Gels and colored light

Learning goal

By changing the color of light with thin colored sheets attached in front of the flash, you will be able both to balance mixed light and to build a deliberate atmosphere. You will calculate through the mired difference how an orange correction gel brings the flash's near-daylight color closer to incandescent lamps, and how a blue correction gel, conversely, brings incandescent light closer to daylight, and choose the gel strength according to this difference. By setting the white balance for the ambient light and matching the flash to it, you will get a blue window behind a neutral face, add color to the background or an edge with effect gels, and take into account that every gel swallows part of the light. You will see how strongly colored light changes detail and color on different skin tones and set the key light accordingly, and complete the module's project with a single flash.

The question

The neighborhood's old barber agrees, after closing the shop, to sit in their own chair and pose. Yellow, old-style bulbs burn on the shop's ceiling; behind the shop window the blue hour of the evening has begun. You take the flash off the camera, put it beside the barber, and set the white balance for the flash. The barber's face is neutral and clean, but the shop behind them is drowned in orange; there is a sticky yellowness over the mirrors, the brushes, the old bottles. When you switch the setting to the bulbs, the shop looks warm and natural, but the barber's face stays in a cold, bluish light.

A photographer friend takes a thin orange plastic sheet out of their bag and tapes it in front of the flash. In the new frame both the barber's face and the shop are in natural color, while the evening behind the window has turned a deep blue. How do you make colored light? Could changing the color of light be a way both to balance a scene and to transform it?

Context

In the natural light module we solved mixed light by deciding between a window and a bulb (Lesson 18.6): you chose which light to make dominant, switched off the other, or moved the subject right up to a single source. When working with flash, one more way opens up. The flash is a light you carry, and you can change its color with a sheet you attach in front of it.

These sheets have two separate jobs. One is correction: matching the flash's color to the light in the setting and bringing all the lights in the frame to the same color. The other is creation: adding a color to the scene on purpose, giving a background purple, an edge blue, a face a warm tone. This lesson takes up both, as the last step of the light module, and closes with the module's project.

Simple explanation

How does a gel change the color of light?

A gel is a thin, heat-resistant, colored, transparent sheet attached in front of a light source to change the color of the light. A gel holds back part of the colors within white light and lets the rest pass. An orange gel holds back part of the blue in the light; the light comes out warmer. A blue gel holds back part of the orange and red; the light comes out cooler.

That is why every gel swallows part of the light. The denser or darker the gel you use, the more the flash light reaching the subject decreases. When a gel is attached, you need to raise the flash's power or set flash exposure compensation again with a test frame. Dark effect gels swallow much more light than correction gels.

Gel strength is the degree to which a gel changes the color of light. Correction gels usually come in strengths like full, half and quarter. A full-strength gel makes the biggest shift, a quarter-strength gel a small correction. The gel's package or edge says how many mireds it shifts the light; this number is the key to choosing the gel correctly.

How is a correction gel chosen?

A CTO (color temperature orange) is the gel that brings the flash's near-daylight white light closer to the warm color of incandescent lamps. A CTB (color temperature blue) does the opposite: it brings an incandescent lamp's light closer to daylight. For both, you can calculate the right strength with the mired unit you met in the color temperature lesson (Lesson 11.1). A mired is found by dividing a million by the kelvin value, and it turns the distance between two lights directly into a number.

If you take the flash as a source of about 5500 kelvin, dividing a million by this number gives a value of around 182 mireds. If the barber shop's bulbs are around 2700 kelvin, the same division gives around 370 mireds. The gap between the two lights thus comes to about 190 mireds. To bring the flash to the same color as these bulbs, you choose the orange gel whose package states the value closest to this difference. If the lamps in the setting are cooler, for example around 3200 kelvin, the difference drops to about 130 mireds and a lighter gel is enough. An exact match isn't essential; a difference of a few tens of mireds is most often closed with a small white balance adjustment in editing.

Balancing works like this. First you set the white balance for the light in the setting, here the bulbs. If the flash stays without a gel, it looks blue at this white balance and cools the face. The orange gel brings the flash to the same color as the bulbs; now both the face and the shop are in the same light and look natural in color. The daylight behind the window, on the other hand, turns a dark blue at this white balance. This is how you build a warm interior and a blue outside world in the same frame.

When is an effect gel used?

An effect gel is a gel, usually saturated and strongly colored, used to add a color to a scene on purpose. Most often it is used to leave the key light neutral and throw a second flash, or part of the main flash's light, in color onto the background, behind the subject or on an edge. A gray wall turns into a purple background when a flash with a purple gel is put in front of it; a blue light falling on the edge of the hair and shoulder separates the subject from the background.

Using two opposing colors together, for example a warm key light with a cool edge light, creates a strong color contrast. This is an aesthetic preference; the same photo can tell something else with a single-colored, neutral light too. What matters is that the color comes as a decision, not by chance.

Colored light leaves strong effects on skin. When you give a strongly colored gel directly to the face, as the key light, the skin takes on that color and loses its own. On dark skin, a dark blue or purple light makes a surface that already reflects little light even darker; the shadows drop to black, and the face's detail can be lost. On light skin the picture flips: the same color looks very dense and saturated on the surface, and the skin looks as if covered with a layer of paint. That is why many photographers leave the key light neutral or with a light correction gel and move the color to the background and the edge. If you want to give the color to the face, enlarge the face in the test frame and check that the detail of the eyes, the lips and the shadow side can be made out.

Visual evidence

In the barber shop you set the white balance for the bulbs, at about 2700 kelvin. You tape the full-strength orange gel your friend gave you in front of the flash and raise the flash power by one stop. In the test frame the barber's dark skin is warm and natural, and the reddish tone in their cheeks and the graying hairs of their mustache can be made out. The shop's mirrors, brushes and old bottles are no longer in a sticky yellowness, but in a warm yet natural light. The evening behind the window is a deep, dark blue.

Then you put a second small flash behind the barber, on the floor, attach a blue effect gel in front of it and aim it at the dark tiles behind. A slight blue halo appears around the barber's head; a contrast forms between the warm face and the cool background. When you turn the blue light onto the barber's face for a moment, their face darkens and loses its detail; you turn the light back to the background.

Common mistakes

Summary

A gel changes the color of light by holding back some colors in the light, and while doing so swallows part of the light; that is why the flash power is set again when a gel is attached. An orange correction gel brings the flash closer to incandescent lamps, a blue correction gel brings incandescent light closer to daylight; how strong a gel is needed is calculated with the mired difference between the two lights, and when the white balance is set for the light in the setting, the face and the place look the same color while the window looks blue. Effect gels add a deliberate color to the background and the edge; because strongly colored light can swallow detail and color on skin, the key light is most often left neutral and the face is checked by enlarging.

Mini assignment

If you don't have flash gels, you can try with a clear colored file folder or a colored candy wrapper; but don't expect color fidelity as with real gels.

1. In a room with incandescent lamps, shoot a person first with the flash without a gel, then with an orange sheet attached in front of the flash; in both frames set the white balance for the lamps and compare the color of the face.
2. Calculate the mired values of the lamp in the room and of the flash, write down the difference, and decide what strength of gel would be needed.
3. Throw light on a background with a colored gel and leave the key light neutral; then turn the same colored light onto the face and compare the two frames.
4. Module project: shoot the same subject with a single flash in three different light characters; for example with a soft light bounced off the ceiling, with a light coming from the side with a modifier off the camera, and with an atmosphere built with a colored gel. Next to each frame, note the flash's position, its power and the tools you used.

Check yourself

• Can you calculate the mired difference between two lights and choose a gel to suit it?
• When you set the white balance for the ambient light and matched the flash to it, did you see the face and the place come to the same color?
• In your project frames, did you check whether the face's detail and the skin's own color could be made out even in colored light?

Glossary

Mini quiz

1. When you set the white balance for the bulbs in the barber shop, how does a face lit by a flash without a gel look?

• A) In natural color
• B) Orange
• C) Greenish
• D) Bluish and cold

2. If the flash is about 5500 kelvin and the lamps in the setting about 2700 kelvin, about how many mireds is the difference between them?

• A) 190
• B) 2800
• C) 50
• D) 1000

3. If the frame goes dark after you attach a dense effect gel in front of the flash, why?

• A) Because the white balance changed
• B) Because the shutter speed got longer
• C) Because the gel swallows part of the light
• D) Because the flash's sync speed dropped

4. When a dark blue gel is given as the key light to a dark-skinned person's face, you see the shadows drop to black and the detail disappear. Which is the most fitting solution?

• A) Lower the ISO
• B) Leave the key light neutral and move the blue light to the background or the edge
• C) Increase the gel's strength
• D) Shift the white balance toward blue

Answers

1. D — When the white balance is set for incandescent light, a flash close to daylight looks blue.
2. A — The flash is around 182 mireds, the bulb around 370; the gap between the two values comes to about 190.
3. C — A gel holds back part of the colors in the light; a dark gel swallows more light.
4. B — A neutral key light keeps the skin's color and detail; the color builds atmosphere in the background and on the edge.

Summary card

• A gel changes the color of light and swallows part of the light; set the power again.
• An orange gel brings the flash closer to incandescent light, a blue gel brings incandescent light closer to daylight.
• Choose the gel by the mired difference; set the white balance for the ambient light.
• An effect gel adds a deliberate color to the background and the edge.
• Strong color can swallow detail on skin; most often leave the key light neutral and enlarge the face.

Bridge to the next lesson

In the module's project you built three separate lights with a single flash: a soft light from the ceiling, a light shaped from the side, a colored atmosphere. Each time a single source was enough. So in a controlled studio, how much of a portrait can you build with a single light and a reflector placed opposite it; when do you really need a second light? How much can you do with one light? The next lesson opens the studio lighting module by exploring the limits and possibilities of the one-light portrait.