Lighting Q and A, 09-12-08

This week's questions are all about gels. Hit the jump for the light-sucking, color-shifting details...




James, who would tell us where he lives but then he would have to kill us, asks:

"If you gel a flash, say a full CTO for instance, does the power output of the flash alter the color thrown? (I didnt know if it had the same effect as say dimming a tungsten bulb where there is a dramatic color shift between full and almost completely dimmed.)"


James, the color of the gelled flash will remain consistent. The only thing that is actually changing as you dial down a speedlight is the amount of energy that is being discharged through the tube. The tube always flashes at (almost) exactly the same color.

For those who have not yet discovered the color shift in incandescent bulbs, take note: If you are trying to balance tungsten ambient up with a CTO'd flash, crank those ambient lights up. They only balance well at full power.

But if you want a deep, ambient red (for effect, say, in a candle-lit room) dim the lights down as far as you can stand it. Awesome colors that way. You can eyeball it, too, as you get close to low power.
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Cheryl, from Toronto, Canada asks a related question:

"How much light does my CTO filter soak up? I would like to know how much to raise the power on my flash to compensate."


Excellent question, Cheryl, and if you memorize a couple numbers, you can really help yourself.

A full CTO gel costs you 1.1 stops of light. For all practical purposes, I think of it as one stop.

Which is to say that if your flash (at 1/4 power) is giving you the proper exposure at a given distance, and you slap on a CTO gel on there, you'll either need to open up a stop or move up to 1/2 power. The partial CTOs follow the trend: 1/2 CTO costs a half stop, 1/4 CTO costs 1/4 stop, etc.

The CTBs are a little less efficient, and weirdly nonlinear: Full CTB = ~1.5 stops, 1/2 CTB = ~1 stop. And a typical fluorescent (green) conversion gels cost about 1/2 stop of light loss.

For the most part, if you remember that CTO fractions equate to lost f/stops and FL greens cost half a stop, your flash workflow will be much more intuitive. A basic familiarity with light loss numbers will help to save you lots of exposure chimping during a shoot.

You can get full light-sucking information on all of the gels made by Rosco on their site. The RoscoSun numbers, for instance, are here.

If you do not know your CTOs from your CTBs, you can get more info in the gel sections of L101 and L102.
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Tim B., also apparently from no fixed address, asks:

"About bouncing strobes off the ceiling - I did this at a recent wedding reception and the light was nice. But the ceiling was somewhat yellow. I wonder if there is a way to set custom white balance on the camera to match the color of the bounced light. Have you ever done this?"


That depends upon a couple of things, Tim. First, how much is the ambient contributing to the photo? If you are nuking the room by bouncing several big flashes into the ceilings, you can shift the whole scene easily in post-processing to compensate. This, of course, assumes your ceiling is at least reasonably close to neutral. If it is chartreuse, your screwed.

If you are shooting RAW (which for a wedding you should be) grab a bounce light shot of a grey card, white sheet of paper, bride's dress, etc. Correct that item to white in your RAW importer and use that color setting for all of the pix in that scene when you import.

But if you are mixing in ambient with the flash (which is done by your chosen shutter speed, of course) then you got problems, my friend. Because when you shift the picture to fix the off-color bounce light, you'll move your ambient color in the opposite direction, too.

Here's the best fix I can offer: Keep a full set of Rosco sample gels in your bag for each flash. Grab the color the most closely complements the color shift of the ceiling. For instance, if the ceiling was a slight yellow, you might blue your flashes just a tad. Maybe a 1/8 CTB. Then your flashes will help to compensate for the ceiling color somewhat and you will not have to correct nearly as far.

This is not en exact science. But you can set your camera on daylight and do a quick test pop or two with your flash and see how much you are helping yourself with the gels.

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Thanks for the questions. Please keep them coming in the comments of this post, or here.