What’s the difference between using CTO gels and CTB gels?

16 May, 2016

Written by: Jared Platt

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). See the diagram below. 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. And yes, I know my chart is not to scale, it is just a fun way to look at the concept.

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.

The original setup

Our lighting set up included one Profoto B2 Off Camera Light, modified by an OCF 2×3 soft box as the main light and one Profoto B1 Off Camera Light modified by a Zoom Reflector and a 20 degree grid as a hair light. The camera settings were set at 160 ISO, 1/200 sec at f 2.3 and we were using a Canon 5D Mark III with a 24-70 2.8L lens at 38mm. After this first shot, we added a gel to our flashes to change the color of light. While the gels will slightly cut the amount of light coming out of the flash, they do not effect the ambient exposure at all, so our camera settings could stay the same throughout the photo shoot. The only thing that changed was the power output of the lights when we added a gel. Each gel can cut the power output of a flash by 1/3 to 1 full stop. The power cut is clearly written on the edge of each Profoto corrective gel for convenience.

Adding a CTB gel

In the next image we used the new Profoto OCF Gel Kit to add a CTB or Blue Gel to our two flashes. By adding a blue gel to the flashes the color of the flash would become the same color as the background overcast daylight. This means that when I changed my white balance in the camera to the overcast setting (6500 k), the entire photo, model and background (because they are both lit by the same color of light) became neutral. You can see in the following image, that the distinct blue cast in the background is removed. Observe though, that the model’s color balance is exactly the same (neutral) because we has chosen the correct color balance for the light that is striking him.

Adding a CTO gel

Now to go the other direction. Rather than trying to match our colors, what if we wanted to exaggerate the color differences between the model and the cold background? Simple. Rather than looking for a gel that was the same as the ambient light source, we needed to find one that was the opposite of the ambient color balance. Remember, the ambient color balance was very blue, so we pulled out the CTO or Orange Gel. By coloring the flash light orange we would make the model very warm! But by setting the white balance on our camera to incandescent (orange light 2500 k) we again neutralized the model’s color. This is because the camera is fixing the model’s overly orange face by “cooling down” the shot. Of course, when the camera adds blue to the shot to counteract the orange on the model, everything else in the shot also goes more blue, including the background. Therefore, you get this next shot.

The conclusion

All three of these shots are perfect. Each one accomplishes a different goal. You can get the first shot without a gel at all. But having a set of CTB and a CTO gels gives you the freedom to alter the mood of the shot to intensify the cold,or neutralize the colors. As long as you understand the principles of the color of light and you have the tools, you can get exactly what you want, whenever you want it.

The Profoto OCF Gel Kit includes a set of corrective gels as well as holders that clip over the outer edge of the Profoto flash head. They can be added inside of a soft box, a grid or even in between the B1 head and an umbrella so that you can modify your light and color at the same time.

Now for something completely different. The shot I really wanted: a day for night shot. By keeping the CTO Gels on the flashes, I maintained the cold feel of the background. Then by underexposing the ambient (changing my ISO to 100, and my aperture to 4.5) and making the needed changes in the flash power, I made my day turn into night and kept my model with the same exposure. The final shot is my absolute favorite from the series!

Now all of this, as you may notice was done in the rain. We didn’t go into this shoot thinking it would be raining. I live in Arizona, it doesn’t rain here. Clouds, yes. Rain? Rarely. And yet, on the day I schedule my shoot, I get heavy rain. So, when the rain started falling, we changed our shoot to include the umbrella and we added a few additional umbrellas and draped from plastic parkas on equipment and worked as fast as I have ever worked! The whole shoot from first shot to last, lasted 8 minutes (not including setup and take down). I’d say we knocked it out of the park for an 8 minute shoot.

I don’t recommend shooting with flashes in the rain, but if you are out while the rain starts, try to keep them covered, but remember, you need to let them breath as well. I didn’t have time or enough umbrellas to cover each light properly, so we simply used whatever we had (plastic) to save our equipment. We finished as fast as possible and then turned off the flashes immediately. Then I took everything back to the studio to let it all dry out for a day or two. It was a very wet photoshoot. But, lots of fun!

Location: Tempe Town Lake, Tempe, Arizona (yes, we have a lake and a crew team at Arizona State University, in the desert)