Most underwater photographers today learn to shoot with TTL (through-the-lens metering). Unfortunately, most also do so without understanding its benefits and drawbacks. Under many circumstances, it works quite well. But TTL is far from foolproof. In fact, there are times when it will most assuredly fail. To best utilize this tool, you have to know its secrets.
Text and photography by Stephen Frink
http://www.stephenfrink.com/sf-tips/TTLmystery/
Coupling Range | How to Make TTL Your Friend
Self-Assignment
What It Is
Joe Liburdi and Cara Sherman, authors of The New Guide to Sea & Sea, explain TTL this way: "When the strobe is fired, light strikes the subject, reflects off the subject back through the camera lens and onto the film plane. A built-in sensor beside the film plane measures the amount of light that has passed through the lens and is reflecting off the film (hence the name: 'through the lens') and signals the strobe to shut off when there has been enough light for a correct exposure."
By using electronics to vary the duration of the strobe's flash, TTL would seem to promise correct exposures automatically. But there are times when TTL delivers on that promise and times when it definitely does not.
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| Photo A illustrates the effects of overexposure from TTL on wide-angle. There was not sufficient foreground to reflect light back and quench the strobe. As a result, the strobe dumped full power, thereby overexposing for the f/5.6 setting. B There's a similar subject in the foreground, but this time the strobe was set at 1/4 power, manual setting, which rendered a proper exposure at the same f/5.6 setting. C An artful blend of strobe and ambient light comes from using the strobe on 1/4 power, manual setting, thereby allowing a wider aperture than would be required with either TTL or full manual. This permits greater ambient light to the scene, perfect for the light lost at depth in this scenario. |
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| A Background makes a big difference in determining whether to trust TTL. The coral reef is nearly on the same plane as the clown triggerfish and therefore reflects back enough light to make TTL accurate. B The triggerfish surrounded by open water reflects back a small portion of the light emitted by the strobe. Choose the aperture as a function of strobe-to-subject distance. |
TTL works best in two very specific situations:
1. When a subject of average reflectance fills at least the center one-third of the frame (the center two-thirds is even better). Since most macro shots offer this kind of composition, combined with the advantage of a background located nearly along the same plane (for uniform reflectance), TTL is an extremely reliable tool for macro work. However, successful TTL standard-lens (fish) photography depends to a great extent on how well the marine life fills the frame and how distant the background might be. A large angelfish against a star coral head will probably do well. A single horse-eye jack in a field of blue water will likely be overexposed.2. TTL works best when the strobe overpowers the ambient light level. Remember that TTL does not take into account the available light on the scene, a fact that can result in overexposure, particularly in shallow water. Think about a Nikonos V, which has its fastest strobe synchronization speed at 1/90th second and its close-focus capability with a 35mm lens limited to about three feet. On a clear, shallow reef at midday it's likely that considerable ambient light will illuminate the scene. The available light level may require a smaller aperture than would be appropriate for the 1/90th second synch speed at a given strobe-to-subject distance. Between the f-stop required by the strobe and that required by the available light, always choose the smaller in order to avoid overexposure.
When It Won't Work
There are three very specific situations when TTL doesn't work:
1. TTL is unreliable when there is a relatively small primary subject, especially when that small subject is isolated in front of a distant background or surrounded by open water. This is why wide-angle photos, which often include significant areas of open (and therefore non-reflecting) water, are notoriously poor candidates for accurate TTL exposures.2. TTL is unreliable when using wide and small apertures. In the first situation, the strobe pumps out too much light for the particular f-stop before light can reflect back and signal the strobe to shut down, resulting in overexposure. Conversely, even TTL can't make the strobe pump out more light than what comes from full power. Using a smaller aperture than is required by a full power manual setting at a given strobe-to-subject distance will naturally cause underexposure.
Coupling Range
The Bottom Line--Even with a subject ideally suited for TTL, there remains a TTL coupling range situated somewhere between these two certain causes of over and underexposure. I find with a Nikonos V and SB-105 speedlight, this coupling range is only about 2 stops. Therefore, with ISO 100 slide film and a strobe-to-subject distance of 3 feet, the coupling range is likely from f/8 through f/11. With a more sophisticated metering system such as that found in the housed Nikon N90S, the TTL coupling range might expand to as much as 3.5 stops, but regardless of the system used, TTL is not a mindless panacea.
How to Make TTL Your Friend
These five simple techniques can greatly improve the likelihood that TTL will work for you, not against you.
Self-Assignment
Your first in-water exercise for testing your TTL system should be to kneel in the sand three feet away from a subject of average reflectance, say a brain coral or your dive buddy. Set your camera on TTL and automatic. With ISO 100 slide film (the exposure latitude of print film is too great for a definitive test), shoot a series of brackets from f/5.6 to f/16 in full stop increments (four shots). Then set the camera on manual (1/90th) and the strobe on full. Do the same bracket test of four exposures. Have the film processed and returned unmounted. Lay it out on the light table to determine two things: the TTL coupling range for your system (as determined by the acceptable exposures) and the correct setting for full power at three feet. Once that is known, it is easy to interpolate for other distances smaller and greater.
