In is simple trig: In this image, A is the throw, B is 1/2 the field diameter, and C is the beam edge or hypotenuse of the right triangle created by A and B. Lets call the beam angle X. Most of these calculations require a good scientific calculator. So you know A and B. To get beam angle: X=2*atan(B/A) NOTE: atan = Arc Tangent or inverse tangent So you know the beam angle (X) and B. To get throw: A=B/tan(X/2) So you know beam angle (X) and A. To get Beam width: A*tan(x/2)

Nicely done, Alex. [user]Tyler[/user], for more, see the CB Collaborative Article: http://www.controlbooth.com/forums/collaborative-articles/7664-mathematical-formulas-lighting.html. One caution: Don't get "beam angle," "field angle," "beam," and "field" confused. When referring to a 26° ERS, the number is referring to the fixture's field angle. Usually, when planning a wash from multiple fixtures, one wants to overlap fields, but not beams. Softening by "running the barrel" or adding a light frost (X114, X119, X132) covers a multitude of sins/errors in judgment. Do they still print trig. tables in the back of textbooks? Edit: Backstage Handbook, Third Edition (Blue Cover). Paul Carter. Broadway Press, 1994, page 226.

FYI 1994 is the most "current" edition of the Backstage Handbook. Maybe if we are lucky 2009 will bring us a 15 year update.

Wow, how time flies. I got mine when I started college in '99 and the book was a mere 5 years old .. that can't have been nearly 10 years ago now, can it?

What in the current (1994) edition of Backstage Handbook is out-of-date? What additions do you feel necessary?