With the advent of LED lights, different moving lights and all sorts of new lensing possibilities, comparing different lights can get confusing.

You may find yourself comparing light fixtures by data sheet, and just be confused when you read “beam angle” and “field angle”.

You see, these 2 measurements are often presented for lights with no other explanation.

If 2 fixtures both have a 25 degree angle, but one is “beam” and one is “field”, which one is wider?  Are they the same?

Let’s look at an example:

At a quick glance, these 2 fixtures look pretty much identical.  Whip up some fancy brochures, put the facts in different locations, and you'll probably convince most people that these are 2 identical lights.

But we're trying to learn here…and that means that we must declare that these 2 fixtures are different, very different.  This is where we get down to the brass tacks- the exact difference between beam angle and field angle.

Beam angle is the measurement of the fixtures light output angle until the output has fallen off to 50%.  This means that you disregard the light out of that 50% and above zone for this measurement, which makes it helpful in knowing how much "usable" light the fixture puts out in a fairly even field. 

Meanwhile, Field angle is the measurement of the fixtures light output angle until the output has fallen off to 10%.  This means that you disregard the light out of that zone for this measurement, though that's not much.  This angle tells you how far the light reaches until it (basically) fades into the darkness.

Definition of Beam Angle:
Half angle(Θ 1/2) (FWHM): 50% light intensity.
Full angle(Θ): 90% light intensity.

What actually matters?
For most uses, the beam angle is the one to go by.  If the manufacturer provides both measurements, it can tell you how fast the light falls off at the edges.  A good leko focused sharp has nearly an identical beam and field angle, but a par can has a good difference between the two.
As you can see in the example above, the FancyLight 364 probably has a wider angle than the GroovyLED Par 600 LXE, but we can’t even make that assumption safely.
Without congruent measurements from different manufacturers, we just have to rely on seeing units in real life and talking with others to find what works best in each situation.  So keep that in mind next time you buy new lights, so you aren’t disappointed later!