Lighting Q.O.T.W. (10/30/09)

[photoatdv]

Experimentty time I think:

Set it to the intensity in question and have an assistant hold their hand in front of the spot and find the focal point in front of the fixture (if it's not out of reach-- if so point it to the side or something), where their hand shows up like a gobo or bunny ears in front of a flashlight :-). Now open it all the way. Is the focal spot in the same place?

I think you're moving the focal spot by changing the size of the source. Somehow... scratching head. Might be time for my flashlight and some props...

[dramatech]

The proposal by fredthe in Post #2 and JChenault in post # 7, are basically saying the same thing, although in a slightly different way, and I think is the answer.

[PeteEngel]

The shutter (douser) in the Topaze is a 4 blade, in plane shutter. it removes light equally from the pool of light without reducing the size of the beam. one effect of this is that it kind of acts like an F-Stop on a camera, so at the distance described (close to the lens) and at a low intensity, it is shifting the focal point a bit, thus drawing the screen into focus on the deck.

[PeteEngel]

also, you wouldn't see this with a S4 or 360Q since the tungsten lamp dimmed by a dimmer acts differently from an arc source with a mechanical shutter.