I need a new lens for my DSLR?

So I want to make Videos using my DSLR I need something hands on quick to use for both manual and automatic usage. I have a Nikon D5100 I was thinking of going for sigma lenses but I have no idea which one to use, most of my lenses are manual and its about time to invest in something different. Any recommendations?

What lenses you use when shooting video has much more to do with the script you are shooting.

Here is a link to a video shot with a dSLR and as you will see at the end of the video, four lenses are listed.

You will also notice that the director also used a small helicopter for two of the shots

As long as you get a Sigma that has a Nikon mount, all is good.