Using CTO colour correction gels with Speedlite flash

Working with off-camera Speedlite flashes on location gives many photographers a new way to light their subjects, yet add a few coloured gels and you have access to a bigger palette of colour than you would find naturally. Some of the popular gels are the colour correction types. These are designed to convert the colour of daylight to that of tungsten filament bulbs, the type we used to find in our homes before the compact fluorescent types became prevalent. CTO or colour temperature orange will change the colour of the light from your near daylight balanced Speedlite to match that of the tungsten bulbs. To convert tungsten lights to daylight the complementary set of CTB or colour temperature blue filters are used.

In the ‘Two exposures in one frame‘ post I described the approach to how a flash exposure is comprised of an ambient lit and flash lit element. By the same token colouring the flash also means that you can light the principal subject with one colour of light and the background with another. If you leave the camera in automatic white balance it will try it’s best to give you a corectly balanced exposure. If you shoot Canon this can mean that different sections of the image are considered as being lit by different light and compensated accordingly. To make the use of gels more effective I usually use the Kelvin temperature white balance, though if your camera doesn’t have a Kelvin setting then you can choose the appropriate preset white balance such as Tungsten.

Speedlite with CTO gel, camera white balance set to near tungsten

The picture above has a Speedlite with a CTO gel on it slightly off out of shot to the left of the frame and directed at the model. To compensate for the orange light (resembling a tungsten bulb) the white balance was set to 3200K, though as above you could just choose the tungsten white balance too. The parts of the frame light by ambient light are given a strong blue cast due to the ‘wrong’ white balance setting. However in the -5C early dusk of a November afternoon, the effect is to add more blue tone in the skies trees and frosted leaves in the foreground.

It’s the contrast between the colour of the light of the flash and the colour of light on the background that gives this effect. In the image right at the top of the post, the background was made to go even deeper blue by setting the camera to underexpose the ambient lit elements of the frame by a full two stops. The flash lit element of the tree and the model are ‘correctly’ coloured but the dark blue comes from the ‘wrong’ white balance being combined with the intended underexposure.

CTO gels are typically available in several strengths of orange ranging from the Full CTO through 1/2 CTO and 1/4 CTO. I have sometimes used a 1/4 CTO gel on a Speedlite firing in to a silver umbrella to create a sunlight effect in dull, overcast or even rainy weather. I’ve found that it makes lots of sense to buy good quality gels from Lee or Rosco since they are stable and consistent from batch to batch. Simply buy a large sheet and cut yourself several gels to fit your flashes. Since this post concentrates on the CTO gel there will be another post in the future to explain the use of CTB gels, since these are less commonly used.