Strobilicous – making a meal of underwater lighting

Colin Gans, www.UnderwaterDisplay.net

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

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.

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 pointing their camera with their strobe lit up and wondered how the photographer managed to capture the moment of the subject’s strobe firing; it’s most likely due to the subject’s slave being triggered by the strobes of the photographer taking the picture. Having a fibre optic cable channelling the light from the master to one or more slaves, rather than using an open sensor, reduces the chance of the slave from being triggered by someone else’s strobe firing close by and increases the reliability of the signal.

Using electrically fired strobes tend to be popular with pro photographers although the sync cables, bulkheads and plugs do require special care and maintenance. Optically fired strobes can only be used as slave strobes and are more commonly used on compact camera housings as a ‘non-invasive’ solution. Some SLR housing manufacturers are offering optical cabled solutions but this has not really caught on in the mainstream. A light signal can easily pass through the clear polycarbonate wall or glass port of camera housing without the need for a bulkhead and pressure sealed plug. Optically fired strobes require more power from the camera battery due to the firing of the camera’s internal flash and consequently a camera’s battery may not last as long. This may require more frequent battery changes and charges during a dive trip.

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

Through the lens (TTL) metering in a camera enables the camera to compute exposure in a scene and control the amount of light emitted by a strobe. While this is a reasonably failsafe solution topside, underwater use of TTL presents problems when too much open water is present. Water absorbs the light providing little reflectivity for the camera to correctly calculate exposure. The result is ‘blowout’ or overexposure in some subjects. TTL may not work well in situations where most of the frame comprises open water. On the other hand TTL can work well for macro photography where a subject fills more of the frame or the background is sufficiently reflective. More recent digital SLR cameras do a much better job reducing ‘blowout’ through more accurate metering of the subject when using TTL. The downside to using TTL is that the camera rather than the photographer is making the decision on exposure for the artificial light.

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.

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.

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.

Master at work: Darryl Torckler aiming a single electrically fired strobe in an estuary

Useful links 

The forums on Wetpixel.com offer a wealth of information on all underwater photography topics. The site is moderated by experienced underwater photographers and chances are that what you’re looking for has already been covered. http://www.wetpixel.com/forums/  

Alex Mustard's definitive UWP text, 'UNDERWATER PHOTOGRAPHY MASTERCLASS' covers strobes and lighting in detail. http://underwaterphotographybook.com/

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/