I'm looking for a good low-light lens, for when I go to rock concerts, aquariums, etc.. I've been looking at these lenses so far: -Nikon 50mm 1.8 (very cheap price, but is it cheap in quality too?) -Nikon 50mm 1.4 (heard great things about it, is it worth the extra $?) -Nikon 85mm 1.8 (I don't know how I feel about the extra mm, but I heard this was a good portrait lens. This is kinda pricey for me though) Should I be looking at Sigmas etc. too? Any input is appreciated.