Gelling your flash, as easy as Red, White, and Blue – Part 1 of 4

One area that I’ve seen photographers have a difficult time in grasping is using gels with their flash, or more generally color temperature and what it really means. There are lots of really good websites out there that discuss the color temperature of light, what it means in relation to your white balance, and using gels, see for example Cambridge in Colour or The Strobist.  What I’ve seen lacking is a good simple explanation of the color temperature of a light source and how gels and white balance interact with the color temperature. Hopefully this series of articles will help you to understand more easily how they interact and make it a lot simpler for you in deciding which gel to use for the effect you want.  By the end of the series you’ll know that gelling your flash is as easy as remembering, Red, White, and Blue.

Most color temperature scales for photography will list the color temperature of various types of light between 1,000 Kelvin and 10,000 Kelvin. Here I’m going to use the values that Canon uses in their camera manual and then add some extra light sources to fill the color temperature scale out some:

Temperature Light Source
 1500K  Candle/Flame
 2000-3400K  Sunrise/Sunset
 3200K  Tungsten
 4000K  Moonlight/White Fluorescent
 5200K  Daylight
 6000K  Flash
 7000K  Shade

We know that the lower the kelvin value the warmer the color temperature of the light source.  And the higher the kelvin value the cooler the color temperature of the light source. This seems counter-intuitive to most people. Normally when we say something is warm or hot, it has a high temperature. And when it’s cool or cold, it has a low temperature. But with light, it’s reversed. A lower temperature number is warm and a higher temperature number is cold.

Instead of trying to remember that, what I want you to remember and be able to visualize is the figure below:

Notice that the warm color/low kelvin is on the left, in the middle it’s white, and then on the right side it’s the cooler color/high kelvin. So when you are out shooting and you need to remember the color temperature scale, don’t think about kelvin and what value is warm/cool. Just remember “Red, White, and Blue”. For people from the US, this will be very easy to remember. Red is warm/hot temperature and Blue is cool/cold temperature. Just remember how you would write that on a piece of paper in a line and you will always remember this figure.

On the left side of the paper you’d write “Red” (warm colors, low kelvin number).  In the middle you’d write “White”.  On the right side of the paper you’d write “Blue” (cool colors, high kelvin number).

Now White doesn’t mean a color temperature that’s white, it actually means the camera and it’s White Balance. So the figure with the phrase looks like this:

Pretty easy to remember right?

It’s all a matter of perspective

Most of the time we are taught that a light source is either warm or cool, but really it’s all a matter of perspective. Think about where you are right now reading this, if you look to your right there are items in that direction. What happens if you turn around and face the opposite direction? Those items are now to your left. The items didn’t change location, you didn’t change location, just turned around the opposite direction. But relative to you the items have moved. It’s the same with color temperature.

Anything to the left of the arrow will appear to the arrow as a warm light source. Anything to the right of the arrow will appear to the arrow as a cool light source. And this remains the same when we move the arrow’s position:

Doesn’t matter where you place the arrow on the chart, if a light source is to the left of the arrow, that light source will appear warm to the arrow. A light source to the right will appear cooler. And the amount that it appears warm or cool to the arrow depends upon the distance that light source is from the arrow. The closer the light source is to the arrow, the more the light source will appear to be neutral white. The further away the light source is to the arrow, the more the light source will appear warm/cool to the arrow.

This is the key to understanding color temperature and how to gel your flash.  Once you understand this figure and how anything to the left is warm and anything to the right is cool you’ll be able to control the color temperatures of your light sources in the image.  In the next part we’ll learn the relationship/interaction of color temperature, white balance, and gels.

Stay tuned for the next part, we’ll get more into white balance and color temperature.  If you have any questions or comments feel free to contact me.  I’ll try to answer any questions you have about this.


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