How does an iris used in Theatre Stage Lighting work?

Question

Here's a link to what I'm talking about:

http://www.stagelightingstore.com/s.nl/it.A/id.2810/.f?sc=25&category=1349

This is also known as a camera iris shutter thing [or something like that].

Here's a better picture:

http://www.turbosquid.com/FullPreview/Index.cfm/ID/256262

A well explained diagram and/or a link[s] is/are greatly appreciated.

Thanx Ya'll!!!

Answer 1

The iris controls how large the beam of light is.  You see this sort of thing clearly with spotlights.  The larger the opening on the iris, the larger the beam of light.  The smaller the opening, the smaller the beam of light.  For example, if you had a spot on a group of people, you'd have the iris set to a wider opening than you would is it was a single person.

Does this help explain it to you?