I thought it might be helpful for those asking questions similar to the ones brought up by Ian and Spoon to expand on ssra30's insights.....
The main feature I would want to look at when buying a strobe is power output
Power output of a strobe can be expressed in several ways. Some manufacturers offer the Guide Number Rating, either topside, underwater, or both. Some measure in feet, others metric, a sensitivity of ISO 100 as an unwritten standard. Other manufacturers include the Watt second rating.
The Watt second rating expresses how much energy the storage capacitor can hold. This measurement is important because although two strobes may have a similar guide number rating, strobe "A" may be designed to spread that intensity over a much greater area (beam angle), necessitating much more reserve power than strobe "B". If your goal was wide angle, a high Watt Second rating combined with a high guide number rating would be desirable. If your goal was macro work, the capacitor's storage capacity would not necessarily have to be as great (smaller beam angle emitted requiring less energy consumed). Some manufacturers share the opinion that Watt second ratings are not as relevant as others. They stress efficiency in how the energy storage is utilized.
Topside guide numbers for strobes are derived by a simple process. An object is placed 10 feet from the strobe and camera in a dimly lit room. The strobe is set to full dump. The camera's sensitivity setting is set to ISO 100. A series of images are captured, varying the aperture while maintaining the proper shutter sync speed. Whichever aperture number offers the best exposure is then multiplied by the distance from strobe to subject (ten feet). The resulting number becomes the Guide Number.
For example, say f/6.3 offered the best exposed image, not too bright, not too dark. We would then multiply 6.3 times 10. That particular strobe would then be described as having a GN of 63, measured in feet (as opposed to measured in meters), ISO 100.
A formula that expresses this, commonly referred to as the Guide Number Formula looks like this:
GN=A*D
Guide Number equals Aperture times Distance
Because it is an equation, it can be written several ways in order to find any third variable when two are known. For instance:
A=GN/D
would also hold true as would
D=GN/A
In theory, the Guide Number formula holds true underwater within certain guidelines. The most powerful strobes can only effect their influence no more than 5-6 feet underwater. And at less than one foot underwater from strobe to subject, I've read that the GN formula is less effective.
If anyone wishes to expand on the guide number theory and it's use underwater, please start a new thread. I'd be happy to contribute what I (believe) I know
....recycle time, number of flashes per charge....
These numbers will vary by type of battery used (NiMH, Alkaline, NiCad, Lithium, etc), age of battery, and ambient temperature. The colder the water, the older the battery, the slower the recycle time, etc....
Also, full dumps are not always the norm, especially in macro work. So in practice, card writing time (with digital cameras) may also be a limiting factor~consideration.
I would also want something that have a bit more than just half and full power setting.
me too
. Especially if you enjoyed macro photography along with wide angle. Flexibility is a key with creative underwater photography. Although strobe to subject distances and diffusers add another level of control, the ease of keeping everything else constant and just turning a simple knob to limit intensity is much easier in actual underwater practice.
Color temp is less important in my mind especially if you are shooting digital with RAW file, I think.
I'd like to add, imho, color temp is more critical when mixing strobes of different Kelvin ratings. IOW, I'm not sure if having a 4800° main strobe with a 5700° fill strobe is a good idea. That said, I did it for a year and didn't lose any friends
.
One other point I'd like to offer. With regards to electronic devices, we dive in a harsh environment, under less than ideal conditions at times (read rocky shore dives, rinse tanks, and filled boats). I'd choose a strobe manufacturer with a record of customer support. I have received excellent support from InonAmerica and Ikelite in that regard. Possibly other divers could offer similar recommendations based upon that aspect of the "choosing the correct strobe" process.
