The Shape(?) Of Light

[TMR Design]

This is a question I was going to ask a while ago when I was trying to make a snoot for my 580EX but I abandoned the idea and forgot to post.

When I took a cardboard poster tube that was about 24" long and put it in front of the flash head and fired the flash I got a tiny, tight circle of light. Then I experimented with other lengths of tube so as to see how short I could make the snoot. I cut off about 8", reducing the length to 16" and instead of producing a round spot of light I got a horizontally elongated patch of light that resembled the shape of the flash head. If I cut the tube down even further the rectangular shape of the flash was even more defined. I can understand why a 4" or 6" piece of tube might do that but I was very surprised when a 16" piece of tube did not round out the shape of the light.

Can someone explain that? I've seen snoot attachments for flash heads and they appear to be no longer than 6" or 8". Do those snoots give you a tight circle of light or do they produce the rectangular shape?

[mpoole]

What was the diameter of the tube and what was the 580ex zoom set at?

[TMR Design]

What was the diameter of the tube and what was the 580ex zoom set at?

The diameter of the tube is 3" and the 580EX was on Manual, set to 1/2 power.

[Emenresu]

I belive if you put a sheet of very translucent material in the snoot close to the flash side it will round out the rectangle shape. I never heard anyone say this or have done it myslef but i belive it will work because it will make the light source round.

[TMR Design]

Perhaps that might do it but I don't understand the physics of what is happening. I thought that anything that would create that column of light would then project the light out the end in that shape. Obviously I was very wrong.

What if the inside of the tube were highly reflective? Or what if it were black?

[DavidEB]

I'll take a stab at it -- the light that comes out the end of the snoot is a combination of light that shines straight out from the flash, without bouncing, and light that bounces around a few times inside the walls of the snoot on it's way out. If the former is vastly greater than the latter, you get a rectangular shaped light. If the bounced light is vastly brighter than the direct light, you get a small round light cone (that diverges a little with increasing distance). More reflective walls will give you a rounder image (and greater divergence of the light cone with distance) and black walls will give you more of the rectangular shape.

As the light bounces off the inner walls, some of it is absorbed with each bounce. The snoot heats up a tiny bit. So a really long snoot would be too dark.

I think the optics follow the Snoot-Hawley law.:eek: