Scientific Gelling

I took some shots last night using my new AB as key light and a 580 EXII as a hair light. I expected the hair light would be a tad cooler than the key light; however, it turned out to be much cooler than I anticipate.

Todays task is to determine what gel to use on my flash to most closely match the color temp of my strobe. My plan is to shoot a Grey card w/ my AB, set my camera to custom WB, take a few shots of the Grey card using various strengths of CTO, import all the files into lightroom, and then use the eyedropper to set the WB in Lightroom. The gel I used in the photo which the color temp changed the least during Lightroom adjustment will be the gel I use on my flash when I'm using it together with my strobes.

My question is this, "does anybody know of a better way to choose the correct gel"?

doidinho wrote in post #6294326
My question is this, "does anybody know of a better way to choose the correct gel"?

I don't know a faster way, but would advise you to follow your procedure repeatedly with different output power levels, since color temperature is prone to change with output power level.

Well, I completed the test last night. The results are that the 1/8 CTO drops the blues down a bit too much, result in slightly warmer light, and the 1/8 CTO doesn't drop the greens down enough, giving a slight green cast.

I think I will end up just using the 580EXII either for a background light or for a hairlight (with the 1/8 CTO) when I'm using my bees.

Understand that the CTO affects the Blue-Yellow color balance axis of the color spectrum, and it does NOT have an effect on the Purple-Green tint axis of the color spectrum.

The AB shift is along the Magenta intermediate 'axis' (between Blue-Yellow and Purple-Green) so adjusting with the CTO partly corrects since it has no effect in te Purple-Green direction.

Wilt wrote in post #6299400
Understand that the CTO affects the Blue-Yellow color balance axis of the color spectrum, and it does NOT have an effect on the Purple-Green tint axis of the color spectrum.

The AB shift is along the Magenta intermediate 'axis' (between Blue-Yellow and Purple-Green) so adjusting with the CTO partly corrects since it has no effect in te Purple-Green direction.

I did my experiment at w/ my bee set to about 1/8 power. The greens didn't seem to be too far off from the reds; it was the blues that were more prominent in the speedlight than the bees. From what I have heard, the magentas don't start coming out until about 1/16 power.

I think a 1/16 CTO might give very good results (if they made one). I may try gelling only half the speedlight with the 1/8 CTO and see if I can do any better.

doidinho wrote in post #6300053
I did my experiment at w/ my bee set to about 1/8 power. The greens didn't seem to be too far off from the reds; it was the blues that were more prominent in the speedlight than the bees. From what I have heard, the magentas don't start coming out until about 1/16 power.

I think a 1/16 CTO might give very good results (if they made one). I may try gelling only half the speedlight with the 1/8 CTO and see if I can do any better.

1/8 gel is as light in density as they make them.

You may need to also put a Color Compensating filter on one of the lights to truly balance the two, not merely rely upon the CTO for matching them.

Also the Magenta drift is the Difference between using the AB at full vs. at 1/16 power, but it does not indicate the absolute neutralness of the starting point, you only know that it 'becomes magenta' than the full power control measurement.

The approach I would take, in trying to gel to match, is to put each light at the power level expected it to be set to, then shoot a white paper or gray card with each light individually, then use PP to determine the WB and the tint value of each light, then balance with filters appropriately.

Or purchase a Sekonic C-500 and measure each light separately.

Titus213 wrote in post #6301089
Or purchase a Sekonic C-500 and measure each light separately.

Thanks for the idea, but I really don't want to go there. I'm sure it is useful necessary for certain types of professional shoots; however, I'm just looking for a hair light thats not blue.

I made a specific gel for my speed light and am liking it. It's a 1/8 CTO, trimmed a bit in the smaller dimension, with a small piece of magenta in the middle. I put it on my flash in vertical direction so that it only covers about 1/2 of the head. I matched it to 1/4 power on the strobe which I hope will be close enough for a hair light.

And I'll bet you it didn't cost you a grand either....good work.