I shoot a lot of sunrise/sunset shots and basically use my Sigma 70-300mm f4-5.6. The problem I'm having is that as the scene gets darker, the lens won't take the shot. I can get the shot up to a point but then the shutter won't click. What lens does anyone use or what is the best lens in your opinion? I've got a 50mm f1.7 that I'm going to try. Thanks!