Nikon D5100 doesn't have a built in autofocus motor.so do AF lenses work on it?

I'm going to be getting the Nikon D5100 soon and also the Nikon Zoom-Nikkor Telephoto zoom lens - 70 mm - 300 mm - F/4.5-5.6 - Nikon F. Will it autofocus in video and in pictures?

If it is the good AF-S VR model, it will autofocus. If it is the cheap model it will not autofocus with the D5100.It would autofocus with the D7000, but the cheap lens still would not be very sharp at the longer focal lengths.

They will but you will have to use the lens in manual mode, something photographers have been doing for decades.

The way Nikon can produce entry level cameras at such low prices with sensors that are high performance is by leaving the on-camera auto-focus motors out.

This is not really a bad thing since all the new Nikkor AF-S lenses are super fast auto-focus so really most people are buying AF-S lenses anyway.

The only lenses that will auto focus with the D5100 are the more expensive AF-S lenses. You can use the less expensive AF lenses but they will not auto focus on the D5100 so you'll have to manually focus them.

The Nikon D90 (gradually being phased out) and the Nikon D7000 (the replacement for the D90) both have in-camera focusing motors. Of course the D7000 is about $650.00 more than the D5100. The D90 is about $300.00 more than the D5100.

I advise against buying any Nikon zoom lens such as the 70-300mm unless it has VR (Vibration Reduction) which is Nikon's version of Image Stabilization.

FYI Sony and Pentax both have a focusing motor in their DSLR camera bodies and they both have their respective versions of Image Stabilization in their DSLR camera bodies. With Sony and Pentax you only pay for a focusing motor and IS once - when you buy the camera. Plus any lens used on either of them will be an IS lens - not just a select few.

Http://www.kenrockwell.com/nikon/compatibility-lens.htm#dslr

AF in video? Check manual.