So, you say that you jumped straight from disposable film cameras into digital underwater photography? Ok, maybe that's extreme but it's a fact that many new photographers exploring the deep don't really undserstand some of the basic principles of photography.

With that in mind, here is a simple primer, covering the three factors that control the exposure of a photograph:

• ISO
• Lens aperture
• Shutter speed

Whether you shoot film or digital, a basic knowledge of these element will improve your images.

ISO is a rating of the light sensitivity of media, either a digital sensor or film. Higher ISO numbers require less light to expose the photo. Film photographers know that you select your film based on ISO numbers, but in the digital world, you set the ISO through the menu system on the camera. Using higher ISO settings comes with a price however. High ISO films exhibit more grain, while high ISO settings in digital cameras result in more 'noise' (stray artifacts) in the image. Both look similar, a gritty, mottled look to areas that should be smooth. The rule:

Aperture is how wide the lens opens during the exposure. Many beginning photographers are confused by f stops, because their numbering seems contrary to logic.... the higher the f stop, the smaller the lens opening.

Here's the scoop... f stop numbers are acutally the ratio of the diameter of the lens opening to the focal length of the lens. So:

• f/4 on a 50mm lens is 1/4 of 50mm or 12.5mm
• f/8 on the same 50mm lens is 1/8 of 50mm or 6.25mm

Because the f stop is actually a ratio, you sometimes see f numbers written like 1:2.8. The rule:

The final element of exposure, shutter speed, is the length of time that the shutter is actually open to expose the photo. Shutter speeds are expressed as fractions of a second, so if you set your camera to a shutter speed of 250, you are acutally setting it to 1/250th of a second. Shutter speeds can be confusing too, but here's the rule:

Shutter speeds have a very unique symbiotic relationship to aperture. Each time you double or halve the shutter speed, the change in exposure is equal to one f stop, either up or down. The following graphic shows the relationship between aperture and shutter speed.

This graphic is based on some hypothetical constant lighting conditions, and does not represent actual real world settings, but is merely intended to illsutrate the relationships between aperture and shutter speed. In this illustration, if a photo is properly exposed at f 2.8 and 1/1000th, it would also be properly exposed at f 8 and 1/125th, and at f 22 and 1/15th.

This brings up another question. Why so many variables and how do you select which combination to use in a given situation?

One of the primary criteria is depth of field. Depth of field is the 'zone' where objects are in focus. As an example, lets choose a hypothetical setting for shooting a picture, say a 28mm lens set to f 8. We're taking a picture of a coral head, with a diver poised directly above the center. If we focus on the the diver who is 5 ft. from the camera, the depth of field for this shot is the zone beginning 3ft. from the camera and extending to 10ft. from the camera where the both coral and the diver are in focus.

If we choose a smaller aperture (higher f stop), this 'in focus' zone grows. Conversely, using a larger aperture causes the zone to shrink. The depth of field for a photo is directly related to the f stop of the lens. Here's the rule:

Another handy tool to help you grasp this concept is the Digital Diver DOF Calculator. Select any camera and lens combination at study the resulting graph to see how aperture affects depth of field. Another factor to consider when selecting one aperture/shutter speed combination over another is whether your subject (or in some instances you and the camera) are in motion. Slow shutter speeds tend to cause moving subjects to blur. While this is less pronounced underwater, it still factors into the equation when selecting exposure settings.

Hopefully, this little tutorial has given you a basic understanding of the relationship between aperture and shutter speed, and gotten you to thinking about the physic involved.

Now, go shoot some pictures!