The Sigma 8mm on a full frame camera (I.E full size sensor) is not ideal for maximum quality as you waste so many pixels to black. The best lens for this camera body is a Canon or Sigma 15mm Fisheye.
That said, the Sigma 8mm is best suited to an aperture of F/8.0. You always should aim for the lowest ISO possible 100ISO.
The camera should be set to (M) manual mode and so should the lens. But first you must fine the "Hyperfocal" distance to give you the clearest possible focus near to far. You can use a Hyperfocal calculator and test with a series of comparable shots.
Quality doesn't come from purchasing equipment alone. Understanding the equipment is where true quality comes from.