Strobilicous – making a meal of underwater lighting

By Colin Gans, www.UnderwaterDisplay.net

If you’ve thought about making rather than taking underwater photos, then wresting control of your camera’s settings is the first step towards your newfound creativity. This means not using auto mode; and as far as lighting goes, using one or more external strobes to allow for artificial light to be aimed more precisely rather than relying on your camera’s internal flash which has been designed primarily for convenience and general topside use.

Although camera technology has advanced dramatically over the past decade, the principles of underwater lighting remain largely unchanged. Whether using a compact camera, mirrorless system or DSLR, success still comes down to understanding how light behaves underwater. 

Consider the following regarding artificial underwater lighting:

 ·         In lighting a subject well, the illumination of particulate in the water (backscatter) is an underwater photographer’s number one enemy. The underwater photographer needs to use quite a different approach to that when lighting scenes topside.

 ·         When positioned correctly, one or more external strobes can overcome the effects of backscatter. The correct positioning of strobes is important and should vary with the focal length of the lens you’re using according to distance to subject. Shooting wide angle underwater requires a different strobe position than when shooting macro

 ·         Compact video lights or dive torches do not work well for still photography as they are simply not bright enough to yield the levels of light produced by a strobe. An external strobe allows for a better quality of light to be concentrated for very a small time; perhaps 1/10,000 of a second per exposure.

Types of strobes

A compact camera set up with two optically fired strobes.

 Two main types of external strobes are used underwater: electrically fired; and optically fired. Electrically fired strobes require a sync cable between the camera and strobe. Optically fired strobes use the camera’s internal flash to trigger the external strobe or strobes. The optically fired strobe takes a signal from the camera’s flash (the master) to an optical sensor on the external strobe (the slave) causing it to fire. Since light travels so fast, the lag between master and slave is negligible in terms of the end result illuminating the scene.  Electrically fired and optically fired strobes may sometimes be used in combination with an electrically fired master strobe triggering an optically fired slave strobe.

Electrically fired versus optically fired strobes

If you've ever seen a photograph of a diver with their strobe illuminated and wondered how the photographer managed to capture the exact moment it fired, the answer is most likely that the subject's slave strobe was triggered by the photographer's own flash. Using a fibre-optic cable to channel the trigger signal from the master to one or more slave strobes, rather than relying on an exposed optical sensor, reduces the likelihood of the slave being triggered by another nearby photographer's strobe and provides a more reliable firing signal.

Electrically fired strobes were once the preferred choice of many professional photographers, although the sync cables, bulkheads and electrical connectors require careful maintenance to ensure long-term reliability. Today, optically fired strobes have become the de facto standard for many underwater camera systems. Rather than carrying electrical signals through immersed cables, an in-housing optical flash converter uses a low-power LED to transmit the trigger signal through a sealed port and into a fibre-optic cable. This eliminates external electrical connections, reducing the risk of corrosion and water ingress while simplifying housing design. Fibre-optic cables are also thinner, lighter and more flexible than traditional sync cords, making them easier to route and allowing greater freedom in strobe positioning. Early optical systems relied on a camera's built-in flash, increasing battery consumption, but modern LED flash converters have largely overcome this limitation while retaining the reliability advantages of optical triggering.
 

Pre(mature) flash

Digital cameras typically fire a sequence of pre-flashes micro-seconds before the main flash and the reflected light is then measured in-camera. This is used to compute the length of time needed before quenching the main flash. Most digital cameras also use pre-flash to compute other parameters such as white balance. This all happens extremely quickly and is barely noticeable to the naked eye; although I have observed some small fish reacting to the pre-flash. It is possible for the pre-flash to set off an optical external strobe prematurely if that strobe is not pre-flash aware. The workaround to this problem is to either stop the camera from emitting a pre-flash or use an external strobe which ignores or mimics the pre-flash.

TTL strobe exposure

Many experienced underwater photographers continue to favour manual strobe control because it delivers consistent and repeatable results. However, modern TTL systems, including those incorporated into fibre optic converter modules, have improved considerably and can produce excellent exposures, particularly when photographing fast-moving subjects or scenes where the camera-to-subject distance changes rapidly.

Strobe size, spread and intensity

The spread or angle of coverage of the strobe you require typically depends on whether you’re shooting wide or macro. Macro usually requires a narrow beam to cover the whole scene and provide enough light for a high depth of field (narrow lens aperture) to capture that tiny critter all in focus. Wide angle on the other hand may require a wider spread of softer light; depending upon subject and technique. Manufacturers provide a guide number to indicate the power of a strobe; however, there is a lack of consistency with regards to guide numbers and strobe power. Each brand of strobe has its own unique characteristics and it is useful to understand these when choosing. If in doubt look at what others are using, read the forums and if it’s still not clear post a question. At the end of this article I provide useful links to websites to help you choose your strobe based on your camera, shooting requirements and budget.

Colour temperature

In practical terms there is usually a trade-off between intensity, colour temperature and angle of coverage for a given amount of battery power. Some more recent strobes are more compact (and negatively buoyant) but the quality of light is not as soft; with less gradual light fall off and cooler colour temperature. The colour temperature is really a matter of personal preference and if you are shooting in raw mode (why wouldn’t you be?) then you can set the white balance when optimizing your images as part of your raw conversion. Circular flash tubes tend to produce warmer (lower temperature) colours amongst the currently available strobes. The Underwater Strobe Finder database mentioned in the link below lists and allows you to compare colour temperatures for each strobe model.

Battery Technology

Modern strobes benefit from significant advances in battery technology. High-capacity NiMH cells such as low self-discharge batteries, and increasingly lithium-ion battery packs, provide faster recycle times, greater shooting endurance and more consistent performance throughout a dive. However, photographers travelling by air should be aware that lithium-ion batteries are subject to airline regulations governing their carriage and watt-hour (Wh) rating, which may influence battery selection and the number of spares that can be taken on a trip.

Strobe arms

Having good strobe arms is even more important to the underwater photographer than a good tripod for topside photography. The positioning of each strobe with respect to illuminating your subject and with a view to eliminating backscatter is best achieved through understanding the theory and lots of practice. Strobe arms should provide stability and the degree of movement necessary to achieve this with minimal effort. The most common setup uses two arms with three butterfly clamps acting as flexible joints to provide the freedom of movement and stability.

Focus Lights

Modern high-output LED focus lights have become standard equipment for many underwater photographers. Besides making composition easier, they significantly improve autofocus performance in low light. Many also offer a red-light mode which is less likely to disturb nocturnal or shy marine life.

Empowering yourself to make the decisions rather than have your camera do it for you will enable you to become more creative. External strobes can help make that happen by allowing you full control of your lighting to make a perfect meal of that scene.

Strobe Positioning

The goal is to illuminate the subject, not the water between the camera and the subject. As a starting point, position your strobes so that the inner edges of their light beams just overlap on the subject. This creates an even pool of illumination while keeping the brightest part of each beam off the water column, reducing backscatter. If the strobes are angled too far inward, the beam overlap becomes excessive, producing a bright hotspot in the centre, harsher shadows and increased backscatter. Conversely, if they are angled too far outward, the beams fail to meet, leaving the centre of the image underexposed and the lighting uneven. Think of this position as your neutral starting point—then adjust strobe spacing, angle and power to suit the size of the subject, lens field of view, water clarity and your desired lighting effect. Experienced photographers continually refine these variables from shot to shot rather than relying on a single fixed position.

The author with two strobes set up for wide-angle.               Photo: Les Martin

Disclaimer: Two of the diagrams in this article were generated with the assistance of AI to better illustrate the lighting principles described. Everything else is based on many years, many dives, and more than a few images featuring spectacular backscatter.

Useful links 

Alex Mustard's definitive UWP text, 'UNDERWATER PHOTOGRAPHY MASTERCLASS' covers strobes and lighting in detail.

Martin Edge’s, ‘The underwater photographer’ Third Edition provides an excellent approach to strobe positioning.

 The author’s strobe tests: http://www.underwaterdisplay.net/rig/