# 6" vs 8" Fresnel lens area width



## shedlight (Jan 28, 2010)

I have always been curious why when an 6" and 8" fresnel are hung at the same height, does the 8" lens produce a smaller area width than the 6". wouldn't the bigger lens size create a bigger circle?


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## derekleffew (Jan 28, 2010)

*re: 6" vs 8" Fresnel lens area width*

Good question, shedlight (and welcome to the booth, BTW). Moving this thread to Question of the Day.

Students-only for one week, please.


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## Morpheus (Jan 29, 2010)

Hmmm... ok, how to make this make sense....

Since the fresnel is 'focused' based on the profile of the lens, i'd think that the 8" would have more... erm.... profile(?) to focus with...

ok, i'll start with what I (think I) know...

Fresnels have the profile they due to reduce weight and thickness of the lens, since each ring is the same angle as a single lens...
So, then, following that logic, (this is kinda where I'm guessing) the larger lens even though it captures more light, focuses it tighter...


Or it could be something simple like the lamp is at a different focal length when fully spotted. Yea, that sounds better.


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## Anvilx (Jan 29, 2010)

Morpheus said:


> Or it could be something simple like the lamp is at a different focal length when fully spotted. Yea, that sounds better.



Yeah that is my initial thought my.

I have never used an 8" frenesl, but I have like 4 Kliegl bros Pat.3230 (I think that's the right # though maybe they were 3518s ) in my garage.


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## JustinTech (Jan 29, 2010)

shedlight said:


> I have always been curious why when an 6" and 8" fresnel are hung at the same height, does the 8" lens produce a smaller area width than the 6". wouldn't the bigger lens size create a bigger circle?



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.


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## DuckJordan (Jan 30, 2010)

JustinTech said:


> 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".
> ...



you hit it right on the nail i believe, at least thats how our play prod teacher taught us.


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## JChenault (Feb 1, 2010)

Expanding the question a bit - why is that in general a larger diameter lens is needed for narrower beam spreads? IE if you look at the lens in a 90 degree source four, it is pretty small. On the other hand the lens for a 10 degree source four is pretty large.

Why is this so?


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## derekleffew (Feb 9, 2010)

As others have said, shedlight, larger diameter=longer focal length=smaller pool at same distance. Larger lenses are intended for longer throws and higher wattage, not necessary larger pools.

From a post by ship:

ship said:


> ...
> 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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## JChenault (Feb 10, 2010)

derekleffew said:


> 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.



My understanding of focal length is that it is a two way street. IE it holds equally well defining the distance a point source should be from the lens to give parallel rays of light out of it. A fresnel lens has a focal length just like any other plano convex lens - it also just has lots of imperfections built into the lens so the light is not hard edged.

If you take a standard Plano Convex lens ( like in an old PC fixture ) and put a lamp in it ( with or without a properly positioned spherical reflector) you can get a beam of light that consists of parallel rays ( up to the limit of optics, the fact that you don't really have a point source, etc). As you move the point source of the filament forward and back you affect the size of the beam and the parallelness of the rays. 

Interestingly enough, you can shutter PC fixtures. IE if you put a shutter at the plane of the lens, you can shape the light just like you can in an ellipsoidal spot. This is why you can shape the light of a fresnel without using a barn door if you create a gel frame that has a cut out of the shape you want to project. It will be soft edged, and it is finicky to set up, but if you have no room, or are running in rep, it is a neat trick.


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