When scanning black and white 35 mm film with my Epson V750 Pro I found the focus not as sharp as expected. I tried scanning as colour positive, then inverted in CS3. Then in Channel Mixer I applied in turn various coloured filters. I found that the blue or green filters produced very sharp results, while the yellow, orange and red filters produced progressively poorer focus. Does this mean that the scanner is focusing the red, blue and green scans in different planes? If so, is this a defect or is it normal?