accounting for temperature changes with gels

[mark_allen6]

<p>I recently bought some CTO and CTB gels for use with my flashes. The Rosco literature describes the change in K that occurs with each gel according to their expected use, but I'm not sure if I can apply the temp difference in scenarios other than described by Rosco. For example, Rosco states that a 1/2 CTB increases the temp from 3200K (~standard tungston) to about 4100K. So, if I want to use the 1/2CTB with my strobe (say, to saturate the sunset) but balance for my foreground subject (lit by strobe) - can I just add the 900K difference (4100-3200) to my 5500oK strobe output and set my WB temp to 6400K for a natural flesh tone?</p>

[mark_allen6]

<p>I just looked in another Rosco bulletin and saw that the temp difference is not linear, so I can't just add or subtrate the same number to get a correct white balance for my subject. The brochure does include a calculator graph of sorts, but I will need to make such adjustments on the fly. I do have an expodisc, so I could just set custom WB for my subject that way, but I would rather be able to use a specific kelvin WB setting so I can warm or cool the subject at my discretion. Other than the use of the Rosco graph/calculator in the field, do any of you have any simple and convenient solutions for this issue?</p>

[picturesque]

<p>Shoot RAW. I generally dial in the K temp on my camera based on the K temp that the gel is supposed to get one to. So if the gel says 3400K, that's what I use, using the K temp control on my camera. But I shoot RAW, so I can fine tune later in post. You cannot judge by the LCD, so in field fine tuning to get everything perfect is just not possible. Even a custom white balance is not completely perfect, because light will spill unevenly...</p>

[pete_s.]

<p>Mark, check out rosco's filter fact. <cite>www.<strong>rosco</strong> .com/includes/technotes/<strong>filters</strong> /<strong>filterfacts</strong> _06.pdf</cite><br>

To see what color temperature a 1/2 CTB will do to your flash you use mired to calculate.<br>

5500K is 1000000/5500=182 mired<br>

1/2 CTB is -68 mired shift according to rosco<br>

So 182-68=114 mired<br>

and 1000000/114=8772 Kelvin.<br>

So a WB somwhere around 8700-8800 K should do the trick.</p>

 

[mark_allen6]

<p>Thanks Nadine, I do shoot raw and thus can adjust (after the fact) the K temp, but of course would rather get it right in the camera, and since the Rosco info only states the change in color temp, not the final output (which depends on the original source), I was still uncertain of how to set my cameras K. <br>

Pete, thanks for the calcs, when I referred to the graphical calculator in the Rosco bulletin I came up with about 8500 (very close to your calc), but when I tried to apply my sorry algebraic mind to the bulletin's actual formula, I came up with 9500K - so your value supports my former estimate. Thanks for the backup estimate (and the simplified calculation method)!</p>