Or it could be something simple like the lamp is at a different focal length when fully spotted. Yea, that sounds better.
Oh, a question I can answer.... (Don't know how much this happens. )
When it comes to Fresnel Spotlights...or lens for that matter...
There are two numbers with them such as a 6"x6".
The first number is the diameter of the lens, the second is the focal length.
The focal point is the more important number when it comes to this question. The example above (6x6) has a focal point that is 6 inches from the lens in question. So as to look like this
Lens Focal Point
__)------X
While an 8" would have this...
__)--------X
And so on and so on up to 20", I think?
Now the main thing that causes the 8" to be smaller (tighter beam) is that the angle the light comes out is smaller. So at the same throw distance the beam diameter of the 6" is larger, but the 8" has a larger throw over all. Meaning it can hang farther away and cast the same amount of light.
This comes in handy depending on the size of your theatre. Our Courtyard Theatre here at Truman State is small, so a bunch of 6" are used nicely. But our University's auditorium is larger so there they use 8" since the light has farther to travel.
The picture below can show it best.
...
Further handwritten notes:
3.1/2" lens has a focal length of 2.1/4"
6" lens has a focal length of 3.1/2"
8" lens has a focal length of 4.3/4" ...
The focal lengths of Fresnel lenses are seldom published, and I'm not certain the above holds true for all Fresnel lenses. Unlike ERSs, since the Fresnel fixture can be flooded/spotted, exact focal length doesn't much matter.
Another issue that comes into play when dealing with stage lighting optics is that focal length is a concept based on parallel rays entering the lens. Ellipsoidal and spherical reflectors do not create parallel rays, only the parabolic reflector does, and those fixtures don't use a lens. While I'm sure much study has been done by fixture manufacturers, not much is written on this topic.
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