Is there a rule of thumb for distance from subject to strobe arm spread?

After a few years of shooting with a single strobe I finally broke down and picked up a second D2000 to go with my EPL2 and Oly housing. Using ULCS tray, arms, clamps I've got two 5" arms on both sides. I've got a zen port and shoot using either a Panny 7-14 or the 14-42 oly kit lens. My strobes are D2000's.

Is there a rule of thumb for how far to spread the strobe arms based on how far from your subject you are? e.g. if you're 5' from your subject you want to have your arms 10" extended on each side or is it merely a function of the angle of coverage of your strobe and playing with arm extension until you get the light coverage you want?

Thx!

First off, if you are five feet from the subject, your strobes are not going to do you any good - the light would have to travel ten feet (five feet to the subject and five feet back to the camera). Water eats light, expecially towards the red end of the spectrum.

The general rule, with strobes pointed straight ahead, is to spread the strobes by the same distance as the subject is from the camera. If the subject is one foot away, the two strobes should be six inches to each side - one foot between them. This arrangement minimizes backscatter. If you cannot spread the strobes far enough, point the strobes sightly outwards to illuminate the subject with the edges of the light cones.

Thanks! The 5' was merely for illustritive purposes, but that does bring up another question. What's the approximate max functional distance from the subject for a pair of D2000's?

What's the approximate max functional distance from the subject for a pair of D2000's?

About 1m in clear water...

... if you are lucky.

You can increase ISO & open up your lens to get more light from the strobe, while at the same time, increase the shutter speed to your strobe's longest burst duration to decrease ambient light. From that, you should be able to take picture of subject much farther away. I've taken pictures of subject up to maybe 2m away with the camera's internal flash (oddly, I took multiple pictures, and some came out completely blue while others came out with its natural colors. I guess it all came down to the camera doing white balance and metering with the pre-flash).

There is indeed an equation that you can apply to calculate the relationship between the distance between your strobes and the distance of the subject

You need to know the angle of coverage of your strobes which is given by the manufacturer

This equation assumes you point the strobes straight forward and that they are in line with the port

the distance between the strobes in your case is 2.85 the subject distance

So if your arms plus clamps are 14" each and the tray is 12" the total is 40" and you can get 14" close to a subject. To go further away you need longer arms as the strobes need to be behind the port