Mirror Slap

On a SLR, when you look through the view finder you are seeing a mirror. This mirror must move out of the way to allow light in. There is a range of shutter speeds at which the small amount of vibration caused by this will blur the photo. Some cameras have a mirror lock, which will eliminate this problem. On those cameras that lack this feature a little experimentation will find a range of shutter speeds that won't blur the photo.