Hi, I started to do some photography. Did a few baby photosessions, a few birthday parties. I am looking to buy a new lens for my nikon D7000 and it is hard to deside which one to take I was thinking about Sigma 24-70mm or 70-200mm? Please help!!!!!!! Right now I have nikon 18-200 mm 3.5-5.6 but for some reason it cannot find find focus somethimes and I also have nikon 50mm 1.8G. Thank you.