John Brawley
Well-known member
Hi,
Firstly, remember that CTO stands for colour temperature ORANGE and CTB stands for colour temperature BLUE.
It might help to think about where the actual temperature part of colour temperature comes from....as a metallic reference object heats up for example, it turns orange, yellow then blue.....
The temperature numbers represent how hot the object was when it was that colour.
The higher the temperature, the more blue it is. The lower the temperature, the more orange it is.
The human eye has an amazing ability to auto white balance, se we don't tend to see the differences in colour temperature as much as a camera does.
A camera also needs to know what colour temperature it's looking at. You can do this manually by telling it and setting a preset, choosing the appropriate film stock or you can also have it do it's own calibration in-camera.
3200K and 5600K are two common colour temperature presets that are assumed, but rarely will you find light sources that actually are this colour when measures with a colour temperature meter.
5600K for daylight is a very generic average. Daylight can be very warm and dawn and dusk, say 2700K or can be very blue 11000K if you're at altitude or have cloud cover.
The point is that it CHANGES throughout the day.
3200K is an ideal colour temp for tungsten based light sources, but again they rarely actually are. They tend to be warmer, closer to 3000K because of things like voltage fluctuations, age of the globe, dust and dirt on reflectors etc.
So with a continuous colour scale that can change and have mixes of different colour temps you have to set your cameras colour temp to either correct for the differences OR to creatively interpret them.
Your camera set to 5600k when shooting a 5600K source will make that source colour appear "white" (assuming it's a continuos spectrum source).
if you use a 3200K source with the same camera settings, then the light will appear more orange than white, because it's towards the warmer end of the scale. The camera is expecting a 5600K light. However, you can make this light appear white if you bump the cameras white balance to 3200K.
Now you have a 3200K source and the camera is expecting a 3200K source, so all is *normal*.
The need to add gel and correction like CTB or CTO comes from when you have both 3200K and 5600K sources in the scene.
no matter where you set the camera, one of the sources will appear either warm or blue. you need to be able to match them to each other so everything appears to be the same.
So a camera set to 5600K would want CTB on a 3200K source to make that source 5600K (with less light level) and looking white.
A camera set to 3200K would want CTO on a 5600K source to make it match the cameras white balance setting (again with less light level)
You might also want to consider using less correction for creative reasons. It's great to sometimes have a slightly cooler or slightly warmer source. Instead of full CTB or full CTB, you can look to start using 1/2 CTB and CTO or even 1/4 CTB and CTO. As well as correcting the lights you can use filters in front of the camera to correct (instead of using the electronic white balance) but this won't take into account a secen that has a mix of colour temperature sources.
White balance, or a better way to think of it, white point is only the beginning of the colour temperature story though, because most sources aren't continuos in the colour of the light they output.
Lights can have the same white point of 56OOk, but be made up of different ratios of red, green and blue wavelengths, meaning that you will get casts of green or magenta depending on how far out they are.
In summary, you need to roughly know what different sources of lamp or sunlight are likely to be, and then make a judgement about where to best set your cameras white balnce to best reflect what is in the scene OR to change it to suit your needs.
You can get a very cheap day for night look by simply setting the whitepoint to something like 2700K in camera and then letting the daylight of a scene go blue. You need to be careful to underexpose though because too much level will give the game away. Have a look at this music clip that I did where I used this technique.
jb
