Janusz Welin , Jan 11, 2009; 09:37 p.m.
Hay,
So I am construction a fairly classy DIY Beauty dish and realized that I can't figure out how far away from the bulb to put the reflector (a convex mirror) inside the dish...
I am working with a classic Elinchrom 500 strobe. I have rigged up a system which allows me to change the hight of the bounce reflector that bounces the light all around the dish. However, I am uncertain of what the optimal distance from the bulb to the reflector should be.
Intuitively, I imagine it should be as close as possible, given that for every inch the mirror moves away that adds two inches of distance the light needs to travel (from the bulb, to the reflector, to the dish.) So I imagine the perfect distance would be, as close to the bulb as possible while allowing full coverage of the sides of the dish on the rebound.
So, if there are any lighting nerds out there (No offense! You make the world a better place!...) please fill me in on how the a beauty dish is designed to modify light. I know I am basically playing pool with light but I'm unsure of the optimal the size that the pool-table should be..
Now, I'm prob over-thinking this, (often the case) but I would be curious to hear the science behind the way this light works to effect light output.
Thanks!!
Janusz
Sheldon Nalos 
, Jan 12, 2009; 12:47 a.m.
I actually think you want to go the opposite way.... get the mirror farther away from the light source, but not so far away that direct light from the flash head or reflected off the mirror can creep out of the system.
The goal is to reflect as much of the light as possible into the pan, not back into the flash head itself. The pan is more of a diffuse light source, so you just want to get light onto the inside of the pan. The more you move the mirror away from the flash tube, the greater the percentage of light that gets reflected away from the flash into the pan that can then reach the subject. If you just stuff the mirror on top of the flash tube, you are blocking light.
One helpful approach is to just take a picture while firing the dish straight on into a plain wall and see what flash-to-mirror distance gives you the best coverage and most even light without any hotspots.
Joseph Wisniewski , Jan 12, 2009; 04:08 p.m.
Actually, you tune the little reflector until its shadow just clears the edge of the dish. The flash tube is far enough back in the dish that 75% of the tube's light hits the dish, so power conservation isn't an issue. So, the "purpose in life" of the small dish is simply stop the 25% or so of the light that would make it directly from the tube to the room. If some of that light bounces back to the dish and gets going in a more useful direction, fine. If not, no small loss.
So, an inch from the tube is probably too close, you'll block light that shoulr be hitting the dish. As far as possible is too far...
Anything more precise, and I'd have to know the parameters of your dish.
Robert Jewett , Jan 19, 2009; 11:31 a.m.
Mine ended up at less than an inch.
Rene Gaviola , Jun 26, 2009; 01:50 a.m.
I am also in a similar situation.
My beauty dish has a diameter of 18" while the convex mirror is 5 1/2". Am using a strobist setup, probably a SB-800 or a Vivitar 285 and would like to know how Janusz and others ended up w/ the flash to convex distance.
Also to ask if attaching a diffuser to the flash would help?
Many thanks!!!
- Rene
