This is a piece of equipment which over the years has been sadly misused due to some basic deficiencies in 'knobology' and some principles.
First rule is to leave it how you found it.
The movie below runs through basic use and FB removal using the slitlamp. The basic principles of slitlamps are all the same so the best way to see what turning that knob is doing is hold your hand in the beam and watch as you adjust.
The right sized beam for the job is discussed in the movie.