Auto Focus Speed

[megan_forbes]

I believe AF (as opposed to AF-S) lenses have their autofocus driven

by the camera body? If this is correct, will the body drive Nikkors

faster than 3rd party lenses? (I'm specifically thinking of the

cheap 70-300G or the Tamron 70-300, but general theory on this would

be appreciated as well)

 

Thanks!

[roto]

The focusing speed of AF lenses depends on three parameters: 1) the torque developed by the electrical motor in the camera (with some AF lenses an F5 can produce a detectable torque reaction when it starts autofocusing), 2) the number of turns of the lens screwdriver coupling needed to go from the closest focus to infinity (this can vary wildly from lens to lens) and 3) how good the AF sensors of the camera are (this affects mostly the settling time and "hunting").

 

To answer your question, 1) and 3) being the same, the mechanical construction and optical formula of the lens will indeed affect the focusing speed. For example, internal focus (IF) lenses tend to be faster to focus: in these lenses only a few, relatively small elements shall be moved.

 

There is no reason why, in general, a Nikkor should focus faster than an equivalent Tamrom or Sigma. It could be true or it could not. It is impossible to generalize.

[alex_lofquist]

I notice no difference in "screwdriver" focussing speed on my F5 whether using a Sigma or Nikkor lens.

[todd peach seattle, washi]

I agree with Roberto's analysis. I'll toss in that I had a Sigma APO 70-300 for several years, and it gave good service on my N90s. When I bought the F100, the Sigma wouldn't focus right (they have to 'guess' when they set up the firmware). I sold the Sigma and bought the roughly equivalent Nikon 70-300 ED. The focus speed was identical between the two lenses.

[eric_hagen1]

The speed of the focusing on standard AF or AF-D lenses is dependent on the amount of torque required to focus and the distance that it has to move. For example, a really light weight 50mm prime can focus lightening fast where a bit 400mm lens that has glass elements weighing in pounds moves like an old dog.

 

Because of this, lenses with a very "cheap" feel may actually focus faster than the ones with a big of good solid glass inside them. That said, the cheap lenses also usually don't have nice smooth motion in the focus rings like a pro-grade Nikkor lens.... depends a lot on the lens. There WILL be differences, but I doubt it will have to do with the brand rather than the overall quality and weight of the lens.

 

Eric