Understanding CTO (colour temperature orange) and CTB (colour temperature blue) gels can be daunting, but knowledge is power! Once you can recognise the lighting environment that you are shooting in and know how to adjust your camera/lighting settings, you will open up your creativity. Below is a beginner/intermediate introduction to understanding Kelvin temperature and CTO and CTB gels, plus some bonus creative lighting examples for you to try!
Your camera's Kelvin (K) settings (white balance) explained
My camera Kelvin (K) settings range from 2500 to 10000 – basically very warm to extremely cool temperatures (white balance). Your camera’s K settings are primarily there to help you match with the ambient light for natural looking tones in your photo. It’s also the perfect platform for creative lighting.
I like to think of Kelvin like this: the lower the K number the closer you are to the sun. If you want to cool off you need to get some distance from that toasty sun and therefore you need more K’s! Candle light is around 1850K, while daylight is about 5000 – 6500K. So think low number warmer, big number colder!
Have you ever photographed inside a house with incandescent lighting (~2400K) and caught a window in the background (~5000K)? The window light would probably have appeared blue relative to the interior when you tried to colour balance the shot. Videographers often add either cooling gels (CTB) to overhead warm lighting or gel the windows with warming gels (CTO) to ensure the scene looks consistent.
Further reading on this topic can be found here.
What are CTO and CTB gels?
Colour Temperature Orange (CTO) and Colour Temperature Blue (CTB) gels are different strengths of orange and blue, respectively, that are mainly applied to your lights to add a warming and cooling effect to your photographs.
Flash is daylight balanced (5500K), so if you are taking a photo of a person during a warm sunset (2,800 – 3400K) you might use a 1/2 or full CTO gel on your flash to warm up the light on your subject and match the setting sun colour. Alternatively you could use a CTB gel (bear with me), this actually would make your subject blue. Therefore in post or in camera you can adjust the white balance to properly balance the skin tones – making your background now even warmer. This means lovely warm dramatic sunsets! And yes, my mind was properly blown when I figured this one out. It’s a bit like patting your head and rubbing your stomach at the same time initially, but over time knowing how to use colour temperature gels becomes instinctual.
Some photographers also use CTB gels over flash when taking portraits in in shade (as shade appears blue). This allows the subject to be coloured to perfectly match the background with no blue casts. If you set your camera Kelvin to around (~7500K or shade) add a CTB gel to your flash (try 1/2 and increase if needed) you should see your subject match the background perfectly. If you wanted your background to really warm up you could use a strong CTB gel and then balance skin through your camera white balance settings or in post. Is this starting to make sense and are you now feeling all the power of light control? If not here’s another more detailed article 🙂
A BIG-ISH note about CT green gels
Most standard CT gel packs will typically contain some CT green gels amongst them. And what is the purpose of these seemingly sickly coloured gels you ask? Green gels are used to balance fluorescent lighting – think 80’s/90’s corporate office vibes. Green casts from overhead flouro lighting (and sometimes green light from green window glass) are the scourge of photographers, they add a sickly cast to the skin. The only way to balance the green in post without an added gel is to add a pink tint – this totally mucks the rest of your photo and rarely looks good.
So if you have the disadvantage of having to grab a headshot under flouro lights and you can’t turn them off, grab your flash and start with the lowest strength green gel until your subject and background are the same colour. Yes, they will look like they’ve eaten a dangerous prawn cocktail for lunch. However, after you get into post, voila, a healthy subject and perfect background.
The plot seriously thickens when you also have in the mix blue window light, incandescent light and flouro’s. Lighting designed by someone who clearly wants to cause you pain. In this green, blue, warm mix scenario your best bet is to gel for the main light source and slightly overpower with your flash. Then you will need to do some selective area tinting in post processing to make everyone look vaguely normal. I have photographed in a lot of factories, gyms etc. before and been faced with this scenario – don’t panic, you can only do your best, you are not Apollo.
Let's apply this goodness: AKA creative lighting
Understanding your camera’s white balance settings alone can garner some creative warm and cool lighting effects, but when you begin to add gelled studio light or flash the world of colour really opens up. The truth is, you don’t need a lot of fancy colour lights – just some basic CTO / CTB gels or even some different coloured cellophane – and a basic understanding of colour theory. If you are starting your journey on colour gels a great article on CTO and CTB gels and would like some further reading, these articles here and a great one here will be useful in filling any blanks.
The below photographs were created by against a white wall. First I adjusted my white balance to the coolest light setting (10,000K) – basically so the whole photograph would initially look very blue. I then set a flash in the background with some layers of red cellophane on it (yes I’m super high tech!). The key light was a large soft box with a CTO gel on it (sorry I don’t remember the strength) to allow me to balance the subjects skin tone while still keep the blue background. I did also have a little bit of window light that behaved like a light cool fill light. I did need to warm the legs a bit in post processing, as the CTO gelled softbox didn’t quite reach the legs and you can see the left hand in the first photo has a cooler tone. See below for the setup and photo.
Mixing red light & window light
In the below scenario, I had a huge window on one side and then pointing towards the backdrop a red gelled flash (acting mostly like a rim light). I balanced my camera to the window light (overcast setting – 6500 to 7500K). This bought out the red in the gelled flash and also allowed me to contrast the red light to a normally balanced skin tone. I did move the flash around a little for effect and have also included an example with no flash for comparison.
Disco lights are underrated
I recently purchased an LED disco light from my local JBHiFi – for the princely sum of $41. It is one of my new favourite toys, it can add interesting lighting effects on its own to a photograph. All you need to do is slightly under expose the photograph to bring out the flecks of light. The below were just shot with ambient window light and the disco light.
Go forth and experiment
So now that you have the basic principles, its a matter of experimenting, there are unlimited options! You can purchase gels in all the colours of the rainbow, you can also just grab some coloured cellophane to get started. Add in some disco lighting for good measure and become a light juggler extraordinaire. Pay special attention to how colours mix and try layering them up to create new colours.