I'm going to be in a completely dark area next month and I plan to try out some dark frame subtraction but I'm a little lost on what the ideal settings would be. I plan to use a Oly 12mm F2 or Oly 12-40mm F2.8 at 12mm. Or maybe the Olympus 17mm F1.8. I also own an E-P5 and an E-M1. Which would be best? What exposure settings and how many of each type of frame would be recommended?