Peter has outlined an excellent example of using the DS 125 in manual mode. As he mentioned, 1/2,1/4, and 1/8th power levels are available when using the Ike TTL sensor to signal the substrobe to fire.
If you'd like to practice a TTL exercise on land, try this:
Set up your complete rig, including housing and strobe arms. I know it's heavy, I've done this myself. Pick a day with blue skies. Go outside and turn your rig on, choosing manual mode (A/S/M). Set the f/stop to f/4 or f/5.6. Doesn't matter for this exercise. Now aim the Oly lens at the blue sky, away from the Sun. Adjust the shutter speed until the EV ( what you referred to as the light meter) reads between 0.0 and -0.7.
What you have successfully accomplished to this point is metering the sky (if you were in scuba gear, it could have been open water).
Now try to find something in your yard that is no more than 3 feet away from the camera's lens. Possibly a tree branch, a flower, or anything else that intrigues you. Remember, for good TTL exposures, your subject should fill most all of your LCD screen. Now try to compose the image so just the smallest amount of that blue sky you've already metered for is visible in the background. Again, for a good TTL exposure, you can't have too much sky in the picture. Aim the strobe properly at the target, not directly at it, and try to shoot upwards.
Press the shutter.
What happens next would make Rube Goldberg proud.....
The Oly will send out a relatively small preflash to aid in determining what it feels an appropriate exposure would entail. The internal flash is blocked by you to minimize backscatter. The substrobe though is aimed at the target and is at a better angle. It is also connected to the Ike TTL sensor, which you aimed at the blocked on board flash. The TTL sensor is designed to mimic the actions of the on board flash, that is, when it illuminates and when it quenches. Oly's brain isn't aware of these tactics. It only cares about the amount of light being reflected back off the intended target, through the lens, and captured by the CCD. When Oly feels it has a good grasp of the situation ( in milliseconds!) it sends out the main flash, which is longer in duration. Again, Oly is fooled. It is not aware of where the effective light is being emitted from. Oly's only concern now is to monitor how much reflected light off the intended target has journeyed back through the lens and onto the CCD. When it feels enough light has been captured for an acceptable exposure, it sends a quench signal to it's internal flash. The TTL sensor notices that the on board flash has stopped firing and it commands the DS 125 to do the same.
That takes care of illuminating the foreground. But the underwater image is said to be broken up in two areas, the foreground, illuminated by the strobe, and background, illuminated by the light present ( ambient light).
When you metered the sky, you took into consideration that the substrobe's power would not benefit you much beyond five feet. Therefore you've allowed the aperture ( f/stop) and shutter to stay open both long enough and wide enough to capture the background color for you instead. This background will not be as vibrant as the foreground because of the filtering characteristics of water. But when it comes to the foreground in your image, the substrobe has the capabilities of revealing all those colors visible in our spectrum of sight.
I have had nothing but great results with my DS-125. It does indeed have manual settings, not as many as with the controller, but enough for me.
