As Arrow Head said, you need to behave like you're shooting in almost bright daylight conditions, as far as Aperture and ISO goes. Shutter must be opened and closed by remote, via Bulb setting. I open and close slightly different than Arrow Head though. I open the shutter as I watch the mortar rise into the sky...then I close the shutter, well, whenever I want. I'll actually leave the shutter open through 3 or 4 mortars sometimes, so they all show up in the same photo. Any more than 3 or 4 and the photo tends to become too cluttered. If you're getting too much light into the camera, by keeping the shutter open so long, you can use a black piece of cardboard to "cover" the lens, between each mortar explosion. Just don't bump the lens with the cardboard.