I am using a cannon rebel T4i and have the basic kit lens. I definitely need something that has a fast auto focus that will stay on it's target. I make many skate videos and also make music videos for my band. What kind of lens would work best?
I would look at the Canon 17-55mm f/2.8, which is expensive but probably the fastest. If it's over budget, you could look at the Tamron 17-50mm f/2.8 VC. Depending on your location you might also need a telephoto zoom.
I have the Canon 17-55 lens and it is solid. Low light is good with a 2.8 throughout the focal range. I use it a lot. With that being said, it is pretty expensive for a S-Series lens. If I had to do the purchase over again, I would go with the 16-35 L series (on a cropped sensor would be ~26-56).
If you can get away with not having a zoom lens, I also highly recommend getting used Nikon Nikkor primes. The Nikkor's that I paid $150-250 are better and sharper than the Canon 17-55 I paid $1,300 for.
[JonRoss Maddox]"something that has a fast auto focus that will stay on it's target"
Here is the thing about auto focus, its a great feature that in most cases should never be used. Especially in your case where there will be a lot of movement, the camera just has a hard time knowing what the 'target' is that it should be focusing on.
Those of us that have shot on traditional video cameras with auto-focus features know that in almost all situations, that feature is turned off.