How to know which lens AF in my Nikon D5100?

I want the 70-200 or the 24-70. 2.8 Which one i have to buy for it to AF with my nikon D5100? Does not matter if its Sigma or Nikon. Please thanks!

Added (1). S can I buy it? What the difference between AFS and.

Both the Nikkor 70-200 mm f/2.8 and 24-70 mm f/2.8 are AF-S lenses, the lenses you want for your D5100

Here is a list of all the Nikkor lenses and as you can see, there are a lot of AF-S lenses from which to choose.

http://www.nikonusa.com/...index.page

In your user manual, those lenses should be listed too

It may be a bit confusing but only AF-S (or AF-I) lenses can autofocus on your camera.AF lenses can't.AF lenses are sometimes called AF-D to lessen the confusion of using the term AF without a suffix.AF (or AF-D) lenses can't autofocus on your camera.

It all boils down to your camera is an entry-level DSLR, and needs an internal focus motor in the lens to autofocus as your camera does not have it's own autofocus motor. All entry-level Nikon DSLRs are like this.

AF lenses have a jack-shaft that protrudes into the camera and do not have a motor built into the lens itself. The the higher end model Nikon cameras have a motor built into the camera body that couples to this jack-shaft on the lens to power the focus gearing.

When you look for 3rd party lenses, you want those that have a focus motor built into them. Each brand of lens has a different designation for this motor. For instance, Tokina uses DX for those lenses requiring the camera to have a focus motor, and DX-2 for those lenses that have their own motors.

Sigma and Tamron have their own designations.

Nikon? Look for AF-S.
Sigma? You need HSM.

Here's a complete list of lenses that autofocus on your D5100.
http://en.wikipedia.org/...cus_motors

Only Nikon AF-S lenses will auto focus on your D5100. Sigma lenses with HSM in their description will auto focus on your D5100.