What's responsible for autofocus

Which piece of equipment is responsible for the autofocus of the lens? Does the camera set the speed of aquisition of focus or is it all based on the lens? Is it a combination of both or how does it really work. I was using the Nikkor 70-200 VR1 and I missed 3 out of 5 on focus of birds in flight. I know alot of that was due to operator excitement and error watching great grey owls but it really made me think about how this works. All comments are appreciated.

Auto Focus is done in the camera by the computer chip in the camera. Information about the lens (focal length, aperture) are transmitted to the chip so both camera and lens matter. AF is faster with a lens that has a larger maximum aperture: a 300/2.8 will focus faster than a 300/4 lens in most cases. If you add a teleconverter to increase focal length the max aperture is reduced and AF speed is reduced as well.

Having VR turned on will also likely decrease AF speed - VR takes computing power and energy to move elements around to "Stabilize" the image delivered to the sensor. The only lens I own with VR is the 70-300 and frankly I turn off the VR a lot of the time because it causes me to miss fleeting action while it tries to stabilize things.

Lindy - There are two main type of autofocus : phase detection and contrast detection.

Phase Detection
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As you probably know, there is a mirror in front of your sensor in your DSLR camera to reflect the image to the viewfinder. But there's another path the light takes too, down (usually) to a separate sensor. This one compares two slightly different paths (phases) of the light to check focus. The two signals from the autofocus sensor are processed in a chip and the output sent back to the cameras autofocus controls and lens. The system adjusts until focus is achieved.

Contract Detection
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This is usually performed right on the imaging sensor. In this case, some point on the sensor is chosen, and the lens moved until maximum contrast is achieved, indicating best focus. This has traditionally been slower than phase detection, due to the dedicated hardware being much faster than trying to use the actual sensor. Some newer cameras have caught up quite a bit though, and are just as fast in some cases.

Speed
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As far as speed goes, there are four things I can think of that affect it, though there may be others :

Amount of light reaching the autofocus sensor (affected by the scene, and the max aperture of the lens) : More light means a more obvious contrast or phase difference detection, and the autofocus will happen faster

Amount of movement the lens has to make : Obviously, if a lens is far out of focus, it will have to move more and take longer to find the correct focus

Speed of lens movement : Lenses with high speed motors are generally much faster to autofocus than those with older motors. And lenses with less weight to move tend to be faster than those with smaller, lighter glass.

Speed of the circuitry making the autofocus choices : This includes both the processor chips that determine how to focus, and the speed of the communication between the lens and the camera


Here're a few good links on the process, but not really the speed... the first two have great little applets where you can move the sliders and see what's happening :

Autofocus : Phase Detection
Autofocus : Contrast Detection
Cambridge in Colour : Understanding Camera Autofocus

Agree with all so far and super info. I'd add that you may wish to consider your focus point selection mode. Are you using only a single center point or multiple points or letting the camera use all points? Single point is my preferred method for most shooting but for birds in flight in a clear sky, allowing the camera to select the point will usually lock onto focus faster than I can. In addition, if you then use continuous (servo) focus the camera will more readily remain locked on focus by continually adjusting. Most modern cameras have multiple focus point options from single point to single point expanded to variations of patterns using however many the camera has. You didn't mention what body you're using but generally the more you pay for the camera, the more focus points available. I'm a Canon guy but I think the top end Nikons have over 60 and as you get closer to the entry level models less and less. Look into this stuff and it may help you.
JR
PS: Yes, turn VR off. Another aspect aside from computing power is that the camera will VR first and then AF. If you're doing birds in flight you're probably using a fast enough SS that VR isn't doing anything for you anyway.

[Having VR turned on will also likely decrease AF speed - VR takes computing power and energy to move elements around to "Stabilize" the image delivered to the sensor.]

I was up in the air about that Rich, didn't think it had any appreciable effect. Thanks for your 'input'.

Thanks for all the great input. I uploaded an image in avian and next time will incorporate all these great suggestions. Thanks again,
Lindy

I recently read that using the minimum amount of AF sensors will speed up autofocus. This seems logical to me, because it would be less computing used to focus, thereby making it faster. However in practice there seems to be no difference to me. I am using a D7000 which has a max 39 AF sensors and a minimum of 11 sensors. Does anyone have any thoughts on this?

I have a thought Greg... unless the camera is hunting for something to focus on, I'd bet it selects the autofocus sensor it wants almost instantaneously (certainly quicker than you could notice).

I wouldn't ever expect autofocus point selection to be noticeable, except in extreme cases where it's hunting because there's either nothing to focus on, or you're in continuous mode and the focus point keeps changing.