Variables that affect auto-focus speed

[Naois]

Hello,

I'm looking for additional variables that my effect the auto-focus speed of a camera.

I have heard/confirmed the following things can effect auto-focus speed:

• speed of the AF mechanism on the lens (faster is better)
• amount of contrast (more is better)

I have heard that these things may also effect the auto-focus speed, but I'm not sure how:

• focal length of the lens
• aperture of the lens
• distance from the subject
• exteral battery pack
• camera processor speed

Do these sound right? If so, how do they effect the auto-focus speed, and are there more?

I've tried using google to search for these answers, (and this forums search tool as well) but I can't seems to find any good answers with details as to the reasons why they do.

[OsmosisStudios]

Focal length doesnt really affect focusing
Aperture DOES affect focus ability in the sense that a larger max aperture allows more light to hit the AF sensor.
Distance: This can affect focus speed, but it's circumstantial. If I focus at minimum focus distance (MFD) and then try to refocus on something at infinity, it'll take the camera longer. The same is true going the other way. Often, though, this is lens dependent.
External battery power has no effect
Camera processor is the big one, but it's mainly just the AF module of the camera that's important. Some are faster than others.

Of course, for those of us shooting with Nikon (or any other screw-drive AF systems), the speed of the motor in the body affects things as well. The same lens will focus differently on different bodies.

[inkista]

You forgot the camera body's AF system and AF mode. Some are definitely faster than others.

The reason a lot of folks will tell you to set the center AF point and use that alone is that on most entry-level cameras, the center AF point is probably the only cross-type AF sensor you have, and is more sensitive. The difference between cross-type and linear-type AF sensors is more one of accuracy than speed, but it can make a difference if your entire array is cross-point, vs. a single point, particularly in tracking modes.

Another one that you may not have run into (you generally only find them on high-end telephoto or supertelephoto lenses) is a focus limit switch on the lens. This restricts the focus distances the lens will "search" through while autofocusing. This can help speed up the AF.

[Naois]

Thanks for all the information.

@OsmosisStudios: Are all Nikons "screw-drive"? I thought only the older non-AF-S lenses used that.

On my D40x, I can't auto-focus with the older AF lens because my camera doesn't have the "screw-driver".

Thanks again!

[OsmosisStudios]

Originally Posted by Naois

Thanks for all the information.

@OsmosisStudios: Are all Nikons "screw-drive"? I thought only the older non-AF-S lenses used that.

On my D40x, I can't auto-focus with the older AF lens because my camera doesn't have the "screw-driver".

Thanks again!

Exactly: The D40/40x/60/3000/5000 dont have the screw-drive motor. The motor is only necessary for non AF-S lenses: AF-S lenses dont use the screw drive.

In the cases where the screw-drive is involved, though, the speed of the motor does impact AF performance, rather a lot I might add.