Quite a few customers have asked me about how they could use their flash gel set in flash photography so I have put together this very quick guide to get you started and give you a few ideas. Please feel free to email me with any other applications you feel it would be helpful to include!
The number of ways in which flash gel sets can be used is only limited by the imagination, but there are some techniques which are commonly used. I'll go through each type of gel and give some basic ideas as how they can be used.
Colour Temperature Orange Gel - CTO
This is a very commonly used set of gels which will shift light from a daylight balanced source (6500 Kelvin, similar to the light produced by your flash) closer to a tungsten balanced colour temperature. The strength of the effect depends on the strength of the filter used:
Full CTO converts 6500K to 3200K.
1/2 CTO converts 6500K to 3800K
1/4 CTO converts 6500K to 4500K
Example usage:
Balancing daylight balanced flash with ambient tungsten light
Take for example a flashlit subject in a large room lit by tungsten light bulbs. If you set the camera's white balance to Daylight, the subject will appear neutrally lit by the flash, but the background will appear as lit by an orange (tungsten) light. If the camera WB is set to Tungsten, the background tungsten lighting will appear neutral, but the flash lit subject will appear to be blue.
To balance the two light sources you can gel the flash with a full CTO filter which means both the flash and the ambient light sources now appear a similar colour temperature. So with the camera WB set to tungsten the image subject and background will appear neutral.
Warming a subject
If a flashlit subject would benefit from a warmer colour temperature but you don't want the ambient background light to shift, you can use a 1/4, 1/2 or Full CTO gel on the flash with the camera WB set on Daylight. This has the effect of adding a warm orange tone to the subject. The weaker 1/4 CTO is likely to be the most suitable for this purpose.
Cooling the ambient daylight colour temperature
A perfect balance between flash and ambient light sources is not always the desired result. In images involving a flash lit subject and ambient daylight, the ambient daylight can be shifted to a cold blue by gelling the flash with a Full CTO and setting the camera's WB to Tungsten. This means the subject will appear neutral but the background will take on a blue tone (useful when using a landscape or other large space as the background which obviously would be impossible to light directly with a CTB gel). You combine these techniques and further extend the colour temperature difference by placing another 1/4 or 1/2 CTO gel on the flash (along with the full CTO) which will warm the subject further beyond neutral while preserving your cold background.
Colour Temperature Blue Gel - CTB
This set of gels was designed to shift tungsten light sources closer to a bluer daylight colour balance. Since most small flashes are balanced towards daylight anyway these gels are not really required to serve their original purpose as colour correction gels in flash photography. However, they remain very popular creatively to add coolness to a subject or background.
Full CTB converts tungsten (3200K) to 5700K
1/2 CTB converts tungsten (3200K) to 4300K
1/4 CTB converts tungsten (3200K) to 3600K
Example usage:
Cooling a background directly
As opposed to the method described above of cooling the ambient background colour temperature, CTB gels are often used in location and studio photography as a convenient means of cooling a background which can be lit directly by flash. With the camera WB set on daylight the subject can be lit normally while a second flash gelled with a CTB gel can be used to throw a cool blue cast into areas of the background.
Adding a cold skim light to a portrait
Many photographers like use a neutral flash as the key light in a portrait, but add a cool skim light from behind the subject (which would be a flash gelled with 1/4, 1/2 or full CTB depending on the desired intensity). Aside from the creative effect this can help with the separation of the subject from the background.
Lee Plus Green Gel (sometimes known as Window Green)
These are another useful set of gels which are supplied here in 3 different strengths. They are designed to balance daylight light sources (i.e. your flash) with fluorescent light sources.
Example Usage:
Balancing daylight balanced flash with ambient fluorescent light
When fluorescent light sources are photographed using a camera with the WB set to Daylight they take on a greenish hue. So to make your flash match a greenish fluorescent ambient light you would gel the flash with Plus Green and set the camera WB to fluorescent, much like using a CTO gel to balance a flash with ambient tungsten light. However, fluorescent light sources are much more variable in the green hue they produce so some experimentation may be required to find the correct combination of 1/4,1/2 or full Plus Green gels to obtain the best match.
0.3 Neutral Density Gel
This gel simply reduces the amount of light that passes through it by 1 stop. Therefore if you are lighting a subject with your flash set at its lowest power output, but it is still too bright (and you don't want to alter the camera aperture or ISO), two of the choices you have are to move the flash further away from the subject, or, more conveniently, gel the flash with a neutral density gel.
Creative Effect Gels
The follow gels are primarily used for creative, not corrective, effect.
Light Red
A popular and strong creative effects gel.
Moroccan Pink
Gives a lovely, warm late afternoon sun tone.
Yellow
Magical Magenta
Primary Green
How to tell the gels apart
I am contacted occasionally by customers who have trouble telling which colour correction gel is which in the supplied flash gel pack. This is easily done once you understand that the full strength gels are the most opaque gels, the 1/2 strength gels are 1/2 the opacity (more see-through), and 1/4 strength gels are only 1/4 the opacity (most see-through). Just lay the 3 gels on a white piece of paper and you can easily see which is which.
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