That is an excellent question. Field angle is the angle of the beam of light where it reaches 10% of the intensity of the center of the beam. It is commonly used to determine the beam spread of a stage lighting fixture. As the field angle narrows, the stage lighting instrument may either be used further from the stage to create a similarly sized beam as a closer larger instrument or it can be used from the same distance to create a smaller beam.
A field angle calculator can be found
here. Look for the large blue button.
While some of that makes sense right off the bat, many of us are visual people An easier way to look at it might be through the diagrams below:
Now, if we take the same fixture, the same gobo, and the same distance, by changing the degree on the zoom fixture, we can change the width of our gobo.
This also impacts how large a gobo is projected. If you happen to have
a zoom ellipsoidal, you have a bit more control over how large your
gobo projection can be even if you cannot change the distance your stage lighting fixture is hung from the surface on which you want to project your gobo. Many times we are asked to resize gobos to get an ideal gobo projection size for a certain fixture at a certain distance. This is often very helpful. However, sometimes venue changes might necessitate a change in where the fixture is placed. This can impact your
gobo. Knowing a bit about beam spread ahead of time empowers you to make the best choice for your production or event.
We hope this has shed some light on field angles and beam spreads. As always if you have any question, please do not hesitate to contact
StageSpot.
Now, if we take the same fixture, the same gobo, and the same distance, by changing the degree on the zoom fixture, we can change the width of our gobo.
This also impacts how large a gobo is projected. If you happen to have a zoom ellipsoidal, you have a bit more control over how large your gobo projection can be even if you cannot change the distance your stage lighting fixture is hung from the surface on which you want to project your gobo. Many times we are asked to resize gobos to get an ideal gobo projection size for a certain fixture at a certain distance. This is often very helpful. However, sometimes venue changes might necessitate a change in where the fixture is placed. This can impact your gobo. Knowing a bit about beam spread ahead of time empowers you to make the best choice for your production or event.
We hope this has shed some light on field angles and beam spreads. As always if you have any question, please do not hesitate to contact StageSpot. 
