Genesis light shots!

Rudi explained it pretty well. But think of the guide number as a specific output at a specific distence. You will see it expresses like... 140 at 10 feet. It does not mean a 140 shutter if the model is 10 feet away. It is 140 on whatever scale is used to rate strobes, measured at 10 feet.

I found this to help....."The GN is found by multiplying the flash-to-subject distance by the f-stop required for a correct exposure of the subject at that distance. Although any distance could be used, 10 foot is the easy standard. "

Light manufacturers print their "GUIDE NUMBERS" to give you a general idea how much light is being put out by the unit, but as Mark1 indicates - you have to be sure you're comparing apples-to-apples. If everyone printed their Guide No. using a 10 foot distance we'd have a true standard ... so you'd know that the light kicking out 160 is brighter than the 125. Problem is sometimes they state their guide number using a different distance (the little buggers) to make their lights seem brighter. Example: If I produce lights with Guide No. 125 at 10 feet ... that's far better than Guide No. 125 at 6 feet. Knowing the guide number AND the distance used for their measurement would technically let you mix-n-match lights, then mathematically figure out the distance to place each light for equal exposure. But to heck with all that techno-talk. Since virtually all modern lights have power dials or LED settings - just place your lights where you want 'em and dial in the correct power amount desired , dialing down the stronger light until it matches the weaker if that's what you're after. (Probably need assistance from a light meter for all that.)

The top shutter speed of each light is indicated by manufacturers as a synch-speed (not the guide number). You can always shoot SLOWER than the rated shutter speed, but if you shoot FASTER than the rated shutter speed you end up with the camera curtain either opening too quickly or closing too quickly for the flash to light the entire frame. That may be what happened on your sample image along the right edge. The curtain was already in the process of closing when the flash went off. If you have lights with 2 different shutter speeds just use the slower synch of the two ... not the faster and you should be okay in most instances.

Please forgive if I've restated the obvious. In these forums you never know how much knowledge the various posters or readers have so I certainly don't intend to speak down to anyone with known info. Heck - sometimes it shows how little I know when I say something stupid. LOL

Jason- all very well explained. Should be pretty clear to 2112 and any others now. Thanks.

To help in the comparing of lights..... You have to quadruple the light output to double the guide number.

mixing strobes from different manufacturers. Requires a GREAT deal of understanding WB, strobes output. Robert and Curtis did some great write-ups for that.

Robert's
https://photography-on-the.net/forum/showthre​ad.php?t=469075

Curtis'
https://photography-on-the.net/forum/showthre​ad.php?t=443418
https://photography-on-the.net/forum/showthre​ad.php?t=429444

Both are a most read for learning. Also the 40D will NOT x-sync higher than 1/125, unless using hardwired SpeedLites (580EX, etc). So you have major issues here. Shutter, WB and exposure.

Ah I see, thanks. Mixing the strobes wasnt really a problem. I had one AB and one Genesis on each side. It was the right side that was a bit off. That had the B400 with the medium softbox and 200 Genesis with umbrella. Will try shooting something other than 250 when I get them and see what happens!