When taking images underwater with an external strobe, think of the composition as having two distinct layers, the foreground and the background.
The foreground layer would be any area of the composition illuminated by the artificial light emanating from the strobe. The background would be any area of the composition not affected by the strobe light, that is, illuminated only by any available ambient light (naturally present light if present........iow at night, there is no ambient light.......relatively speaking).
In order to control the foreground exposure a relationship between strobe power and aperture combine to determine desired results. The background exposure is controlled by aperture and shutter speed.
For topside practice, try this technique. I would begin by metering the background exposure. Let's assume the sky was background water of the composition. Point your lens to the sky. Set your aperture manually to f/4. Next vary the shutter speed until your EV readout found in your LCD reads -1.0 when aiming at that clear blue sky. If diving, that might possibly render a nice blue background to your images (not washed out due to overexposure). The faster you set the shutter speed, the darker the background would become........whether sky or open water.
Sync speeds and motion blur are components that I am purposely omitting from the conversation for the moment............
Next step in the practice technique would be to find a subject such as a tree branch that would have the previously metered sky in its background. The tree branch should be no further than @ 5 feet from strobe to subject, preferably closer. I prefer to aim at branches above my head because it instills the concept of "shooting up" in the water column. Otherwise the appearance that your main subject is "plastered" against the reef possibly exists.
Also remember that the closer you are to your subject, the less chance of backscatter. Aiming the strobe so that only the edge of it's projected light beam illuminates the subject is another technique in minimizing backscatter that should be explored. That said, if auto mode is chosen on the strobe to control output, you may have a problem with the technique of "painting with the edge of light" since it is important that the sensor on the strobe head is directed at the subject the strobe is illuminating.
If you are shooting a "macro" composition and there is no "background layer".......in other words the total composition is within reach of the strobe light, and therefore none of the composition will be illuminated strictly by ambient light, then there will be less need for the above step and just proceed to the next.......
Properly exposing the foreground, that is all parts of the composition within reach of the strobe's output, is a little trickier than exposing the background. Much depends on how you control the influence of the strobe light. If your YS 90 is set to auto, then match your aperture value to the corresponding strobe setting and go from there..........shoot, review, adjust, shoot again to quote digital pioneer Jim Watt, until a desired result appears in your LCD. If your YS 90 is set to manual mode, choose a middle power level (duration) followed by the digital mantra once again..........shoot, review, adjust, shoot again. If ever you modify your rig to allow the camera to control the strobe's output (some form of digital ttl), make sure that your composition is filled with subjects of "average reflectance"......otherwise the TTL metering system at times might be "fooled".
I prefer shooting full manual strobe control on daytime dives but enjoy the consistency TTL offers on night dives.........when less open water tends to exist in my compositions (open water has poor reflective qualities).
Aperture settings other than f/4 may be chosen to accentuate characteristics influenced by the size of the iris, such as depth of field, image sharpness, etc. Just remember that a change in aperture not only influences foreground exposure, it also influences background exposure, if present.
There are other variables that influence exposure............there also exists other opinions as to how the photographer controls exposure of the total composition............in the end what works for you is the correct method.
btw, if it were easy, I'd be bored
hth,
b
