Blue gels absorb so much. at DVinfo.net

Leslie Knox · April 8th, 2009, 05:57 AM

I was wondering how often people use blue gels for colour correction as I often find them to be a pain? I thought it was the quickest and easiest option but the full ctb and diffuser absorbs so much of my lights power (Tota light) that I'm stuck with the bare minimum iris setting. I think I might go with covering the windows with cto instead, despite the extra time.

Brian Drysdale · April 8th, 2009, 07:57 AM

It's not that common for people to gell the windows for colour correction and large windows can be a problem. One advantage about the UK power supply is that you can plug a 2k straight into the wall, which works pretty well with a CTO and diffusion - it's about the same output as a 575 watt HMI. Although it can get warm under the lights.

I suspect part of your problem may that you're shooting HDV and those 1/3 cameras aren't the most sensitive. The 800 watt CTB & diffusion combo is fine with the more sensitive SD cameras or 2/3" HD cameras..

Leslie Knox · April 8th, 2009, 08:24 AM

Brian,

Yes, I'm using the Z1 which is a real pain in low light. As you say with a better/different camera I'm sure it would be less of an issue. I'm going to play around with a half ctb and see if that helps.

Shaun Roemich · April 8th, 2009, 08:35 AM

Leslie: I routinely use 1/2 CTB if I can get away with it. Transmission factor is somewhere in the neighbourhood of 50% (it changes across the light spectrum) versus about 25% for the full blue, based on my Roscolux swatch book. SOME variation in colour temp in a scene is ok by today's standards, depending on the nature of the work and if you are mixing in overhead "daylight" fluorescents, the half blue is closer anyway (just watch out for green spikes if the fluoros are older...)

Shaun Roemich · April 8th, 2009, 08:38 AM

As well, my experience with the Z1 leads me to believe that +3 to +6dB of video gain isn't THAT noticeable, depending on the application. Or a 3 way colour corrector in post allows you to claim back SOME exposure through careful use of the highlight and gamma controls, if you're still a little dark after lighting with correction, ASSUMING you are doing your own post.

Brian Berg · April 14th, 2009, 07:10 AM

Warm balance your camera.

Bill Pryor · April 14th, 2009, 09:08 AM

I prefer to gel the windows when possible, but if they're too big or there's not enough time, I'll gel the lights. Often I'll use 1/2 CTB instead of full and white balance for that, then let the background go just a bit more blue. I found the Z1 to need about 1/2 - 2/3 stop more light than a DSR250 under the same circumstances.

Dean Sensui · April 14th, 2009, 05:06 PM

"Blue gels absorb so much."

If you look at the spectral qualities of a tungsten light, and compare it to daylight, you'll see the tungsten light most of its energy in the red end of the spectrum. Whereas daylight has much more in the blue end of the spectrum.

Filters work by absorbing energy. A red filter absorbs blue. A blue filter absorbs red. A neutral density filter absorbs everything equally.

To get the spectral distribution of the tungsten light to be a closer match to daylight, a filter has to subtract out red. And unfortunately that's where most of the tungsten light emits most of its energy.

So the end result is that the gel absorbs most of what the tungsten light emits to emit a light with spectral qualities roughly similar to daylight.

That's why I'm starting to buy daylight-balanced lights. It's easier to filter a light to match tungsten, when the situation probably needs less lumens, than try to get a reddish light to match daylight, when you need all the output you can get.

Technical correction: Red filters out cyan. Blue filters out yellow.

Richard Andrewski · April 14th, 2009, 06:20 PM

Just pick the color opposite to the color on the color wheel that you want to filter for.

4096 Color Wheel Version 2.1

April 8th, 2009, 05:57 AM	#1
Leslie Knox New Boot Join Date: Apr 2007 Location: London, UK Posts: 20	Blue gels absorb so much. I was wondering how often people use blue gels for colour correction as I often find them to be a pain? I thought it was the quickest and easiest option but the full ctb and diffuser absorbs so much of my lights power (Tota light) that I'm stuck with the bare minimum iris setting. I think I might go with covering the windows with cto instead, despite the extra time.

April 8th, 2009, 08:35 AM	#4
Shaun Roemich Inner Circle Join Date: Sep 2002 Location: Vancouver, British Columbia (formerly Winnipeg, Manitoba) Canada Posts: 4,088	Leslie: I routinely use 1/2 CTB if I can get away with it. Transmission factor is somewhere in the neighbourhood of 50% (it changes across the light spectrum) versus about 25% for the full blue, based on my Roscolux swatch book. SOME variation in colour temp in a scene is ok by today's standards, depending on the nature of the work and if you are mixing in overhead "daylight" fluorescents, the half blue is closer anyway (just watch out for green spikes if the fluoros are older...) __________________ Shaun C. Roemich Road Dog Media - Vancouver, BC - Videographer - Webcaster www.roaddogmedia.ca Blog: http://roaddogmedia.wordpress.com/

April 8th, 2009, 08:38 AM	#5
Shaun Roemich Inner Circle Join Date: Sep 2002 Location: Vancouver, British Columbia (formerly Winnipeg, Manitoba) Canada Posts: 4,088	As well, my experience with the Z1 leads me to believe that +3 to +6dB of video gain isn't THAT noticeable, depending on the application. Or a 3 way colour corrector in post allows you to claim back SOME exposure through careful use of the highlight and gamma controls, if you're still a little dark after lighting with correction, ASSUMING you are doing your own post. __________________ Shaun C. Roemich Road Dog Media - Vancouver, BC - Videographer - Webcaster www.roaddogmedia.ca Blog: http://roaddogmedia.wordpress.com/

April 14th, 2009, 05:06 PM	#8
Dean Sensui Inner Circle Join Date: Oct 2001 Location: Honolulu, HI Posts: 2,054	"Blue gels absorb so much." If you look at the spectral qualities of a tungsten light, and compare it to daylight, you'll see the tungsten light most of its energy in the red end of the spectrum. Whereas daylight has much more in the blue end of the spectrum. Filters work by absorbing energy. A red filter absorbs blue. A blue filter absorbs red. A neutral density filter absorbs everything equally. To get the spectral distribution of the tungsten light to be a closer match to daylight, a filter has to subtract out red. And unfortunately that's where most of the tungsten light emits most of its energy. So the end result is that the gel absorbs most of what the tungsten light emits to emit a light with spectral qualities roughly similar to daylight. That's why I'm starting to buy daylight-balanced lights. It's easier to filter a light to match tungsten, when the situation probably needs less lumens, than try to get a reddish light to match daylight, when you need all the output you can get. Technical correction: Red filters out cyan. Blue filters out yellow. __________________ Dean Sensui Exec Producer, Hawaii Goes Fishing

April 14th, 2009, 06:20 PM	#9
Richard Andrewski Major Player Join Date: Jun 2006 Location: Shenzhen, China Posts: 781	Just pick the color opposite to the color on the color wheel that you want to filter for. 4096 Color Wheel Version 2.1 __________________ Richard Andrewski - Cool Lights USA - RED #114 http://www.coollights.biz

April 8th, 2009, 07:57 AM	#2
Brian Drysdale Inner Circle Join Date: Feb 2006 Location: Belfast, UK Posts: 6,152	It's not that common for people to gell the windows for colour correction and large windows can be a problem. One advantage about the UK power supply is that you can plug a 2k straight into the wall, which works pretty well with a CTO and diffusion - it's about the same output as a 575 watt HMI. Although it can get warm under the lights. I suspect part of your problem may that you're shooting HDV and those 1/3 cameras aren't the most sensitive. The 800 watt CTB & diffusion combo is fine with the more sensitive SD cameras or 2/3" HD cameras..

April 8th, 2009, 08:24 AM	#3
Leslie Knox New Boot Join Date: Apr 2007 Location: London, UK Posts: 20	Brian, Yes, I'm using the Z1 which is a real pain in low light. As you say with a better/different camera I'm sure it would be less of an issue. I'm going to play around with a half ctb and see if that helps.

April 14th, 2009, 07:10 AM	#6
Brian Berg Regular Crew Join Date: Apr 2009 Location: Clare Michigan Posts: 71	Warm balance your camera.

April 14th, 2009, 09:08 AM	#7
Bill Pryor Contributor Join Date: Jan 2003 Location: Kansas City, MO Posts: 4,449	I prefer to gel the windows when possible, but if they're too big or there's not enough time, I'll gel the lights. Often I'll use 1/2 CTB instead of full and white balance for that, then let the background go just a bit more blue. I found the Z1 to need about 1/2 - 2/3 stop more light than a DSR250 under the same circumstances.