What’s the Difference? is a series of lighting tutorials. Each article responds to a single question. In this post, Jared Platt shows how to achieve different effects with CTO and CTB gels.
Color of light is a critical part of photographic lighting, but most photographers do not pay much attention to it when using flash. Each light source has a particular color cast to it, which is why your camera has white balance settings. When you choose the proper white balance for the color of light you are photographing, the color in the exposure will be neutral, and look correct. In mixed lighting conditions, where you have multiple colors of light, the light you white balance for will be neutral, while the other light will end up either too warm, too cool, too green, etc. In most cases, you will have one primary light source and color and if you want to keep your image color neutral, you will need to alter the color of your flash to match it. This is where color gels come in.
By covering your flash with a colored, translucent gel, you effectively change the color of the light to match the light source that currently exists on the set. The key is to know the colors of your common light sources. For instance, the sun is a slightly blue light source (measuring at 5500 K), while an incandescent bulb is an orange light source (measuring at 2500 K). Most “warm” LED light bulbs are 2700 Kelvins and overcast daylight is even more blue than daylight (measuring at 6500 K). I am going to be noting a lot of kelvin measurements in the article, but I will attempt to reference the general color as well. But you don’t have to get too exact with these temperatures, you just need to know the basic idea.
Understanding how to match your flash light to the ambient light in your shot also opens the possibility of changing the color of your lights to create color contrast between the two light sources, but first learn to match the color of the light and then experiment with creating contrast with color.
Let’s begin by identifying the color of your flash. Your flash is the same color as full daylight, so if you took a photograph at mid day, in full sun and set your white balance to daylight and added a flash into the image, you would end up with completely matching color. This is your starting point. Full sun is a slightly blue light source and so is your flash. They both sit on the scale at 5500 K.
For this blog post, we started with a known color of ambient light, the color of overcast daylight is blue (more blue than daylight). My flash is going to be the color of daylight, which is only slightly blue. See them as about 1000 kelvins apart on the scale. If I light the model with flash (5500 K) and white balance for the flash, my model will be neutral, but the background, lit by the cooler light of the overcast sun (6500 K) will appear more blue in the photograph.