I am sticking to my promise and delivering content that many of you requested in the survey a few months ago on blog topics. This is an interesting one that not many people talk about. It is absolutely one of the most essential tools for a cinematographer. Color correction gels can be used many ways, but what I will discuss is how to use them in subtle ways to create color contrast with the new digital sensors.
When I first became a cinematographer, there were two companies in the gel manufacturing business, Rosco and Lee. A few years later, a new kid on the block emerged, GAM Filters. They had been big on party colors, aka theatrical colors, and jumped into the color correction gel business as well. Back in the day, CTO was the only way to warm your lights up in increments. You had: 1/8, 1/4, 1/2 and Full CTO.
In the mid-nineties, Rosco introduced a warming gel called CTS, which had more yellow than orange in it. I prefer yellow over red and orange as a color on skin tones. The CTO line seemed to deliver more red and orange. With the digital sensors, I find that the CTS works the best on skin tones, giving that vitality and golden glow.
When I use HMI lights, they all tend to be a little cold to my eye so I warm them up a little bit to start. Mornings are more calm and clear, which gives you that greener, cleaner look at sunrise. Sunset is a more an orange feel and tone, mainly because all the particles that have been stirred up during the day add a warmer feel. Obviously this is just my personal observation. I prefer to use the best aspect of CTS and CTO and not just one gel for all looks.
When I lensed this scene in The Greatest Game Ever Played, I used CTS to deliver a more yellow tone, which I felt fit the morning light that we described above, with that green at sunrise.
It is really important to experiment with your color temp meter and watch the sunrise and see what your Kelvin does. Then do the same at sunset. Notice your green levels as well. This is where I came up with the whole thing, baseing decisions on science and observation, not just creative choice. Great lighting is the synergy between Art and Science.
When it was time to pull off a late afternoon dinner scene in Greatest Game, I turned to CTO with its little extra orange to deliver the mood and tone. This required me to combine Full CTO and ½ CTO on my 18Ks out the windows. This is where GAM Filters got really inventive. They did gel combos where instead of using Full CTO and 1/2 CTO, they made one gel that did that combo called extra CTO. Better for the environment and cheaper. Now one gel did the work of two. It also helps with heat. When you put two gels together without an air gap, they burn up, plastic to plastic. This costs more money as well because you have to replace more gel, so it is a win win. Save the environment and save production money.
Using CTS and CTO on tungsten sources is so important. As mentioned previously, I feel most HMIs are a little cold to my eye. I feel the same with tungsten lights. They are too cold, so warming them up a little bit is key.
This has been my mission for years — to find a gel pack that emulates the High Pressure Sodium street lamp, the warm looking one. You can get close, but it never looks totally right. (See a previous post on color temp.) So I engineered these lights to give the exact color. Urban vapor is a match to low sodium lights, which do not exist much anymore because of their power inefficiency. So that one is no good. You can play with it, but I have found that when using the light that you are trying to match, gels can only go so far in matching the Metal Halides and the Sodiums.
Before Kino Flo existed, (wow I am dating myself) you needed to use a specific gel called minus green. This gel would color correct your cool white flo and take the green out. By doing this, it would also warm up your flo. I remember crews going into office buildings and for days adding minus green gel to all of the fixtures. What a time consuming, inefficient process. Then some manufacturers started to make some flos that were tungsten based, called Optima 32s, and daylight balanced, which were Chroma 50s. You would now send a crew in to change all the bulbs instead of cutting the gel and placing them in the fixtures — still very inefficient.
I am a big fan of using the color of the lights that exist and timing the green out later in post if that is what is best for your story. Maybe you want the scene to be a pea green or a minty green to assist the look, mood and tone of your film. Whatever the case may be, if you want it neutral, then using the same colored bulbs that are in the ceiling and correcting it later is one approach.
The other approach is using the beauty of plus green color correction gels to aid your look and feel. How do you do this? Say you are shooting in Walmart. They have seven thousand cool white flos in the ceiling. If you do not have cool white Kino Flos at your disposal, you can use HMI lights with Full Plus Green on them. It gets your light very close. It will not look promising to your eye, but it will look good in the camera.
An HMI is what you want when you are doing any interior store photography with Cool White or Warm White flos. Tungsten units take way too much gel to get them to the color temp of a cool white and then when you get there, you won’t have much output. It will take Full Plus Green and usually 3/4 blue to match a cool white tube, which is usually around 4400 Kelvin. So imagine a tungsten unit being 3200. When you add the Full Plus Green, your color temp drops to 2800 Kelvin and now you have to cool the light to reach 4400. If you are matching a Warm White flo with a tungsten light, this will require Full Plus Green, which lowers your Kelvin to 2800, then 1/4 and 1/8 CTB, which is Blue gel, to get you to the 3400 Kelvin.
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