EF kit lense Auto Focus

[stixstudios]

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.

[KeithB]

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.

[emanresu]

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

[Bob Atkins]

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.

[emanresu]

Got you!  Thanks Bob!