Without discussing water clarity, at least four other variables will control the light influence from of a strobe:
1. User defined (manual) strobe output, including the use or non-use of a diffuser
2. Distance from strobe to subject
3. ISO
4. Aperture
Shutter speed plays no role in influencing strobe power as long as you choose a speed slower than the fastest sync speed allowed by the camera (varies by camera model, but speeds up to 1/250 should be safe). Shutter speed will however influence background exposure........that is, any part of the composition that is not influenced by the strobe itself. The choice of 1/1600 sec will "clip" most strobe outputs, especially if a full dump is dialed in.
Try this simple test on land. Place a camera in manual mode (not auto, not shutter priority, not aperture priority). Set the following three constants:
1. Choose an object to photograph in a dimly lit room. Aim your camera lens and the strobe at the subject. Maintain the same distance from lens to subject and from strobe to subject for each image capture (not to confuse, but the lens to subject distance can vary from the strobe to subject distance......but each distance must remain constant for this series of captures). I would choose a distance of at least five feet considering light travels much easier through air than water. Therefore a longer than "normal" distance from strobe to subject should be chosen for this practice test.
2. Set ISO to 100 (or any other constant).
3. Set strobe to any output setting (I recommend the lowest output setting for this practice test).
If any of these settings are left on "auto", it will alter the test.
Now take a series of images, varying the aperture setting from wide open (f/2?) to the smallest opening available (f/8?) with your camera.
Upon review of the images, you should notice the pictures go from very light at wide open apertures (f/2) to very dark at the smaller aperture settings (such as f/8).
You may run other similar tests by choosing to vary any one of those four factors while constantly maintaining the levels of the remaining three. By doing so, you will learn what variables influence (settings) control strobe power.
Bottom line, it's not just the dial on the strobe.
As for underwater use in this particular case, I would recommend putting black electrical tape inside the housing, in front of the onboard flash (not on it!) , to block it's rays from influencing the composition and potentially creating backscatter, especially if no other blocking device (i.e. opaque diffuser) is standard on your particular housing. I would also recommend installing a fiber optic cable to insure that the signal your strobe receives is from your own camera's onboard flash and not from someone or something else. You may find an Epoque FOC works although I would ask questions first to whomever you choose to buy it from (I have no personal experience with the Epoque FOC).
btw:
With all other factors discussed remaining constant, for every foot of increased distance between strobe and subject underwater, your strobe loses half it's power. For every foot closer, your strobe potentially is twice as powerful!
Switching from ISO 100 to ISO 50 will reduce strobe influence by half. Going from ISO 100 to ISO 200 will effectively double the strobe's influence, but not double it's actual distance effectiveness.......iow, it's light will still not be effective underwater much past a travel distance of five feet or so from strobe to target due to the filtering effect water has on light, which remains constant despite the ISO setting. I'd always choose the lowest possible ISO setting for underwater use.
Installing the diffuser will broaden the beam and also reduce the strobe output. diffusers vary in their knockdown influence depending on opacity, but generally between -0.5 and -3.0 stops of light will be absorbed by the diffuser
The power level numbers found on a strobe are not necessarilly intuitive, especially if they represent Guide Numbers. If the numerical settings refer to guide numbers, a setting of 6 will definitely yield more light output than 3, but it does not represent a doubling of light. It is closer to 4 times more!
hth,
b
