A theatrical fresnel is a soft edged circle of light that can zoom in size from spot to flood by changing the relationship of the lamp with the interior reflector. They are simplistic fixtures that have stood the test of time in their ability to provide basic controllable light. They range in size from a 3" diameter lens to an 8" diameter lens. They range from 100 watts to 1000 watts. In the film industry they can become much larger.
Read on to learn more about fresnels.