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Understanding Aperture, Shutter Speed, and Depth of Field

Words and images by Ken Ashman. Copyright © 2005.

Let's face it, underwater photography isn't easy. Knowing how your camera settings control your image outcomes is required before going underwater. My purpose in this article is to cover the basics of how your camera settings work together to create the image you end up getting.

Aperture controls how wide your camera's "eye" opens.

Shutter speed controls how long the "eye" stays open.

Understanding how to effectively use aperture and shutter speed will make you a better photographer. Aperture and shutter speed settings are quantified as numbers. A lower number means more light will reach your camera's sensor. A higher number means less light in your image. Once you learn to use shutter speed and aperture to your advantage, you will gain a high degree of control over your photographic outcomes.

We will start with shutter speed, since it's the easiest to understand. If you cut the amount of time your camera's eye is open by half, you will get half the amount of light in your image.

Light from a strobe is not affected by shutter speed! A strobe flashes for only a scant fraction of a second. Your camera's fastest sync speed is usually not as fast, so changes in shutter speed do not change how much strobe light appears in your image. Changing shutter speed will affect any areas of your frame beyond reach of your strobe. Speeding up the shutter will cause these areas to go dark. The faster the shutter, the darker these portions become.

Blackbar Soldierfish

In this image at right of the Blackbar Soldierfish Myripristis jacobus, the contrast between the subject and darker background helps the fish stand out dramatically in the frame. The shadow adds to the sense the fish is hiding in the coral. D200 with 60mm Nikkor Micro lens at F18, shutter set at 125th.

The next area we want to cover is aperture settings. A smaller aperture number means more light will get to the sensor. Unlike shutter speed, adjusting the aperture will affect ALL of the light in your frame; strobe and ambient light alike.

Your camera's aperture works like a valve on a water faucet. The amount of water is analogous to the amount of light. The wider you open the valve, the greater the amount of light. Extending the analogy, shutter speed determines how long a time the valve is open. Aperture determines how wide you've opened the valve. Both determine how much water actually ends up in your cup, or how much light gets into your image.

Now we can talk about depth of field.

Your aperture controls the depth of field (DOF). To understand depth of field, think of a string extending from your camera's lens to your subject, and then beyond to infinity. As you travel down the line, the closer you get to your point of focus the sharper the focus will become. As you pass beyond your subject, the focus will get softer. The "focal point" is the exact point of perfect focus along the line (i.e. the point at which initially collimated rays of light meet after passing through a convex lens.)

A little of the space in front of the focal point will be in acceptable focus, and comparably more of the space beyond. You can control how thick this in-focus zone will be with the aperture setting. The smaller the aperture number, the smaller the depth of field. A larger aperture number will provide a bigger/thicker zone of acceptable focus. This effect is more pronounced in macro/close up situations.

The amount of DOF you get out of any given aperture setting is increased if your subject is further away, and decreased the closer you are to your lens' minimum focus distance. You have to concern yourself with DOF most when you are very close to your subject. If your subject is relatively farther away, even a wide open aperture can nonetheless provide adequate DOF.

NOTE: A very high "stopped down" aperture setting will yield an overall softer focus. A lower, more "wide open" aperture setting will provide a sharper focus. A lens is always sharpest when the aperture is near wide open, and softest when the lens is "stopped down" to it's highest aperture setting.

Fine tuning your exposure:

If there are areas of the frame beyond reach of your strobes, one approach is to use a wider open aperture to get the right exposure in the ambient-lit portions of your frame. Another option is a higher ISO setting.

Slowing down your shutter speed is another way of adding more ambient light, but this only works to a point. If you set your shutter too slow, you'll get a blurred image due to movement. Some UW photographers get away with 1/60th of a second in certain conditions. For me, 1/125th or even 1/250th is best for macro, especially when the water is moving and diving conditions are more challenging.

You can usually set your strobe output high enough to allow you to use a high aperture and thus achieve good DOF, but remember a high aperture setting also means the reduction of ambient light in your frame, and some loss of sharpness in focus.

Blackbar Soldierfish

In the photograph at right of the Yellowfin Fringehead Neoclinus stephensae, I wanted to show how the bottom dweller uses camouflage to match it's surroundings. Using a wider open aperture tightened the DOF placing the Fringehead's face in focus, but keeping the (very nearby) background completely out of focus. This separates the fish from the background, helping it stand out in the frame. D200, 105mm Nikkor Micro lens at f11, shutter at 125th.

It's my hope this article has helped you understand your camera better. As I said at the start, underwater photography is not easy, but with today's digital cameras it's a lot easier to improve your skills than it was in the "film days". My advice is get a big memory card for your camera and shoot, review, adjust, and shoot again, and again, and again. Experiment, and take lots of images on every dive.

Good luck in your animal hunts and safe diving! -Ken Ashman

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