Hello,
My name is Fred. I started my underwater shooting using manual strobe and by the mid-nineties had changed over to TTL with a housed Nikon F4. I'm now shooting virtually everything TTL (yes including wide-angle with 17-35mm, 14mm & fisheye) with housed F5's. I believe TTL allows me to get much more consistent results, especially on the one-shot opportunities, than non-ttl ever did. And I have reasonable photo credits to back up the statement including a number of dive magazines, a few hit with German GEO, and five underwater stories in National Geographic.
TTL is not magic. It requires some careful testing and thinking as does any good photography. There are few general factors you need to consider.
Is the primary subject the same general brightness as the background? If its lighter (as on dark sand) you need to add a little minus compensation.
Is the primary subject quite small in the picture? If it is use some minus compensation.
Is the picture really three dimensional and where is the prime subject? If the subject is in the forward part of the picture it will tend to be rendered a little overexposed when TTL averages the whole scene, so again use a little minus compensation to darken the average and render the prime subject properly.
Is ambient light a factor? Determine if it is by metering the subject itself and if it is more than three f-stops darker than the water color setting you are using, it can be treated as a flash only subject with consideration for the three factors above. If the subject, like most fish on the reef, is only a stop or two darker that your water color camera setting, this means it is getting a significant amount of its light from ambient light, so add considerable minus compensation to reduce the flash exposure to offset the amount of light contributed to the subject by ambient light.
Example: I am shooting a school of jacks and my water color setting for a nice blue color is 1/125 @ f8 which shows up as -.3 on the analogue metering scale of the camera. The jacks read about 1/125 @ f5.6 (about -1 on the analogue scale). This means the jacks are already getting at least 50% of the light they need from ambient light. A full TTL flash will give them another 100%, resulting in at least 150% or a half stop overexposure. And this doesn't consider that some of the jacks are closer than others.
In this type of situation I find that two quick shots, one with the compensation at -1.7 or -2.0 and another with one more full stop of minus compensation (-2.7 or -3.0) will almost always give me at least one professionally usable picture. the camera is operated in the manual exposure mode. It is important to meter the water color and jacks with the compensation set at 0.0. And then once the ambient light setting is determined, change the compensation to control; the flash output. I make dealing with the two different compensations even easier by setting the cameras to a normal and -1.0 shot bracket pair, and use the custom functions to apply auto-bracketing to only the flash exposure. Finally, after ambient metering change the compensation to either -1.7 or -2.0. Pairs of exposures can then be made quickly and easily by simply pushing the trigger twice.
Doesn't a whole f-stop flash differnece seem like a lot. Do the math. If you're getting 50% or even 60% of the light from ambient, a -2.0 flash adds 25% more and a -3.0 adds 12.5 % more. Both of these give the jacks something like 75% and renders then just right to go with a nnice blue water color. This allows you to really control the balance of the ambient light part of the picture with the flash part, regardless of whether the jack come a bit closer or not, because the TTL will simply cut the flash output a bit. And you will have incredible difficulty matching this with any manual flash guide numbers or tables for different distances.
And this same technique can work on reef scenes or anywhere ambient light makes a major contribution to the exposure. All you need are good TTL strobes.
In an earlier post Dr. Mustard posed a situation of a wide-angle shot with a yellow sponge on the left side of the shot and a darker red one on the right. And said How can I do it with TTL? The answer is fairly straight forward. Since the yellow sponge is already brighter than the red one, position the strobe more to the right side of the picture and at enough of an elevation so that the distance to the front part of the sponge and its farther parts are more equal. Even easier if you use twin strobes for wide-angle and simply move the strobe on the left side much farther back that the one on the right.
Eric Hanauer in his post said, "Its that 2 1/2 stop latitude on the dark side that got me hooked." James wisely stated later that even with digital you can't really miss the shot by two f-stops and still not pay a price, which is something I've confirmed playing with a Fuji S2. If you simply use digital so that you can better correct your bad exposures, then I feel you'd do a lot better learning how to use your camera better.
This does not mean that I don't like digital. In fact I've just ordered a D2X. But I feel I'll be using it mainly topside until someone comes up with a really good underwater I-TTL strobe. In the meantime, I expect I'll continue to do most of my underwater shooting with a film camera and good TTL strobes.
divegypsy
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