Flash Test Circles

Flashes have a shape and that shape is expanded by distance we know that the amount of light is diminished by the distance, and by observation I know that the size of the shape gets larger the further away.. Alien Bees 800 light I have, I have a reflector ring of 80degrees, I'll measure the diameter of the reflector, place the flash on a stand and then measure diameter of the circle as I move the flash backwards. Pictures is taken at an angle because I couldn't take it straight on because the light was to close to the wall. There is clearly different zones, so I took out my which is a Polaris light meter, used incidental light metering, and reflective light metering, and actually got the same values when metering the same zones. The value I left out of the picture is ISO 200 at shutter speed 30.

I took another picture with a tape measure put on the wall where the light circle was located found that each zone was smaller in the size at the one before by one inc, I'm going to name the zones 5 for the middle (funny how there is 5 zones here) anyways.. in inch. zone V (the middle one) is 7inc, zone IV is 6 inc, zone III is 5 inc, zone II is 3 inc, zone I is 1inc. Zone V: 7inc Zone IV: 6inc Zone III: 5inc Zone II: 3inc Zone I: 1inc now I move back the light to 2feet from the wall. I think that zone one will be half as bright and will be 2x bigger in size. wrong again. it actually went down 2 stops

the zones are bigger, but the power went down by 2 stops not just one. Zone V: 16 inc Zone IV: 13 inc Zone III: 12 inc Zone II: 10 inc Zone I: 8 inc moving to 3 feet back from wall.

fstops are 2.8 2, 1.4, 1 zone sizes: Zone V: 31inc Zone IV: 25inc Zone III: 20inc can't measure the other to hard to tell difference between wall and light. move back to 4 feet. fist all I realize I need a bigger test area, and I'll need to test at night. can't do that right now.. so I tried to use the flash itself, more light, and smaller fstops.. maybe will work better, because it's not very bright day today. light in the room without flash is at shutter 30 fstp 1.4.. that is a problem with low light test. redoing testing with flash... first I set flash to lowest power, because that metered at 125/f32 problem i have here is I don't have f/32.. oh well.. I take picture anyways at 125/f22

basically same values just higher flash power... see how zone V looks like it blasted it's way into the middle only other problem now is I can't measure ... oh well. now going to shot at 2 feet it meters at f11... just for fun I'm going to keep my fstp at 22... so to keep the picture the same.. only change is the distance from the wall. I see there is a ring in the middle for the 2foot mark, I think that is a reflection from the wall to the ring on the flash, and then reflecting back. also I see that each zone seems to have a brighter ring around it.

now I'm going to move back to 3feet middle meters at 125/f8 but still taking the pictures at 125/f22

then at 4 feet

now going back to 1 foot but using a 12inc umbrella I'm placing the rim of the umbrella at 1 foot. the max fstp is f11 so that is where I'll place my fstp.

now at 2 feet

3 feet

what I'm seeing, is not much that using an umbrella defusses the light, it actaully makes the light bigger. what my theory light and more on, strobe lights, is that I would decript the way the light reaches the subject is more like in the shape of an ice cream cone which would explain why in the middle of the light circle it's brighter, and furter away from the middle the less light there is.

I think I have a formula down now to calculate the size of the circle. center Zone 5 say = f11.. so now at 4ft and the guide number for the flash is 32. (yes crummy flash)... f11 is gn / feet. and we know gn = 32.. so 32/ft = 11, what is feet? so it would be 32/11 which is about 3ft.... however the flash unit itself is 4ft away from the wall in this case... so now we take the next formula size = sqrt(3 ² + 4 ² ) ... size = 5ft. so f11 would be 5 feet from the center of the flash circle.

I've perfected the formula (I think anyways) because we already have the Guide Number / feet. so really all we need to know if the Guide Number and distance to subject. so formula is simply. S = ( Sqrt ( (Gn / Ft) ² + (Ft / Ft) ² ) ) * 2 S = size Gn = Guide Number of Flash Ft = feet then to get to the next f stop down you take first part of the formula and add in ((Gn/Ft) / 1.4) here is a java applet that I programed to calculate the Size of the zones of the flash. I've based it off my Studio lighting, I've found that yes there is set - F/stops for power, however I still can set the value in between... one more thing I realized after making the java applet, was that while my testing seems correct, and my formula seems correct based on my theory, one thing I didn't test is that I has tilt or turn on the flash, I only had the flash square with the wall (being my subject). So there is not any tilt or turn values into the formula. I'll find those equations and add those into my formula. My guess is, I can use the same formula as used for large format camera.