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Thread: CTO, CTB and their partials

  1. #1
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    Default CTO, CTB and their partials

    Can someone help me better understand the use of partial correction for balancing with CTO or CTB gels? I understand the basics well enough but haven't had much chance to use or see the results of using a certain known correction value. How would you judge using 1/2 CTO instead of full or 1/4, etc? I'm in the dark here! Also, why correct a light instead of a window, or window instead of a light, for example?

    Thanks for sharing any experiences!

  2. #2
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    Default Re: CTO, CTB and their partials

    Well, it depends on a few things, mainly the amount of correction you want as well as the amount of light you have available. Remember, CTB gels will cut your light more than a CTO. The same goes with camera filters #80A as compared to #85. A full CTB will cut your light two stops, a half CTB one stop, and so forth. A full CTO will only cut your light 2/3 of a stop, and so forth down the line.

    If you're lighting indoors and it's daytime outside, you should be fine using tungsten lights and using 1/2 CTB gels on them. Any color differences can easily be timed out during your telecine. If you have more light available tho, you could definitely go with a full CTB.

    You could also go with partial correction if you want to create a certain look. Say it's daylight outside and you're lighting inside to match daylight, but there are some tungsten practicals. You could cool them off a little bit by putting a 1/4 CTB on them.

    Regarding on correcting windows vs. lights, it'll depend on a few things. It's generally cheaper to gel lights as compared to entire windows. You'll want to match everything to your primary light source tho. For example, if you were shooting indoors during the day with tungsten film, you could gel the windows or get an 85 filter for the camera to convert the daylight to tungsten light. If you had any tungsten lights indoors and wanted to warm them up, you could use some amber gels or something.

    I hope this helps and isn't too confusing!

    John-Paul

  3. #3
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    Default Re: CTO, CTB and their partials

    Thanks for the info. But I'm still a little confused on the color correction side of it. So CTO/B rated at 1/4 1/2 etc. reflect the stop change in whatever source they convert? But for color, a 1/4 CTB on a lamp for example, would partially correct the red light to blue and a full CTB would change it all the way to blue (Daylight)? This is where I'm still not 100% clear. The rating of the gel is related directly to how far a color change they make, as well as how much light they absorb? I guess someone just has to get used to what's right for a scene then... like, how red do I want those lamps to stay, etc? Must be an experience issue then.

  4. #4

    Default Re: CTO, CTB and their partials

    The number on the gel does not refer to stop-loss, just strength of the effect as a fraction of the Full version.

    They are designed to alter color temperature mainly, although CTO (Orange) versus CTS (Straw) vary in terms of shade of orange -- Straw is more yellow, less reddish. But a Full CTO or Full CTS will convert daylight (5500K) to tungsten (3200K).

    A Full CTB (Blue) will convert tungsten (3200K) to daylight (5500K).

    But gels are also used to warm up or cool down lights for artistic effect, and there, you're more likely to pick a strength by eye or experience (like whether you feel 1/4 CTS is good for a warm night interior effect).

    In terms of stop loss, the Full versions are the same as their camera filter equivalents. The 80A blue filter, which converts 3200K to 5500K, is nearly 2-stops loss and is the same as the Full CTB gel. The 85B orange filter, which converts 5500K to 3200K, is slightly more than 2/3's of a stop loss, and is the equivalent of Full CTO or Full CTS.

    So probably a 1/2 CTO is like a 1/3 stop loss, or between 1/3 to 1/2.

    Now if you really want to exactly understand the amount of color shift in each strength of gel, you have to learn the MIRED system -- each manufacturer lists the MIRED shift for each color-correction gel (like in the swatchbook.) You can correlate a MIRED shift to the color temp, or vice-versa, if you want to. For example, if you need to gel an HMI or tungsten to 4700K.

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