does anyone know of a graph or chart that compares the shutter times and exposure with lumens as the output. I.e. I wanna see how much difference a 1.8 opening will do compared to a 3.5, how much i would be able to lower the shutter time and still get the same lumens(brightness). Thinking of getting a 1.8 50mm nikkor and I wanna know exactly how much I can lower my shutter speeds.