Putting it All Together: Exposure Techniques That Work

Exposure Modes

Modern electronic cameras usually come with 4 different exposure modes. Different cameras may call them different things, but they all do basically the same 4 things.

'A' Mode

'A' Mode refers to "aperture-preferred mode." What this means is that you set the aperture, and the camera picks an appropriate shutter speed for you, based on the ambient light and whether or not the camera detects a strobe attached to your camera. If you're going to use an auto-exposure mode underwater, this one is probably it. The aperture is probably the singe most important control underwater. It affects all the light coming in to the camera, both ambient and strobe. It controls your depth of field. But most important, when you're using a manually-controlled strobe, the aperture setting is what decides how far you have to be from the subject for proper strobe exposure.

In all of the auto-exposure modes (A, S and P), the camera has to know the film speed in order to pick a matching shutter speed. Modern cameras have DX coding, which means that the camera can read the film speed from information on the outside of the film canister. Older cameras have a wheel you turn in order to tell the camera what speed film you're using.

'S' Mode

'S' Mode refers to "shutter-priority mode." This mode is the inverse of A mode. In S mode, you pick the shutter speed and the camera picks a matching aperture for proper background exposure. You will probably never use S mode underwater unless you are running your strobe in TTL mode. With manual strobe, S mode will require you to set up the shot, trigger the camera's light meter, note the aperture setting, and the change your strobe-to-subject distance or strobe power setting accordingly. That's just too much trouble. Even if the strobe is in TTL mode, you will only have a small range of shutter speeds to choose from--usually between /160 and 1/125 or maybe 1/250. 1/60 to 1/250 is only 2 stops worth--hardly enough range to make it worth using S mode in the first place.

'P' Mode

'P' Mode stands for "full program" or just "program." In P mode, all you do is point, focus, and shoot. The camera picks whatever combination of aperture and shutter speed it thinks will work, and goes from there. P mode is useful when you're getting started, because you really ought to concentrate on composition and framing rather than exposure, but it suffers from limitations similar to those of S mode. You can't really use manual strobe with P mode, because you won't know what aperture the camera picks until you trigger the camera's light meter. If you are shooting in TTL strobe mode, doing fish portraits or macro work, P mode will probably work OK for you. If you try to do wide angle work, though, P mode is likely to disappoint you.

'M' Mode

'M' stands for 'Manual.' Your camera, your settings, you're in control. Pick your own aperture and shutter speed. You can still use TTL strobe in manual mode, but why would you want to? If you're in manual mode, just go for the whole enchilada and run your strobes on manual, too. Manual exposure is handy when you think the camera is likely to make a mistake regarding the background exposure. For instance, if you're shooting out the porthole of a wreck, you don't want the camera to get fooled by all the black around the porthole. You want the image on the other side of the porthole to be properly exposed, even if that means making the inside of the wreck completely black. With auto-exposure, the camera will likely try to balance the whole scene, resulting in a too-bright porthole. So meter on the open water from close up to the port hole, lock those settings in on manual mode, and pull back to compose the shot the way you want it.

TTL strobe

TTL strobe is a godsend when it works. There are plenty of situations when it doesn't work, however. Don't expect TTL to work when the subject is very shiny, or very dark. Don't expect it to work when the subject occupies only a small portion of the frame. Also, don't expect it to work if you are very close to your subject unless you stop down the aperture fairly tight. The problem is that the strobe's intensity on-subject can get so high that by the time the camera has quenched the strobe, it's too late. Inside 2 feet, expect to have to set your aperture at f/11 or higher. This will give the strobe enough time to quench that it won't overexpose your subject.

In general, you can use TTL strobe in all exposure modes discussed above. When you're just getting started, shooting fish portraits in P mode and with TTL strobe will get you some results that are sure to please. It's a good way to learn about the other parts of your camera system before you try to complicate matters with manual controls.

Also, TTL mode works pretty well for macro photography. There you want to use A mode or M mode, because you want to set the aperture manually in order to control the depth of field. See the section on macro photography techniques for more details.

Manual Strobe Control and the Modern Camera

Modern cameras have all sorts of nifty exposure modes, but if you're using manual strobe, only one of them is really useful. If you're taking wide angle shots, especially close-focus wide-angle (CFWA) shots, manual strobe control will be an important part of taking successful pictures. If you're shooting macro, that's not as true, but there are still limits.

Wide Angle

These days, I do almost all of my wide-angle work in A mode. If you have a good camera with the ability to work in A mode, I recommend it highly. A mode gives you almost all of the benefits of manual exposure control, without the hassle of having to re-meter every shot. Most of the time, what I do is this: set the aperture at f/5.6 or f/8, set the strobe power to the right setting to place my subject at 3-4 feet, and then just shoot everything from that distance. I can swim around for almost an entire dive without deviating from that plan. It makes strobe exposure a snap; as long as the subject is anywhere close to the right distance, I get proper exposure. If I need to take a picture from a different distance, I have two options: open/close the aperture a little, or change the strobe power. Usually I change the strobe power.

The only danger with this approach relates to the way the camera picks shutter speeds. The camera will not pick a shutter speed too high for proper strobe sync. If the scene is very bright and you have the aperture set too wide, the camera can easily reach the max sync speed, in which case your background will come out too bright. To avoid this, be sure to re-meter the scene at the start of every dive, and every time you change depth by 30 feet or so. My camera (Nikon N90s) has a meter in the viewfinder that tells you the shutter and aperture combination, and then whether that's too much or too little light. When I re-meter, I just point the camera at blue water, and press the shutter release halfway. Then the meter in the viewfinder tells me what I need to know. If the scene is too bright, the camera will show me whatever aperture I picked, 1/250 shutter speed (that's the max for the N90s), and then +/- meter will read way over to the + side, telling me that I've reached the fastest shutter speed and there's still too much light. Then I have to stop down the lens a bit, and try again. When I finally get the meter to agree that it can get a good exposure for the background, I have to set my strobe power to match 3-4 feet at whatever aperture made the camera's meter happy.

Similarly, there's a danger in too-slow shutter speeds. Depending on your camera model, the auto-exposure system may not want to set the shutter speed below 1/60 or 1/30. In this case, if the ambient light is too low, you can easily pick a 'normal' aperture like f/8 and not have enough light for proper background exposure at the minimum shutter speed. If this happens, you can still get a proper strobe exposure but the background will be too dark. The procedure for avoiding this is just the same as the one for avoiding too-birght backgrounds. Every time you start a dive or change depths by 30 feet or more, meter the background water and set the aperture to a value that lets the camera pick a good shutter. Then be sure to remember to set the strobe power for proper exposure at whatever distance you've picked.

It is entirely possible that you will not be able to set a wide-enough aperture for good background exposures. In other words, you're at f/2.8 and the meter is still telling you "too little." This mostly happens when you're diving deep and/or with slow film. If you find yourself in this situation, there's not much you can do about it on that dive. So think about what sort of dives you're going to make, and pick a film speed accordingly. If you plan to go deep or are diving in low-vis water that eats up the sunlight, don't load the camera with ISO 50 film. Pick something higher, like ISO 100, 200 or even 400. There are 3 stops between 50 and 400. That means that f/2.8 at ISO 50 would translate to f/8 at ISO 400...which in turn means you have three more stops (f/5.6, f/4, f/2.8) to play with in order to satisfy your camera's meter.

There is one more thing you can try. If your camera allows it, you can blow through the minimum shutter speed and shoot really slow. My N90s, for instance, has a "slow sync"
mode available that tells the camera's computer "don't stop at 1/30 seconds." The camera will then pick whatever shutter speed it wants, even if "5 seconds" is the right answer. That's great as long as you think you only need an extra stop or two, but I forgot I had it on that mode in Devil's Throat once, and the shutter stayed open for almost the entire time I was in the swimthrough. I thought the camera had broken, until it finally closed the shutter. I then immediately took it off slow sync, and I haven't used it since. :)

Macro

If you're shooting macro, you'll still want to use A mode most of the time. The reason for this is depth of field. You can read up on depth of field in the macro section, so I don't want to cover it in depth here. But basically, depth of field is the range of lens-to-subject distances that will be in focus on a single picture. DOF changes depending on what lens you use and how far away the subject is, but there is one rule that is always true: the higher your aperture number, the greater depth of field. As a result, you will shoot most of your macro shots at extremely small aperture, often the smallest aperture your lens can manage: f/22, f/32, or even f/64! Because of this, you can't afford to have the camera pick some random aperture/shutter combination. You have to be in control of it yourself. Thus, if you are going to use an auto mode for your macro shots, A mode is going to be it.

So that's it for manual strobe and the modern camera: use A mode when you can, and you'll have a great time of it.

Manual Strobe and Manual Camera

So you have a camera that doesn't have any auto-exposure modes, or you have one with auto modes but you want to shoot manual for some reason. What do you do?

First off, you need a light meter. If your camera has one in it, great. If not, there are several dedicated light meters you can buy to do the job. The old standby is the Sekonic Marine Meter II, an amphibious light meter that you'll see in use all over the world. Nikon used to make a housing for a different Sekonic meter, and Ikelite makes a digital light meter for use underwater. I'm sure there are others.

So you have a meter. Great. What do you do with it? First off, you're going to simulate A mode here. Pick an aperture that works for your strobe and strobe-to-subject distance. Then pick up the meter, dial in your film speed, and take a reading on the background light. The meter will then give you a range of aperture / shutter pairs that will work. Find the aperture you selected, and then the shutter speed that matches it is the one you want. If your aperture is not on the list, something is wrong: either the scene is too bright or too dark for that aperture. If you have a small aperture (like f/22) set, pick a larger one (f/8 is a good place to start, usually). Similarly, if you picked f/2.8 and your aperture isn't on the list of acceptable aperture/shutter combinations, try again at f/8.

You've got an aperture, you've got a shutter. Now the only thing left to do is dial in the strobe power setting you want. You're ready to go!

Just like with A mode, any time the ambient light changes due to depth, clouds, etc., you'll want to re-meter. In truth, you want to re-meter for every shot, but that's a real pain, so I'd just stick with the same shutter until you suspect there's reason to change it.

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