I love the Sigma 15mm and 20mm lenses. With my Nikon D100 I can always shoot in any light. Especially in shady areas, where I set up the ASA 800 - 1000, it would create more depth of field. I don't see any flaws in the Sigma 15 and 20mm lenses.
Canon EOS SLR Camera Lens EF 24-105mm f/4 L IS USM
124 Review
Nikon 35mm f/1.8G Auto Focus Lens for Nikon DSLR Cameras - Black (Model 2183)
125 Review
Canon EF 50mm f/1.8 II Fixed Lens - Discontinued by Manufacturer
93 Review
Nikon 50mm f/1.8D Lens: Perfect for Nikon DSLR Cameras!
97 Review