Brian Caldwell

  The horizontal dimension of DX and BMCC frames is 23.6mm and 15.81mm, respectively.  So the horizontal crop factor would be:   (23.6)(.71)/15.81 = 1.06     If you calculate it based on the diagonals (28.4mm and 18.133mm, respectively for DX and BMCC) you get:   (28.4)(0.71)/18.133 = 1.11   Either way, you wind up cropping just a small portion of the DX format image circle.   On the other hand, if you mean crop factor relative to full-frame (24mm x 36mm) format based on the horizontal you would get:   (36/15.81)(0.71) = 1.62