Hi Ron,
I'm sorry for the length of this post, but underwater, so many things become intertwined. Combine that with the fact that we all have different shooting goals, and it becomes apparent that any comment that closely resembles a "blanket" statement can come under question.
I agree with you that 1/250 is more than adequate to stop action. But the shutter speed has at least one more important purpose underwater.
Background exposure.
( I know you know these principles, I add the following only for any newbies following our conversation) The combination of shutter speed and aperture controls the background exposure. They accomplish it by influencing how much ambient light is allowed to pass through the lens. The background exposure I would define as any area of the composition that is not influenced by strobe light. Ambient light is light which does not emanate from the strobe.
If our goal is a very dark background in our image to either highlight the foreground dramatically or simply enable the viewer to ignore an otherwise uninteresting or worse yet, confusing "salad", then we must decide whether to increase the shutter or stop down the lens, or possibly even a little of each.
An instance when the option of a faster shutter speed could be beneficial is when shooting into the Sun (sunball) underwater in a close focus wide angle composition.
Fast shutter speeds can also be used to "capture" or freeze sun rays. Chrism is a master at capturing underwater sun rays at speeds up to 1/1000 sec.
Except for the sync speed and potential clipping* issue, the shutter speed has no bearing on the foreground exposure.
The aperture however is influencing the foreground exposure along with strobe intensity. If the lens opening must diminish in size to help control background lighting, then the need for more strobe light increases. As long as the strobe is up to the job, then no problem. On the plus side of a smaller aperture there also comes depth of field. But along with the potential advantage of an increase in depth of field comes a downside, diffraction.
If the digicam being used has a cropped sensor, then dof advantage of a small aperture is diminished somewhat. But there also exists times when dof is not desirable. Say if you wish to accentuate a specific critter from its surroundings or even feature of that critter like the eyes for instance.
Therefore it could be beneficial at times to have the option of a faster shutter in order to decrease the amount of ambient light illuminating the background exposure. This is especially true now that some underwater strobes are able to dump their energy up to five times faster than models of just a couple of years ago. That should negate the clipping issue.
BTW, if your point and shoot digicam is like mine, we do not even have a sync speed issue. But we still are faced with the clipping issue with certain strobes, especially if a full dump is required. BTW, I should explain clipping* for anyone not familiar with the term.
*Strobe flash tubes burn at set "brightness" level. They influence the foreground exposure by how long they remain illuminated. IOW, the strobe is not controlled by a rheostat, the total light output is ultimately controlled by how long the beacon shines. "Burn" time. In order to achieve their full potential, strobes must remain "on" for an "extended" period (we're talking fractions of a second, though)**. If the shutter closes prematurely while the strobe is still "burning", the action may "clip" some of the strobe's output. iow, if the "curtain" comes down too soon on the act, then the exposure will be adversely affected.***
[qoute]you'll never see that shutter speed underwater if you are balancing the flash with any type of background.[/quote]
I commonly shoot with fast shutter speed to completely black out unwanted backgrounds (see image). But then again, and as you suggest, I'm not trying to balance anything. Which is part of my point, sometimes balance is not the goal.
That's something to consider, but it also burns batteries.
True. But in actual use inconsequential. Personally I have never run down a DS 125 battery. My typical profile: two repetitive shore dives (@ 1 hr each, 35+- "average" shots per dive, one hour 15 minute surface interval). Then recharge.
A fully charged DS 125 can illuminate over four rolls of 36 exposure slide film with a full dump on each image, one second apart if necessary (but not advised!!!read: poor fish). If you are not shooting with full dumps, even more flashes per charge are possible. The strobe will fully recharge in 90 minutes using the quick charger. At night the modeling light can be also used as the primary night dive light.
vkalia,
The DS 50s could do what you're attempting. And some divers have gone that route. If you're hesitant to go the DS 125 route due to size, then you might be interested in waiting a month or so for the new DS 80 to be introduce. The 80 stands for watt seconds. Not sure yet how the Ike techs will choose to "spend" those watt-seconds. They could opt for a wider beam angle than the DS 50 or more intensity, that is closer to the guide number rating of the DS 125.
I should point out that you mentioned GN's of 10 and 9 for the DS 125 and DS 50, respectively. Those stated GN's represent the underwater GN measured in meters, ISO 100. To translate that to feet, ISO 100, they would be 32 and 28, respectively. To translate that into topside GN's the Ikelite site suggests doubling those figures. If we are to remind ourselves that this thread theoretically is about the Sunpak, I might as well add that the 22 GN rating supplied by Sunpak for the G Flash is the topside GN, measured through air in meters, ISO 100. The underwater GN should be lower if you take into consideration the density of the medium.
SeaYoda,
Thanks for the images! You may wish to read the article that Jeff Farris authored (link below). In it you will see a topside test he conducted to determine beam angle and flash intensity. Although it does not combine the effects ultimately that water will play with the beams of light, it may help give you and the rest of us an understanding as to how much wider a beam is created with the addition of that diffuser. Good luck and keep up the good work, bud!!!!
**Matthias Heinrichs is compiling a list of strobe full dump speeds:
http://www.heinrichsweikamp.net/blitz/indexe.htm
***Jeff Farris documented the effects of clipping and shutter speed in his strobe survey:
http://www.digitaldiver.net/images/strobearticle/strobearticlescrn2.pdf
note to self: put down the coffee!
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