Q: Can I take underwater photographs without using flash?

A: There are many situations in which the use of natural light underwater can provide good, or at least interesting, results; especially if the camera has the facility for manual white-balance adjustment. In answer to the general question "should I go diving without a flash unit" however, the answer is "only if you are absolutely sure that it won't be needed". The point is that most underwater photographs do need flash, and prior evaluation is needed in order to establish whether conditions favourable to natural light photography exist at the time of diving. If you have a flash unit with you, you don't have to use it; but if you don't have a flash unit, photographic opportunities may prove to be be very limited.

At very shallow depths (0 - 4m), with bright sunlight above, there is usually plenty of light; but the ripple pattern of the surface is projected on to the bottom and often has greater contrast than any potential subject. Hence, if the intention is to photograph some particular object rather than record the general ambiance, it is better to stop down the lens until natural light is insufficient for an exposure and use flash to provide even illumination.

At greater depths, all is gloomy; with impenetrable shadows, and filtration of the light by the water gives rise to a strong blue-green colour cast. You can compensate for the colour shift by applying white-balance adjustment in conjunction with an underwater colour correction (UWCC) filter or a Magic Filter; (see, for example, Mike Deaton's photographs in the Gallery); but filters absorb light, necessitating large apertures or long exposure times; and large colour shifts necessitate high gain in the red imaging channel and a consequent increase in electronic noise. At great depth also, or for very long light paths, red light all but disappears, and you might as well revert to black and white. Hence, if the situation allows flash to be used, then flash should be used.

Q: How do I work out which strobe to get?

A: The first thing you should consider is the angle of coverage of the flash. Ideally, a flash should provide even illumination of the entire subject field, but this is difficult to achieve in practice. For a dome-ported 20mm lens used on a 36 x 24mm (35mm) format camera you need a flash with at least 95 degrees of coverage; but note that the coverage is usually defined such that the illumination at the edge of the field is 50% of that at the centre. This 1 stop difference in illumination between centre and edge is an accepted but noticeable feature of wide-angle flash photographs; and can only be improved upon by using a flash with much wider coverage than the lens (which means more power for a given light level), or by using multiple flash units, i.e., by using bulky and expensive equipment. In general, you should get a flash with at least the same coverage as your widest lens, and opt for wider coverage and more power as your budget and diving skill allows.

The next consideration is the guide number or output of the flash unit. You should be aware that the underwater guide number of a flash is about 1/3 of its guide number in air. Thus, for example, a flash with a guide number of 24 in air has a guide number of about 8 underwater. A guide number of 8 implies that you can use an aperture of f/8 at a distance of 1m for an ISO/ASA setting of 100. If you increase the distance by a factor of Ö2 (i.e., 1.4), you must open the aperture to f/5.6, and if you increase the distance by a factor of 2, you must open up to f/4. Similarly, if you reduce the ISO speed to 50, you must open up by 1 f-stop, and if you increase the ISO speed to 200, you may close the aperture down by 1 stop. Thus it transpires that for a particular ISO speed, the flash guide number enforces a maximum distance at which effective illumination can be obtained with a particular aperture setting. To obtain the best depth-of-field in a picture, a small aperture is required; and hence the larger the guide number, the better the depth-of-field. In general it is best to use f/5.6 or smaller in wide-angle photographs; and so, assuming ISO 100, a flash with a guide number of 24 in air has an underwater illumination range of 1.4m for a reasonable depth-of-field. Increase the air guide number to 33, and the illumination range goes up to 2m. Trying to illuminate objects more than 2m away is generally not worthwhile, because the water will absorb most of the red light from the flash, and so users of flash units with very large guide numbers do so mainly in the interest of using a very small aperture. For macro photography, best results are obtained using apertures of f/16 and smaller, but since the illumination range is also small, a modest flash unit can do the job. Conclusion: flash units with guide numbers in the range 24 to 33 in air, or 8 to 11 underwater (approximately), are good for general purpose underwater photography. A smaller guide number implies depth-of-field compromises in wide-angle photography. A larger guide number implies heavy equipment.