thanks lairdb........
me? quickly point something out? huh?
First I'd like to mention that imho, the DS 50 does not get the respect it deserves. I think this is at least in part due to it's name. If Ike had named the strobe the
Galaxie 500 Stun Gun or the
DS 5600 Stargazer or any other Madison Avenue type splashy moniker, I think some would actually take the powerful strobe more seriously.
But since the DS 50 presently competes with other "flashier" named flash guns with higher numerical name tags, some I believe assume it is not as powerful. Some of the DS 50's competitors named their strobes after their top side (metric) guide number rating (typically a number 2x or more than underwater rating would be). Another strobe company placed an extra zero (or two!) after it's topside metric GN. And while other strobe manufacturers prefer to name their strobes after the Greek Alphabet, Ikelite chose to name their strobes after the amount of energy the capacitors within each strobe could hold.* btw, just in case anyone misreads my comments, I'm not talking about the quality of any of these other strobes, only their names. imho, most if not all of these other strobes are very well made. So much so that I don't think those strobes have to hide behind the sexier monikers their marketing departments branded them with.
So the reason the DS 50 got it's name is due to the watt-second energy rating of the strobe (the DS I believe stands for digital substrobe). The strobe's capacitor is capable of storing 50 watt seconds of energy. It uses that energy to spread a beam of light, measured around 70° (more with a diffuser), with an underwater guide number rating of 28 (ft. ISO 100).
I'd like to compare the major difference between the DS 50 and the DS 125 for a moment. As you can see, the Underwater Guide Number Ratings are very similar. The DS 50 at 28, and the DS 125 at 32. Relatively speaking, close, somewhere around a 1/3 of a stop of light. The major difference however is that the DS 125 spreads it's light over a much greater area, around 100°. The DS 125 must create and store, and eventually emit more that twice the energy (measured in watt seconds) to achieve this. And that is why the DS 125 is physically larger than the DS 50. It must be in order to accomplish this goal.
To simplify, if you consider the DS 50 a spot light and the DS 125 a flood light, that may help to explain the main difference. And as you can see, the Guide Number Rating of a strobe does not tell the whole story. The energy rating, once again measured in watt-seconds, is also considered an important consideration by many.
The DS 50 is designed to illuminate the field of view of your typical point and shoot camera. So if your camera lens zooms from roughly 28° to 105°, the DS 50 will be a good match. If you wish some day to add a wide angle conversion lens to your rig, a second DS 50 or a DS 125 would be two options to consider (although I've seen pool tests where the DS 50 successfully covered the field of view of a WAL).
Now to answer your question, laridb
The DS 50 as mentioned has an uw gn of 28. The standard Ikelite diffuser reduces the intensity by one stop of light (in essence, reduces the output influence by one half). To determine the new guide number of a DS 50 with a diffuser installed we divide by 1.4 (for an excellent explanation, I recommend Dave Read). 28/1.4=20. The new guide number will be 20. So if you are using the manual controller to limit the DS 50 output and you set the controller to F (full), the gn of the strobe with the diffuser installed will be 20 (ISO 100, ft). If you prefer to set your digicam to ISO 50, then your new guide number, still with the diffuser installed, will be (20/1.4=14.29) rounded off to 14 (once again if you would like to know why I divided by 1.4, please google Dave Read).
From here we can again visit the guide number formula to determine aperture and distance combinations.
GN=A*D
the Guide number should equal the aperture (f/stop) multiplied by the distance from strobe to subject
14=f/4*3.5
An aperture of f/4 and a distance of 3 and one half feet (strobe to subject) should result in a properly exposed foreground when the manual controller is set to F and the DS 50 has a diffuser installed and the ISO setting is 50.
One foot closer( 2.5 ft), two clicks clockwise on the man con (setting 1).
Another foot closer (1.5 ft), two more clicks (setting 2).
etc.........
Keep in mind that the Guide Number Formula is exactly that, a
Guide. Prior to the digital age, photographers would bracket to insure the results they wished for. Today we have LCD screens to speed up the process and shorten the learning curve. Shoot, Review, Adjust, Shoot Again. Even the best subscribe to the mantra. And to borrow another saying, this one from Chrism, Shoot 'til your fingers bleed......
hth,
b
btw, I have no affiliation with Ikelite. Just a user of their products.
*DS 125 according to Ikelite actually is rated at 110 WS
