The importance of an external strobe for anyone wishing to do underwater photography cannot be over emphasized. Strobes provide underwater photographers with the much needed light that is the key to getting sharp and colorful images underwater. For someone looking to purchase an underwater strobe for the first time there are so many features and specifications to choose from, that often, the first time photographer is deterred from purchasing an external strobe kit simply out of fear of selecting the wrong one. This guide will help you understand what the common strobe specifications listed by manufacturers actually mean which should help you in your decision making process.
You will often see the specifications of a strobe/flash unit listed somewhat in the below manner:
Specifications
* Guide No. 60/20
* Coverage Angle : 60 degree
* Color Temperature: 5700 K
* Cycle Time: Approx 7~8 seconds
* Rated Depth: 54M (180 Feet)
* No. of Pre-Flash Settings : 4
* No. of Power Output Level Settings: 4 ( 10% > 25% > 50% > 100% )
featured above is the Sealife SL708 DC1200 Elite Set Digital Camera available at Leisurepro.com
Guide Number
The power of an underwater flash/strobe is usually given by a guide number. The higher the guide number, the stronger the strobe. The guide number is also used by photographers to calculate exposure settings while using a strobe. Normally your strobe’s TTL (Through The Lens) setting can usually automatically provide decent exposures calculating the distance of the subject and aperture and automatically adjusting the flash intensity accordingly. However while using manual settings or wide-angle lenses, or even photographing small subjects on dark backgrounds, one can calculate exposure based on the guide number. Many strobes have an “exposure guide” printed on the side of the strobe., this table gives the appropriate f-stop (aperture setting) for proper exposure at a given distance. Guide number divided by distance equals f-stop. So to take a picture of a diver at 3 feet with a guide-number 27 strobe, you would calculate 27 / 3 = 9 and set the aperture to f-9 for the perfect exposure.
Guide numbers are usually given in meters, at ISO 100 but some manufactures may specify guide no in feet.
Coverage Angle
Coverage Angle means exactly what it says, it defines the angle or arc that your strobe will illuminate when fired. It is important to keep in mind that the flash coverage angle should match the angle of coverage of your widest lens otherwise your shots end up lit very unevenly. Most strobes cover either a standard or wide-angle lens so If you buy a strobe to match your widest lens then you will cover all eventualities. Strobes meant for wide angle usually have an angle of coverage of 90-100 degrees or more in both directions. So when purchasing a strobe, opt for a higher coverage angle which will allow you more versatility if you switch to wide angle lenses. However strobes having smaller coverage angles do not necessarily make them a poor strobe, but only more specialized to regular lenses.
featured above is the Ikelite Substrobe DS-160 Kit with Sync Cord & Ball & Socket Arm System for Ikelite Housings
Color Temperature
Color temperature of most strobes will range from 4700 to 5600K. Increasing the color temp of a photo or warming it up brings out yellows and oranges. Decreasing the color temp of a photo or cooling it down, brings out blues in your images. Some of the higher end strobes have adjustable color temperatures which allow the user to adjust the strobe color temperature according to the subject and ambient surroundings.
