Gel advise

jambredz

jambredz

Veteran
Hey guys...i'm looking to improve my cinematography skills and would like some advise on gels. I know the basics and how to white balance b4 or after for either colour correction or to add effects ect. Most of this is theory, so i just bought some gels (full ctb, 1/2 ctb, and ND gels) to practise on some worklights (i dont have a light kit as yet). What i want to know is:

1.what are situations where you would use 1/2, 1/8, 3/4 CTB of CTO gels. like what situations would require u to want these graduations instead of a full gel. Time of day and the particular colour temp at that time of day??

2. the different ND gels and how much light they cut.

3.And are gels universally labelled. I see codes like 201 and 218 ect for lee filters online. are these lee specific or universal codes.

If u guys could point me to a link to help with my questions i would really appreciate it.
 
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1. I choose a specific gel density to best match the other sources I am trying to mix with. Maybe I want a light to be a little warmer, so I only use 1/4 instead of a half, I rarely use full. My other consideration is how much light I will lose from each density.

2. ND cuts 1 stop per every .3

3. The numbers are Lee specific. It is common practice to write the density in the corner of the gel. The only time I hear anyone refer to the numbers is when talking about diffusion (216, 250, 251).
 
Get yourself a Lee samples (what would you call it? Its a ring with small swatches of all their gels). Anyway good to have handy when youre looking at how youre going to shoot something.

Remember that your blues and oranges will depend on your white balance. If you use a full ctb on tungsten your going to end up with a daylight balance. And your light will look white. If you stick a full CTO on a HMI your going to get about a 3200k balance, and your light will look white and its the reverse for those lights without gels.

But as kkeller said, if you have total light control (no sunlight coming in or other uncontrollable source), do what suits. This can help you control youre light output as well. So say your 1k light is too hot, throwing a half ctb on will knock it down, white balance to that as a start point. Then when you add a full CTB to another source it will look blue.

Theres so many things that can be said on this topic, but I always have the following on hand.

- .3/.6 ND
- 1/4 - 1/2 - FULL CTB and CTO

Thats your start point and then I like

- straw yellow
- magenta and a couple of other magenta variations
- green (have to check swatch for exact name.
- couple of forms of diffusion.

But I go with primaries, and then you can mic them for other colors.
 
The sample books help along with lots of testing, but one really needs a color temp meter &/or an on-set CC monitor do this effectively.

Unfortunately the human eye/brain adjusts for color too well and as a result we can end up having difficulty judging color properly. The whole Color temp situation is a perfect example. The human eye/brain doesn't need a color filter. Why?, because we automatically adjust for differences and intrepret white as being white no matter what it actually is. This is an excellent adaptive survival trait but it can make lighting a little tricky.
 
You should label your gels with a circle for full, a circle with 1 line down the middle for half, then cut the circle into quarters for quarter, so on and so forth. I've found this method is much better than just writing, 1/4 or 1/8th. It's much easier to distinguish the symbols than distinguish someones crappy handwriting.

As far as ND gels, for every .3 it's 1 stop of light. So, if you have an ND .9, it cuts 3 stops of light. A stop is a halving of the light, so if you put a ND .3 gel on a 1k lamp, it now becomes a 500 watt lamp. ND .6 would make it a 250 watt lamp. I generally don't like to use ND gels, and over my few years of freelancing, I've only seen one person ever call for an ND gel for a lamp. Of course for windows and other thing it's a different matter, but if you need to bring a lamp down, just use scrims or back it up. Much cleaner that way.

Also, just remember full CTB corrects 3200 light to 5600, full CTO corrects 5600 to 3200. I once saw a DP throw up a bunch of HMI's behind someone with some 1/8th or 1/4 CTO to give the talent a sort of sunny summer day sort of rim. You could also throw 1/4 CTO or something on a light that's supposed to be from a desk lamp or table lamp at night. Just to give it a little bit of a tungsten feel...

I've also used 1/2 CTB when the only source was moonlight, or when I wanted a source that had to look like moonlight. Some will say that's too much, or too little, but it's all personal preference.
 
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