Basics of Strobe Photography

"Wide macro" sounds like a contradiction in terms. Wide photography captures big scenes; macro photography captures tiny details. How can you do both at the same time? By moving close to your subject with a wide-angle lens.
The wide-angle lens magnifies the close-up subject in the foreground to bring out details, reducing background and providing a sense of depth. The wider the lens, the greater its depth-of-field, so it's possible to capture both the subject in front and its background in clear focus. (Compare that to macro photography with a very narrow depth of field that forces focus to just one part of the image.)

The built-in flash of a compact digital camera doesn't provide the flexibility you need for successful strobe photography. It's usually located near the lens and can't be moved. Shooting with the built-in flash evenly illuminates all (or sometimes just part) of the subject. You can't angle it for other effects as you can an external strobe, which is mounted on an arm extending from the camera.
When you use a compact digital camera's built-in flash underwater, you'll see a lot of backscatter in your images. Backscatter looks like falling snow, and is caused by particles in the water such as plankton or kicked-up sediment that reflect the light of the built-in flash.
Because a built-in flash is right next to the lens, its light reflects directly back to the lens, exaggerating the backscatter. In situations with backscatter it's often better to turn the built-in flash off (suppressed flash mode) before you shoot. Natural light alone, though, saturates your photo with blue or green (whichever color the water is around you).
An external strobe lets you avoid backscatter and fill your subject with light, bringing out true colors even in distant or wide subjects.

As light passes through water, the water absorbs different colors at different rates. Reds, oranges, and yellows disappear first, while blue (and to some extent green) persist much longer before being absorbed. The deeper you go, the more the light is filtered until nothing is left but blue, leaving you with monochromatic images. Shooting without a strobe at this depth can only produce photos like those below.

Why not just use a built-in flash to fill in colors underwater? Why use an external strobe? There are three big drawbacks to the built-in flash:
Power. As you learned earlier, water filters out light, so an underwater flash must put out light strong enough to travel to the subject and reflect back again without losing too much of its strength. A built-in flash is designed to work on land, in air that absorbs very little light. When used underwater, the built-in flash lacks the power (measured by its Guide Number) to adequately light up your subject. It may work for close-up macro subjects, but typically fails to light up anything over an arm's length away.

Backscatter. Because the built-in flash is usually located right next to the lens, it's reflected by particles in the water and adds backscatter to your images, sometimes forcing you to give up on flash for the picture.

Lens shadowing. If you put an add-on lens on your housing for macro or wide-angle work, the built-in flash isn't usually located high enough to completely clear the lens tube. The lens tube casts a shadow on your subject, sometimes just darkening part of the image, other times completely blacking it out. The closer you get to your subject, the more pronounced the shadow.

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Watch a movie of strobe photography in action.

Strobe
Supplemental lighting equipment that emits a high-intensity burst of light synchronized with the camera's shutter. Also called a flash. A compact digital camera's built-in flash produces low-intensity light and is almost useless for photographing distant subjects underwater. An external strobe is much more effective.

EV (Exposure Value)
A value that specifies the amount of light entering the camera (the brightness) as determined by the shutter's aperture value and its shutter speed.

Color temperature
A value expressed in units of degrees K (Kelvin) that specifies the color of light. The color temperature of sunlight on a clear day is typically 5500K and is considered "natural" color. Higher color temperatures are bluer, lower color temperatures are redder.

Slave function
A function of a wireless secondary strobe that detects when the main strobe fires so that it can simultaneously fire the secondary strobe.

GN (Guide Number)
A value that expresses the amount of light that a strobe emits. A GN is usually calculated as the aperture setting that sets correct exposure for a subject 3.3ft / 1m distant when shooting with ISO100 film and the strobe set to maximum intensity. The larger a strobe's GN number, the greater the amount of light emitted by a strobe.

Strobe beam angle
The maximum angle of the light emitted by a strobe. The beam angle determines the area illuminated by the strobe's light. If the strobe beam angle is narrower than the angle of view of a camera's lens, the edges of the picture will be dark. To increase coverage and get rid of dark edges, use two strobes to expand the lighting range.

Recycle time
The length of time after firing that it takes a strobe to regenerate a charge for another flash. A strobe must store a significant charge of electricity before it's powerful enough to trigger another charge. Recycle time is determined by battery power, power of the flash, and other factors.

Diffuser
A filter for the front of a strobe that reduces and spreads emitted light. A diffuser is useful when you can't reduce a strobe's power any further and you're shooting close up so the strobe is too powerful. A diffuser typically removes enough light to drop exposure by one full F stop.

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