We use 3 × 3 × 3 Sobel operators to compute both spatio-temporal derivatives s_x, s_y and s_t and their certainties (scalar number) in a number of image sequences and then use Lucas and Kanade's weighted least squares framework to compute optical flow (image velocity) in 5 × 5 image neighbourhoods, where the weights are the derivative certainties. We model the certainties in the derivatives as proposed by Spies [Vision Interface 2003] and analyze them quantitatively by evaluating the flow computed using them. For a number of synthetic image sequences with the correct answer known, we perform a quantitative analysis using either weights of 1.0 or weights computed from the derivative certainties (2 ways) and show that using a good estimation of the derivative quality in an optical flow calculation leads to better quality optical flow (both more dense and more accurate).