copyright by Julie Seitel
Field angles are not angled fields (that would be rather difficult to fit in a stage light), but rather they are the term to describe the angle of the beam of light where it reaches 10% of the intensity of the center of the beam. These are often commonly used to indicate the ellipsoidal fixtures beam spread, the options range from 5°, 10°, 19°, 26°, 36°, 50°, 75°, to 90°. Older fixtures are described by the width of the lens and the focal length. For example, a 6x9 ellipsoidal has a 6” diameter lens and a focal length of 9”. These fixtures have a field angle of 37 degrees. As field angles narrow, the instrument can be used further from the stage. Each field angle describes a certain focal length at a fixed distance.
It’s easy to find out which field angle you need based on the distance between the fixture and the surface, as well as the desired beam spread. A handy calculator can be found here (in the form of a handy blue button called field angle calculator).
At a distance of 10 feet, the following field angles will be expected; a 36-degree will yield a beam spread of approximately 6.5 feet, a 26-degree will yield a beam spread of about 4.8 feet, and a 10-degree will yield a beam spread of nearly 2 feet. If using a gobo, the image area on the gobo can also be adjusted to help get you the correct projection size.
But what if you want to move the fixture around the grid? Do you have to buy a bunch of spare lens tubes so you can get the field angle you need each time?
Answer NO. This is the really great part, there are zoom ellipsoidals. These fixtures have lens tubes that can adjust the field angle in order to achieve the desired focal length.
Understanding how field angles work can help you achieve the look you want. We hope this little brief has helped you understand a bit more about one of our favorite theatrical lighting fixtures, the ellipsoidal.
