Equation for Beam Angle

In is simple trig:
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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.

Tyler said:
beam angle when I know the throw distance and beam width
So you know A and B. To get beam angle: X=2*atan(B/A)
NOTE: atan = Arc Tangent or inverse tangent

Tyler said:
throw distance when I know the beam width and beam angle
So you know the beam angle (X) and B. To get throw: A=B/tan(X/2)

Tyler said:
beam width when I know the beam angle and throw distance
So you know beam angle (X) and A. To get Beam width: A*tan(x/2)
 
Nicely done, Alex. Tyler, 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.:)

...Most of these calculations require a good scientific calculator. ...
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.
 
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FYI 1994 is the most "current" edition of the Backstage Handbook. Maybe if we are lucky 2009 will bring us a 15 year update. :rolleyes:

From your keyboard to whatever you believe in's ears.
 
...Maybe if we are lucky 2009 will bring us a 15 year update.
What in the current (1994) edition of Backstage Handbook is out-of-date? What additions do you feel necessary?
 

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