Text and Photography by Stephen Frink
"So, what's the big deal about shutter speeds?" a student asked me in a recent photo seminar. "Getting the right exposure is all about how close I can get and what strobe power I use, and then I just set the right f-stop, right?"
"Well," I answered, "getting good color underwater is strobe-dependent, so at a very basic level, you're right. But if there's any measurable ambient light on the scene, creative control of all the light management tools is an essential skill." It's what underwater photographers can use to elevate their work from a simple record of a scene to true art. Shutter speed, strobe power, f-stop and ISO settings together determine every aspect of the exposure, including foreground, background, motion blur and depth of field. Creative composition aside, being technically savvy can pretty well assure the right exposure. But true creative control comes from previsualizing and controlling the balance of these variables.
How Shutter Speed, Aperture and ISO work together
There is a clear relationship between shutter speed, aperture and ISO that is well known to topside shooters. Underwater photo expert Jim Church explained this with a metaphor about a herd of cows in a pasture. Imagine the cows are the light. Doubling the shutter speed setting (1/100 sec to 1/200 sec, for example) halves the time the shutter stays open--the number of cows getting in depends on how long the gate stays open. Similarly, doubling the aperture setting number lets in half as much light--the number of cows getting in depends on the size of the gate. And each time the ISO number is doubled, the effective speed at which the sensor absorbs the light and records the image (film or digital) is doubled--the faster the cows move, the more that get inside given the size of the gate and the length of time it's left open.
For topside photography, this translates into a standard rule called "Sunny-16." The Sunny-16 rule states that when the light is good and falls on the subject (usually between 10 a.m. to 3 p.m. on a bright, sunny day with the sun at your back), the correct exposure setting is f/16 and a shutter speed that is the reciprocal of the ISO. So, with ISO 100, shoot f/16 at 1/100 sec. For every one-stop change of the f-stop, the shutter speed requires a one-stop change in the opposite direction, and vice versa. For our 100 ISO example, shooting f/11 at 1/200 sec or 1/50 sec at f/22 will render the same exposure. Because of the relationship of shutter speed, ISO and aperture, all three of these settings are exactly the same in terms of light transmitted to the sensor.
Why would a creative photographer choose one over the other? A faster shutter speed paired with a wider aperture is good for stopping fast action or purposely restricting depth of field to give added emphasis to a specific subject. Conversely, using a smaller aperture and a slower shutter speed will increase depth of field, meaning a greater proportion of the scene will be in focus. A slow shutter speed can also help give a sense of motion by allowing a photographer to pan on a moving subject and purposely blur the background. Using a higher ISO setting will increase the light sensitivity of the camera's sensor, so the photographer can have greater freedom in selecting specific aperture or shutter speed settings. The downside: As the ISO setting increases, the digital noise can increase. This is the equivalent of grain in an analog photo, but it's also an area where modern digital cameras are steadily making great technological advances. Noise in a modern camera like a Nikon D3 at ISO 1600 is probably similar to the noise artifacts in the previous generation Nikon D2X at just ISO 400.
Because it's 600 times denser than air, water is a serious light-sucker. Rough surface conditions, suspended particles and the angled sunlight of early morning or late evening will remove even more light from the scene. But under the right conditions and in shallow water, you can apply a variation of the Sunny-16 rule to underwater ambient light photography. With slick-calm seas, that same 10 a.m. to 3 p.m. time slot and 100-foot visibility, about every 10 feet of depth will suck about an f-stop's worth of light up to about 30 feet. So, f/16 on the surface becomes f/11 at 10 feet, f/8 at 20 feet, and f/5.6 at 30 feet.
Let's say a photographer knows from previous experience that shooting a subject of average reflectance three feet away, with ISO 100, shutter speed 1/100 sec and a strobe at full power, will require a setting of f/8. This properly exposes the strobe-lit portion of the image, but properly exposing the background still requires consideration of the modified Sunny-16 rule.
10 feet deep--The background will be one stop overexposed because it requires f/11 for proper exposure. This will actually overexpose the whole image.
20 feet deep--There will be an exact blend of strobe and ambient light because the background requires f/8, just like the strobe. This gives a nice overall exposure with a light, blue-water background and a well-lit subject in the foreground.
30 feet deep--The background will be one stop underexposed, but this should still provide a nicely exposed photo with a well-lit subject in the foreground and a background that's darker blue compared with the shot taken at 20 feet.
Here's where it gets interesting. Imagine a photographer shooting 80 feet down on a shipwreck, on a cloudy day, late in the afternoon. There's a model kneeling on the bowsprit with the ship's wheelhouse making up the background 30 feet away. With the model three feet away, the strobe light at full power will do its job, and the skin tones will be spot on at f/8 and 1/100 sec. But shooting at that exposure will create a very dark background, probably almost black, and the wheelhouse will be invisible.
In this situation, changing the shutter speed can add some light. Shooting at 1/50 sec will open up the background by one stop; 1/25 sec will open it by two stops.
If the background needs more than two stops, dropping to something like 1/4 sec would brighten up the image, but the shutter speed would probably be too slow for the photographer to hold the camera steady, causing a blurry image.
A better solution is increasing the camera's ISO, which is very simple with today's digital cameras, even on a shot-by-shot basis. Because this is a frequently accessed control for underwater photographers, most housings give easy access to the ISO setting. Going to ISO 400 will open up the background by two full stops, shutter speed and aperture remaining constant. Kicking the ISO up to 400 and slowing the shutter speed from 1/100 to 1/25 sec gives a cumulative background adjustment of a whopping four stops!
But what about the foreground? The subject was perfectly exposed at ISO 100. Now, at ISO 400, the foreground is two stops overexposed. There is a simple solution for that as well: Reducing the strobe power from full to 1/2 power will take the foreground exposure down one stop; reducing it to 1/4 power reduces it two stops. By consciously manipulating the light for every aspect of a shot, a photographer can effectively control the exposure, no matter what the situation.
Guidelines for creative light management
Extremely high ambient light In very shallow water, you may need a fast shutter speed, at least 1/250 sec, to keep from overexposing. Imagine a wide-angle shot with a model four feet away, requiring f/8 for the strobe value. A normal shutter speed like 1/100 sec would likely require f/11 to keep the ambient light correct, but the strobe would not be strong enough to use f/11 with the subject so far away. Solution: higher shutter speed for ambient light, wider aperture for strobe-to-subject distance.
Low ambient light Conversely, in deep or dark water you'll need slower shutter speeds. The foreground exposure is controlled by strobe power, strobe-to-subject distance, and subject reflectance, but given the correct aperture for all of that, to open up detail in the background you'll need to leave the shutter open longer.
Fast Action When shooting a fast-moving subject, the best way to freeze the action is with a fast shutter speed. For a topside shot that may mean 1/1000 sec or faster. But for underwater work, we are usually limited by the synch speed of the camera, given our dependence on artificial light for color. Some modern cameras can synch as quickly as 1/320 sec -- go as high as you can to freeze the action.
Panning A very slow shutter speed combined with a panning motion (swimming or moving the camera along with a moving subject) will suggest the speed of a subject, even if they aren't really moving all that quickly. Take a slow shutter speed, say 1/15 sec and move the camera in the direction of the subject's motion. The strobe will stop the action and give nice detail on the subject, while at the same time totally blurring the background. Ideally, the camera will synch with the rear-curtain shutter release so the implied motion is behind the subject.