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Sunday, May 29, 2011

••◊ 5500k fluorescent lights...yeah right!

My previous post about the color shift of the Fader-ND neutral density filter got me thinking about another issue I've run into in the past.  I was setting up a shot last year after buying the Promaster 3-in-1 Cool Light shown below.  It's a simple soft box with four 85-watt fluorescent bulbs.  The bulbs *say* they are color balanced for 5500k, but in actuality the color balance of the camera doesn't come out at 5500k when lit with this light.  Sort of like the previous post, I wanted a short cut to white balancing if I didn't have a white balance card with me and the scene was dominantly lit by this light.

So...  I set up my Gretag White Balance Card and lit the card with the fluorescent light and read the white balance using the histogram on the back of the camera.  Because I didn't have a picture of the histogram for this blog post I pulled the footage into Premier/Photoshop and inserted the histogram.  You can see that blue is much less dominant than red or green.  As expected, green is dominant because this is a fluorescent bulb and that's what phosphorus does.  Even in the expensive professional Kinoflo lights, they have a nasty green spike in the spectral response.  Given all the downsides of tungstens and the bling-worthy prices of LEDs, I'll take the problems of fluorescent lights any day of the week.     

Starting with a white balance setting of 5500k I experimentally adjusted the white balance shift to (B2,M5) to adjust the white balance back to neutral.  As you can see in the last picture, the white balance card is now back to a neutral color.  So now when I'm in a pinch I know to set the white balance to 5500k and shift the white balance to (B2,M5).  No white balance card required.

There's always a downside to shortcuts, right?  Here's the deal with this method - it only works as a full system including the lens.  In a previous post I showed how different lenses from Canon render not only color but also exposure differently, especially if you use color temperature for white balance.  So if I was to shift to a Zeiss lens, for example, the white balance shift may be different.  It will also be different for a Tamron, Sigma, or non-L series Canon lens.  So this cook book white balance shift value doesn't always hold the same value.  The experiment has to be repeated on a lens by lens basis.  Pain for some, but my lens selection is fairly limited so no big deal here.

1 comment:

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