Conventional Fixtures Source 4 aperture calculations

[ohnoimabear]

Hi all, I'm new to the forums, so I apologize if this has already been covered. I did some searching and wasn't able to find help.

I have a 90° source 4 ellipsoidal used to create a circle on stage. It is pointing straight down and I have dropped an iris in. Unfortunately, the irises that we have aren't large enough, and I'm left with an 18' circle, instead of about 20'. Without the iris though, I'm at about 26'. I thought about building/cutting an aperture gobo to make the circle the right size, and my first thought was trial and error, but because I want to make the circle look as perfect as possible, I'm hesitant to do that. I'm wondering if anyone knows of a resource through which I can calculate the proper gobo aperture.

Here is what I know so far.

A 90° will build about 26' of circle. If I make it 80° (in VW) I get a circle about the right size. Knowing that the aperture of a S4 is 3", I figured that a 2 2/3 in. aperture would suffice. However, I don't know if that 3" aperture is the aperture at the focal point, or what.

Anyway, I'm not sure exactly where to go from here, because, like I said, I want to make sure that I get the most perfect circle possible.

Thanks all!

-Shane

 

[church]

Hi all, I'm new to the forums, so I apologize if this has already been covered. I did some searching and wasn't able to find help.

I have a 90° source 4 ellipsoidal used to create a circle on stage. It is pointing straight down and I have dropped an iris in. Unfortunately, the irises that we have aren't large enough, and I'm left with an 18' circle, instead of about 20'. Without the iris though, I'm at about 26'. I thought about building/cutting an aperture gobo to make the circle the right size, and my first thought was trial and error, but because I want to make the circle look as perfect as possible, I'm hesitant to do that. I'm wondering if anyone knows of a resource through which I can calculate the proper gobo aperture.

Here is what I know so far.

A 90° will build about 26' of circle. If I make it 80° (in VW) I get a circle about the right size. Knowing that the aperture of a S4 is 3", I figured that a 2 2/3 in. aperture would suffice. However, I don't know if that 3" aperture is the aperture at the focal point, or what.

Anyway, I'm not sure exactly where to go from here, because, like I said, I want to make sure that I get the most perfect circle possible.

Thanks all!

-Shane

The genuine S4 iris has two positions: one lets you reduce the aperture to the minimum dimension and the other allows the maximum aperture. On the flat plate side of the S4 iris you will find three small phillips screws and three extra holes. If you remove these screws you can rotate the iris relative to the backing plate until the next set of holes align with the tapped holes in the iris and refit the screws. You should now get the largest aperture possible.