Is Full Power Always the Best Power?
A number of strobes that are designed primarily for macro photography only have one power setting. But when it comes to strobes built to accommodate the needs of wide-angle photography, almost all offer the feature of multiple power settings. So it is not uncommon for an underwater photographer to encounter the feature of multiple power settings as they move from macro photography into wide-angle work.
A little background information about strobes will help you understand why using the full power setting on a strobe will not always produce the best results. On the whole, strobes intended for macro work are more compact than strobes made for wide-angle photography. The smaller size helps with maneuverability and admits camera systems into the best possible shooting positions when operating within the restrictive confines of a reef. Despite being physically smaller, many macro strobes are as powerful as many wide-angle strobes. The major differences between ma-cro and wide-angle strobes are that wide-angle strobes must create a wider beam of light, and almost all strobes that are primarily intended for use in wide-angle photography offer multiple-power settings. The increased angle of coverage comes from more, or larger, batteries and results in bigger strobes.
As a rule, in macro work we want to get as close to our subjects as possible, and we want to maximize our depth-of-field-a goal which is accomplished by closing down the aperture (f/22 as opposed to f/2) as much as we can. A powerful strobe helps us close down the aperture, and as a result, in most cases strobes used in macro photography are used at full power.
However, when it comes to wide-angle photography, the intensity of light from a strobe is often not as important as the angle of coverage and the ability to turn the power up or down as needed in order to help balance the intensity of strobe light with that of natural light. During the day, a wide-angle strobe is used primarily to paint color into the foreground of a wide-angle scene and to provide the proper exposure in the foreground elements in a photograph without overpowering the natural light exposure in the background.
In brightly lit water, using a wide-angle strobe on full power is necessary in order to properly expose foreground elements while balancing the light emitted by the strobe with the intensity of sunlight. In darker settings, such as when you are working under a canopy of kelp, on a deep wall, in the early morning or late afternoon, it is often necessary to open the aperture to an f/stop in the neighborhood of f/2.8 or f/4 in order to properly expose a dimly lit background. However, you still want to get close to the foreground subjects in order to maximize image sharpness and enhance colors. Getting close to a foreground subject without overexposing the subject and overpowering the background exposure means you must be able to turn the power down on your strobe.
In wide-angle photography depth-of-field is often an important consideration but the problem is overcome in large part by the use of wide-angle lenses, which have an inherently greater depth-of-field than macro lenses.
The bottom line answer to this month's question is that in macro photography using a strobe on full power is usually best. However, in wide-angle work it is quite common to use a strobe on minimum power for one shot and maximum power for another shot. Which power setting you use depends upon the power of the particular strobe, the lighting conditions of the moment, how close you can get to your subject and the effect you are trying to create.