bobf:
couple of points....
As ssra30 stated, backscatter is created by capturing out of focus suspended particles in the water column between the lens and target. These particles are illuminated undesirably by the underwater strobe. The light emanating from the strobe is at such an angle that the reflection of light that bounces off the particles head directly into the camera lens. Objects that are captured out of focus are recorded larger than actual size.
Generally speaking, light travels in a straight line. Moving a strobe closer or further away, with or without a diffuser, and without changing it's angle with relation to the lens, will have no major effect on the ability to reduce backscatter. Placing a strobe a different angle from the composition will. This is why you will often hear people discussing the technique of "painting with the edges of light" emanating from your strobes.
You will see many underwater rigs, especially those set up for wide angle compositions, with relatively long strobe arms. Their goal is not to illuminate the water between the lens and the target. The strobes are aimed straight ahead, so that only the edge of their beam comes in contact with the target.
Very close macro distances naturally reduce the potential for the capturing of backscatter.
With regard to strobe power, the DS 50 has an underwater guide rating number of 28, ISO 100, measured in feet (9, iso 100, meters). With the supplied diffuser the intensity is cut by one stop of light, which brings the gn down to 20 (ft) or 6.5 (m). At full intensity, the DS 50 is capable of illuminating a subject 4 feet away from the strobe head when the aperture is set to f/5, sensitivity set to ISO 100.
Indeed Bob, ssara30 and yourself have some very good points. Unfortunately I have seen too many wide-angle shots with under exposed centers and over exposed outer edges. I have to some how sneak in some light from the strobe to get the correct exposure.
The long arms on the wide-angle setups made it possible to place the strobes hotspot far enough away, out of the frames wide-angle reach (SLR 14mm fisheye lens etc.) to cut down on backscatter.
This is really the reason I use a diffuser on my strobes, to cut down on the hotspot but still keep the strobe as close to the lens as possible. When I say close to the lens I mean about 9 inches or so not right on top of the lens port, except when I shoot in macro, which then the strobe is right above the lens port.
Which brings up your other good point:
Very close macro distances naturally reduce the potential for the capturing of backscatter.
This point is my first line of defense against backscatter; getting the cameras lens as close as possible to the subject. In clear waters I try to limit the subject to camera distances to less than a foot, even if I cannot include the entire subject (wide angle shots) in the frame. I try to keep the strobe at the same plane as the cameras lens, to reduce the shadows.
So, I can only get the top/middle part of a bright red soft coral and part of the cobalt blue water in the background (due to a 28mm lens in flat port)... As long as I get the razor sharp focusing (at less than 1 foot, a very good chance) and evenly lit soft lighting (diffused) with minimum amount of shadows. The bottom half of the coral attached to the reef gets very cluttered anyway.
Shadows of absolute black nothings are my main concern. Shadows that are a 1/3 stop darker I do not have a problem with.
The other reason for reducing the dark shadows, is that the dark black shadows makes a great color contrast for light objects therefore a great black background to highlight the out of focus white Backscatter!
Thanks for the guide number info.
Dive Safe
