But what value to use is a bit puzzling for me. When stacking, the focus distance interval is related to DOF. As I understand, DOF calculation requires a value for COF. I see recommendations for 0.013, and 0.010. Have used this but sometimes think the results sub optimal. Probably my technique is at fault, but should I be using a smaller value. My crude calculations indicate that if COF were to be the size of a typical, fair quality monitor (1024 ? wide), then the COF value should be about 0.003. It means 3 times more exposures, but as that part of stacking is not the greatest time consumer, it would be OK if results were good.