 Different field angles are needed for different venues with different catwalk and electric systems (and, therefore, different throws).A lens tube with a smaller field angle will light an area from far away, whereas a large beam degree such as a 90 degree can be much closer in order to light the same area.A 90 degree Source Four might be used to project a gobo from the rear only 5 feet away on a scrim at the back of the stage, while a 10 degree could be used in the back of the house, for example, in the technical booth where a technician could access it to refocus or change gobos during a show.