens and feed a signal to actuators in the lens which move a small group of lens elements by the right amount calculated to compensate for any image shift. The "smarts" are built into the lens itself.

Got you!  Thanks Bob!