Underwater light

Learning goal

You will be able to bring back the red, orange and yellow that water swallows by giving light to the subject from close. While lighting a subject with strobes standing on arms on either side of the housing, you will take into account that flash light also travels through water, and so can color only nearby subjects. By putting the strobes in the right place, you will prevent the flash from lighting the particles between the camera and the subject and filling the frame with white specks. In shallow, bright water where you don't use flash, you will know when to fit and when to remove a filter that balances the water's blueness. You will make sure the flash light falls on a buddy's face, on light and dark skin, at the right power and softness.

The question

Your friend shoots the sponge at the foot of the rock again from an arm's length. The sponge is still brownish, their buddy's skin still leans toward green. The instructor shows them the two round strobes standing at the ends of long arms on either side of their housing, and shoots the same sponge: in the frame the sponge is bright red, the stones at its foot full of yellow and orange patches. Your friend tries with their own camera's small flash; this time the frame is covered with hundreds of white specks. Where do the reds disappear to?

Context

In the red buoy example you saw that water swallows red first, and that color loss grows with the distance from the surface to the subject and from the subject to the lens (Lesson W.1). In the flash and ambient light lesson you set the difference between the subject lit by the flash and the background lit by ambient light, the ambient-to-flash ratio (Lesson 19.4). In the one-light portrait you also used feathering the light, moving the light's center off the subject and lighting it with the light's soft edge (Lesson 20.1).

Underwater these three come together. The flash brings back the lost colors, but its own light also travels through water and is swallowed. Ambient light gives the background's blue, and the ratio of the two decides the frame's mood. Lighting with the light's edge, meanwhile, is the most effective way to fight the particles in the water.

Simple explanation

What brings the colors back?

An underwater strobe is an external flash with a waterproof body, fitted beside the housing on flexible arms, that gives full-spectrum light to the subject from close. Sunlight passes through meters of water before it reaches the subject and leaves its red on the way; the flash's light, though, comes to the subject from close and carries the red before losing it. That way the sponge gets back its red, the fish its yellow, the skin its warmth.

But the flash's light also travels through water: from the flash to the subject, from the subject to the lens. If the subject is a meter away, the flash light passes through about two meters of water and keeps most of the colors; if it is three meters away, it passes through six meters of water and the red is lost again. That is why an underwater strobe is for nearby subjects; beyond a few meters the flash only lights the subject a little, it doesn't bring the colors back. Getting close to the subject is the first rule here too.

The background, meanwhile, is water the flash can't reach, and is lit by ambient light. As the shutter speed shortens, the water in the background darkens and turns navy, as it lengthens it turns light blue; the subject in the foreground is decided by the flash's power and the aperture. That way, in one frame, the nearby subject can stay in its own color and the water behind it blue.

Where do the white specks come from?

Backscatter is the flash lighting the particles in the water between the camera and the subject, and these particles returning to the frame as white specks, blotches or a hazy glow. Because the camera's own small flash stands right beside the lens, it sends light right into the water the lens sees; every particle in between shines like a star.

Backscatter is reduced by moving the strobes away from the lens. The strobes are placed at the ends of the arms on either side of the housing, slightly back and pointing outward; not the center of the beam but its inner edge falls on the subject. That way the water between the lens and the subject stays outside the beam. Getting close to the subject also reduces the number of particles in between. The camera's small flash is turned off or used only to trigger the external strobes.

One source of backscatter is the photographer themselves: sand stirred up by fins, or a hand touching the reef, adds new particles to the water and the flash makes them shine too. That is why neutral buoyancy is also about light. A few small specks falling in the frame can be removed in editing, but a frame covered with hundreds of them often can't be saved; the solution is at the moment of shooting.

What is done in shallow water without flash?

An underwater color filter is a colored filter fitted in front of the lens or the port that balances the water's blueness or greenness by adding back a little of the missing red. In blue water reddish filters are used, in green water magenta-toned ones. The filter works in shallow, bright water shot with ambient light and no flash; as you go deeper, there is no red left for it to balance and the filter only reduces the light.

A filter and flash aren't used together. The filter adds red on top of the flash's already full-spectrum light, and nearby subjects, skin especially, are painted red. The same care applies to skin: if an underwater strobe hits light skin too close and at full power, it leaves shiny patches on the cheeks and forehead; dark skin, on the other hand, loses its depth if it doesn't get enough light. Diffusers fitted in front of the strobes soften the light, the power is set on each skin tone with a test frame, and an angle is sought where the flash doesn't reflect in the mask's glass.

Visual evidence

The instructor shows your friend how to place the two strobes on either side of the housing, at the ends of the arms, facing outward. They approach the sponge from an arm's length. In the first frame the strobes are close to the lens and facing forward; the sponge is red, but the water in front of it is full of white specks. When they open the strobes to the sides and turn the inner edge of the beam toward the sponge, the specks almost disappear.

Then they shoot the same sponge from three meters back. The flash's light reaches the sponge, but the frame is brownish again. The instructor reminds them that the light passes through six meters of water going and coming back, and calls your friend back to an arm's length. When they shorten the shutter speed, the water behind turns navy, and the sponge stays in its own red.

Finally the instructor shoots their buddy on a shallow beach. First with a red filter and no flash; the water is balanced, their buddy's light skin natural. Then they turn on the flash but forget the filter: the skin is bright red. When they remove the filter and fit diffusers in front of the strobes, their buddy's face is soft and warm; the instructor has their own dark-skinned face shot with the same settings too, raises the flash power by one step, and their skin is recorded together with its depth.

Common mistakes

Summary

Underwater, the lost colors come back with an underwater strobe that gives light to the subject from close, but because flash light also travels through water, this works only for nearby subjects. Backscatter is prevented by moving the strobes away from the lens and turning the beam's edge toward the subject, an underwater color filter is used only in shallow water without flash, and the flash's power and softness are set on each skin tone with a test frame.

Mini assignment

In this assignment you will try underwater light on a small scale; a pool or an aquarium and a flash or a strong flashlight are enough.

1. Mix a little dust or fine sand into an aquarium or a clear container filled with water; shoot two frames, giving the light from right beside the lens and from the side pointing outward, and compare the backscatter.
2. Shoot a red object in the water, putting it close to the light and far from the light; note how the color changes with the light's distance.
3. With their agreement, shoot a frame of someone in a pool or from right behind the water, with diffused and undiffused light; set the light's power for their skin and compare the two frames.

Check yourself

• When you gave the light from the side and pointing outward, did the white specks decrease?
• Did the red object lose its color as it moved away from the light, and can you explain why with the light's path going and coming back?
• Was the person's skin lit at the right power and softness, without shiny patches and together with its depth?

Glossary

Mini quiz

1. Why does an underwater strobe bring back the lost colors?

• A) Because it changes the water's color
• B) Because it sets the camera's white balance
• C) Because it carries light to the subject from close, before losing its red
• D) Because it cleans the particles

2. If the subject is three meters away, about how many meters of water does the flash light pass through?

• A) Six
• B) Three
• C) One and a half
• D) Nine

3. How is backscatter most reduced?

• A) By using the camera's own flash
• B) By moving the strobes away from the lens and placing them to the sides and pointing outward
• C) By raising the ISO
• D) By moving away from the subject

4. Why isn't a red filter used together with flash?

• A) Because the filter turns off the flash
• B) Because the flash melts the filter
• C) Because the filter only works at depth
• D) Because it adds red on top of the flash's full-spectrum light and paints nearby subjects and skin red

Answers

1. C — Sunlight loses its red on the way, nearby flash light doesn't.
2. A — The light goes three meters to the subject and comes three meters back to the lens.
3. B — When the water between the lens and the subject stays outside the beam, the particles don't shine.
4. D — Because the flash already carries red, the filter makes it too much.

Summary card

• An underwater strobe brings back red, but only up close: going and coming back doubles the water path.
• The background's blue is set with the shutter speed, the foreground's color with the flash.
• Backscatter: strobes to the sides, back and pointing outward; turn the beam's edge toward the subject.
• An underwater color filter only in shallow water without flash; remove it when shooting with flash.
• Soft light on skin with a diffuser; set the power with a test frame.

Bridge to the next lesson

Strobes, arms, a housing and dive training: your friend sees that all of these won't come at once. The instructor suggests a simpler start: a mask, a snorkel, a shallow, sunny cove and a simple waterproof case. Is photography with a snorkel possible? The next lesson takes up starting with a snorkel, in a shallow, sunny cove.