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What’s the simplest way to bracket exposure with a housing system for those pictures that are really important, such as that rare whale shark or a macro shot of a cleaner shrimp in the mouth of a moray eel?




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Questions and Answers (Q & A's)

From Alan Broder (from Ocean Realm Magazine - October 1996)


You don’t mention which camera you have so I’ll guess that it’s one of the Nikon N8008 or N90 cameras. This puts the odds in my favor to the tune of better than twenty to one, considering what’s out there these days. The general principles will apply to just about any camera.

I’ll assume that your whale shark photos will be of the ambient lit variety, since possession by a civilian of a strobe putting out the amount of power needed to light a critter of that size would certainly be a capital crime in virtually any country in the world. Your basic exposure can be manual or automatic.

If you choose to use a manual exposure, you’d select an aperture/shutter combination which would give you a "best exposure" using your light meter to guide you. Generally, when you’re doing wide-angle photography underwater, ambient light exposure and background water color are interchangeable. You’d select a light meter reading using aperture and shutter speed controls which would yield the depth of blue desired in the shot. Generally, this would be average (zero on your light meter) to minus one-third or two-thirds of a stop, although in some situations you might like a full stop under. Underexposing water yields a deeper, richer blue. Overexposing slide transparencies is generally no desirable, although there are some situations when a slight foray in this direction might be considered. So you make your decision as to the depth of blue you want for the water. You select your shutter speed and aperture. If the light is coming from behind your subject, you’ll have a silhouette in some measure. Increasing exposure will brighten the water and allow more detail to show in your shark. If the light is coming from behind you, separating the shark from the water becomes a major issue. Without a bunch of experience, it’s hard to know exactly what exposure will look best. Time to bracket!

You bracket in manual by moving either the f-stop or the shutter speed. To bracket under, you’d either select a faster shutter speed or a smaller aperture. If your original exposure gave you enough depth of field and a shutter speed fast enough to adequately stop movement—both yours and the shark’s—you can’t screw this up. If you’re bracketing to lighten the photograph, you’ll either open the aperture or slow the shutter speed. Opening the aperture decreases depth of field. Slowing the shutter can allow movement to blur the image. This is especially true when you’re hauling derriere, swimming like crazy trying to keep up with this majestic, graceful, humongous, and seemingly slow-moving object de photique. Concentrating on holding your camera steady on your subject under these conditions is a serious challenge at the very least. You’re generally safer to open up the aperture to increase exposure, since depth of field is generally less of an issue with a subject of this size. Naturally, this depends on the angle you’re shooting from.

It might, in fact, be easier to shoot on one of the automatic settings. When shutter speed is the more critical variable, shutter priority might be the best choice. You should be watching the aperture that’s being selected by the camera for the speed you set and be prepared to back off if the speed selected causes the aperture to become unacceptable. You can now bracket with the exposure compensation control. Remember that it might be easier to bracket with the aperture control since the exposure compensation method may require moving two controls at once.

Macro is another story. Your subject will, almost certainly, be entirely strobe-lit. I’d suggest that you use TTL flash exposure control. Your shutter speed will be set to a convenient sync speed—say 1/60 or 1/125 of a second. Shutter speed has no effect on your photograph as long as you select a speed slower than the maximum sync speed for your camera and are not shooting into the sun, or nearly so. Your aperture will be selected to yield the optimum depth of desired for the shot, generally around f22 for most macro. The strobe, or strobes, will be positioned at a distance from the subject that is within the maximum distance as which the power of the strobe is sufficient to provide a proper exposure of the subject at the aperture selected (you paid for the magazine; you’re allowed to read that twice, I did!). When you use TTL, the resulting exposure will yield an "average" exposure equal to 18 percent gray as measured and weighted by the meter and computer. It’s generally dam close, if not right on. To assure that you’ll get a proper exposure of a specific part of the frame that’s darker or lighter than its 18 percent gray surroundings, you can bracket either by adjusting the exposure compensation control or the ISO control. Since you’ve set you f-stop and shutter speed manually, these won’t change when exposure compensation is adjusted. If you have a light subject, such as a flounder on sand or an albino eel with a cleaner shrimp in its mouth, you might start with a base exposure of plus two-thirds of a stop or so. TTL will yield 18 percent gray at zero exposure compensations. You’d then do your bracketing up and/or down from there.

You could expose manually with your strobes. In this case, you can bracket by adjusting your aperture or by moving your lights in or out. This requires practice, record keeping, and disciplined systematizing in order to achieve predictable results. If these things don’t bother you, then maybe sacrificing ideal depth of field for exposure and/or arm-wrestling with you strobes while your subject leaves the scene will.



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