For this stuff I use a 28-70 mm zoom for the flexibility. For my documentary work I usually just my 50 mm but I also had a 28 mm and 75 mm canon lenses. They all have pretty low f stops so that you can use them in low lighting or to get nice depth of field. If you are just starting to get equipment I would suggest buying a 50 mm with a low f stop and just using your feet to zoom in and out. Wow, that was really long, but I hope it helps!