Posted 12 July 2003 - 09:44 PM
For the rear gelatin holders you buy the red or magenta color correcting gels and cut them to fit. If you also want warming you add a warming gel. I haven't had any trouble putting two gels in the holder although it's a little fussy. There are no flourescent gels to my knowledge but I'll explain how to make one below..
Red and magenta gels are specified by their strength (usually in increments of 10) and either an "R" or an "M" Alex recommends a "CC 30 R" or "CC 40 R" I believe (CC means color correcting). I've used the "CC 40 M". The M filters pass more blues and violets but attenuate yellows more than the R filters. M filters are definitely better between 30-70 feet. From 30 feet to the surface I don't know, but you've seen Alex's results and they're hard to argue with. Apparently Nikon sells a CC R filter for underwater use if that helps you choose.
The warming gels are the 81 series and the 85 series and are used to lower color temps. Their values are specified in "mireds" and their effect is given by the following formula:
CCT(new) = 1 / (1 / CCT(old) + mireds/10^6)
Mireds are much handier than Kelvin when talking about color temps and filters since you just add them up. Instead of saying daylight is 5500K you say it's 182 mireds. Halogen lights are 313 mireds. If you want to convert halogen to daylight you need a -131 mired filter. The 80A is just that. Easy, eh?
The 81 series filters consist of the 81, 81A, 81B, 81C, 81D, and 81EF with values of 9, 18, 27, 35, 42, and 53 mireds.
The 85 series is stronger and consists of the 85C, 85, and 85B with values of 81, 112, and 131 mireds.
A Singh-Ray FL-B is 147 mireds plus CC 50 M. A Hoya FL-D is 34 mireds plus CC 30 M. These are my measurements. You can see that a variety of flourescent class filters are easily constructed with a combination of two gels. That's what you do with a rear holder.
The filtering behavior of water varies with quality and time of day, but I believe that blue water at noon on a clear day acts like a -5 mired + CC 2 G (green) filter PER foot. Some believe that a higher value of CC 4 G per foot is closer. Since an M filter is the direct opposite of green, a Singh-Ray FL-B is a nearly perfect blue water filter at 30 feet.
As if this wasn't enough to think about already, I'll mention that in Adobe Camara Raw you have two controls that adjust white point, color temperaturre and tint. Color temperature corresponds to mireds and tint corresponds to the magenta/green balance. Moving left on tint is like adding a CC G filter. Moving right is like adding a CC M filter. Since filters modify light before the sensor they are actually better than digital adjustments since they balance exposure beween R, G, and B more evenly. If anyone using ACR is noticing that they are consistently at +20 or more on tint they should consider trying a B+W FL-D. It'll move your tint 20 points left I promise.
I love it when a plan comes together.
- Col. John "Hannibal" Smith
------
Nikon, Seatool, Nexus, Inon
My Galleries