Controlling Depth of Field to Give Photos That 3-D Look

The most effective underwater photographs are those that portray three dimensions. A photo is a two dimensional image, flat, trying to convey the three dimensional underwater world. Three dimensions in a photo is suggested in one of three ways: object size differences (i.e., the closer object is larger), background objects appear in a haze or with less color, and background and/or foreground objects are out of focus. It is the throwing out of focus the foreground or background that is a function of "depth of field."

Depth of field is simply the area in which the subject will appear in focus. With that in mind you'd think that maximum depth of field would be most desirable. Not so. Remember, you are trying to add three dimensions to the photo by putting either the foreground or background, or both, out of focus. This is accomplished with a narrower depth of field. Too narrow, however, and it will be difficult to get anything in focus.

Your depth of field is controlled by a number of factors. First and foremost is your lens. Wide-angle lenses have the widest depth of field with the wider the lens (smaller the number, 15mm vs 28mm for example) the larger the depth of field. Macro lenses have the narrowest depth of field.

Depth field is also controlled by how close the subject is to the lens, the closer the subject, the narrower the depth of field.

But the control factor that divers have the most ability to manipulate is the lens aperture. Every lens has a small hole with which the amount of light to delivered to the film is controlled. The hole is opened wider for more light and constricted for less light. Hole sizes are expressed as "f-stops." The larger the hole, the narrower the depth of field (f2.8 and f4 are examples of large holes). Conversely, a small hole will have a wide depth of field (example f16 or f22). By controlling the amount of light added by the strobes, the underwater photographer can manipulate the aperture and, consequently, the depth of field. With automatic TTL cameras, hooked up to TTL strobes, one has only to manipulate the aperture and the strobes will adjust automatically.

Exactly how you will control your depth of field to get the desired "3-D" effect with background and/or foreground out of focus will depend on your lens, strobe, film, and exact desired effect. You'll need to experiment. Here are some tips to get some interesting results right off the bat.

- With fish, put the eye or eyes in the center of your depth of field.

- In macro-photography, do not use apertures of f2.8, f4, or f5.6. The depth of field will be far too narrow to be effective. Also, your strobes will most likely over expose the subject.

- With wide-angle work, pay careful attention to the depth of field guides on your lens. They can be very helpful, particularly when photographing subjects at close range.

- Be very conscious of your foreground and background. Position your camera so that an out-of-focus kelp frond will frame your subject or blue sunburst water will illuminate the background.

- Position your camera to take advantage of three dimensions. Rather than just shooting a fish against a flat rock, get low and aim up, placing the fish in front of an effective background. Shoot the fish head on with its tail out of focus, maybe even its nose, too.

The goal is to make your underwater photos jump out and become alive. That can only happen if it conveys a sense of three dimensions. Controlling the depth of field can help you do that.

Dale Sheckler is editor of California Diving News, coauthor of the book Southern California's Best Beach Dives, and producer of the largest consumer dive expo in the western U.S., SCUBA Show 2000, June 3 & 4 at the Long Beach Convention Center.