Wouldn't it be nice if your fish stopped and posed when you wanted to take a picture - maybe perching conveniently on top of a particularly photogenic coral? Alas, that is a rare event unless your tank is full of Clown Gobies!

The real challenges for photographers are fish that, like Tangs, Angels and Wrasses, never seem to stay still for even a second. The following information will help you learn how to get nice, crisp shots of your fish.

Understanding how a camera works will go a long way toward helping you maximize its, and your, fish-photography capabilities. The camera is basically a fancy box that holds the film, or sensor. Attached to the box is a lens. Behind the lens is an opening, whose size is adjustable, called the aperture. Between the lens aperture and the sensor is a shutter; think of the shutter as a curtain that opens and closes for a precisely measured amount of time, allowing light to pass through the aperture, hit the sensor, and be recorded as a picture. That measured amount of time is the shutter speed. Shutter speed is measured in fractions of a second. The smaller the fraction, the more rapidly the shutter must open and close. Consequently, the faster the shutter moves, the less time it is open to allow light to pass. Aperture size and shutter speed changes are calculated in ratios related to the amount of light allowed to pass through the lens. These ratios are referred to as "f (or focal) stops." Each doubling or halving of the shutter speed or aperture size represents a one-stop change. Contrary to what seems logical, the larger the aperture f-stop value, the smaller the actual size of the aperture. This is because the aperture value is the ratio of the focal length of the lens to the diameter of the aperture. Thus, an aperture of f4 on a 100mm lens is a 25mm opening.

One other important part of the camera is the light meter. The light meter measures the amount of light hitting the scene in the viewfinder. The scene is broken up into sections, or zones, with each zone given a value. The center zone is given the highest value and the outer edges the least. Without getting too technical, let's just say that the light meter in most cameras may be thought of as a series of small photovoltaic cells wired in a predetermined manner. The total current generated by the cells is "read" by the camera to indicate exposure. Depending on the wiring, and the sophistication of the meter, various portions of the image may be read or weighted differently to provide different exposures. The current value is simply read and compared to predetermined values and that gives the meter value.

On all but the most automatic point and shoot cameras the three variables - aperture, shutter speed and ISO - are adjustable. ISO stands for International Standards Organization and is a measure of the light sensitivity of the film or sensor. Adjustments are measured in "f-stops" or just plain "stops." One stop is double or half the previous value. For example, that means that a shutter speed change from 1/60^th to 1/30^th, or from 1/60^th to 1/125^th represents a one-stop change. Moving from f4 to f8 is a two-stop change (f4 to f5.6 to f8). Changing the ISO from 100 to 800 is a three-stop change (100 to 200 to 400 to 800). Camera exposures are generally adjustable in 1/2- or even 1/3-stop increments.

Typically, common aperture numbers are f2.8, 4, 5.6, 8, 11, 16, and 22, although some lenses will have larger and smaller apertures than these. This is a good place to discuss the property known as "depth of field" because it is directly affected by the aperture size. Depth of field can be described as the length of the image, measured from foreground to background, that appears in sharp focus. To increase the depth of field, use a smaller aperture. An image shot at f4 will have much narrower depth of field than an image shot at f11; for everything to be in sharp focus, try a very small aperture (f8 on the point and shoot cameras and f16 or 22 for SLR's (Single Lens Reflex cameras), as a starting point).

It's a real challenge to get good depth of field on close-ups. One reason is that the closer the lens is to the subject, the smaller the depth of field. Compensating for this requires using extremely small apertures to achieve decent depth of field when shooting close-ups. The other reason is that with each halving of the aperture its size doubles, so the shutter speed has to double to keep the brightness of the exposure consistent. For example, assume that you are trying to take a photo where the light meter reads f4 and 1/500^th is an accurate exposure. To get more depth of field, the aperture is reduced to f8, a two-stop change. The shutter speed must then be slowed down by two-stops to 1/125^th of a second (from 1/500^th of a second to 1/250^th of a second to 1/125^th of a second) to allow the same amount of light to hit the sensor. With SLR's an aperture setting of f8 results in a narrow depth of field when the lens is only a few inches from the subject. If an aperture setting of f22 is used to increase the depth of field, a three-stop difference from f8, the shutter speed drops all the way down to 1/15^th of a second (1/125à1/60à1/30à1/15). At 1/15^th of a second any movement will blur the image- this is the main reason that close-ups are such a challenge. It's almost impossible to hold a camera by hand at such slow shutter speeds without moving; using a good tripod or providing more illumination by the use of a flash or strobe is key to getting a shot in sharp focus.

Back to our challenge of getting sharp photos of the Tangs and Wrasses and Angelfish - since the fish are moving quickly, it's best to use shutter speeds around 1/125^th of a second or faster, if possible. That usually means shooting "wide open," at the lens' maximum aperture, because even bright metal halides simply don't provide enough light to accommodate really fast shutter speeds. The maximum aperture setting allows the most possible light through the lens. To achieve this setting, put the camera in aperture priority mode and select the smallest available f-stop number (largest aperture), then take a few pictures. If the shutter speed was fast enough, and if the focus was accurate, the fish should be in sharp focus. There is a downside, however, to using the largest or larger apertures: the depth of field will be very shallow. Sometimes this is a good thing, such as when trying to isolate the subject against a background with a lot of distracting detail. When shooting fish it's best to focus on their eyes, as that's where the viewer's attention is drawn. As with portrait photography, it's imperative that the eyes of the subject be nice and sharp.

If the tank's light is insufficient to allow adequately fast shutter speeds, then a couple of options are available. One is to raise the ISO. A film's sensitivity is a set value, but on digital cameras this sensitivity can be adjusted. It's important to remember that the higher the ISO, the "grainier" the image will be. As with aperture and shutter speed adjustments, each halving or doubling of the ISO corresponds to a one-stop change; changing the ISO from 200 to 400 buys one extra stop of speed or depth of field. If the fish is moving too fast to get sharp shots at 1/125^th of a second using an ISO of 200 and the maximum aperture, try changing the ISO to 400 and use a shutter speed of 1/250^th. Most modern cameras will provide very good images at an ISO of 200, but at a setting of 400 the grain becomes somewhat apparent. At 800 it can be very obvious, so it's a matter of compromise - image sharpness versus overall image quality.

A second option in situations where the tank's light is insufficient is to supplement the lighting with a flash. The obvious benefit of a flash is that it provides the extra light needed to support the faster shutter speeds. But extra light has other benefits, too: it brings out the fishes' bright colors, and it evens out the shot's overall lighting. With only the tank lights working, the photo will show very bright spots and very dark spots; the flash helps to fill in the dark areas. When done well, the overall effect is a more balanced exposure.

An external flash (a flash unit that fits into the hot shoe at the top of the camera) is far superior to a built-in flash. Most of these tips, however, apply even to a built-in flash. The first thing to keep in mind is that the camera must be pointed either slightly downward or upward or to the left or right relative to the aquarium's glass to prevent the flash from reflecting off the tank and back into the lens.

If the camera allows both the shutter speed and aperture to be set (manual exposure mode) the flash's full benefit can be utilized. Set the camera to manual exposure; a good starting point is f4 and 1/90. The actual burst of light from the flash is extremely fast (1/500^th of a second or faster), which is why it freezes the fishes' movement, and it's an effective way to enable a shutter speed fast enough to avoid "camera shake." Experiment with different combinations of shutter speeds and aperture sizes to find out which work best.

Experimentation may reveal that the flash is putting out too much or too little light. If so, then the flash exposure compensation (fec) can be adjusted. The camera's manual should explain how to do that. Negative fec will reduce the amount of light the flash generates; conversely, positive fec increases it. If the camera does not have a manual exposure mode or flash exposure compensation, the best bet is to make any necessary exposure adjustments by setting exposure compensation. Exposure compensation is different from fec in that it increases or decreases the brightness of the entire image as opposed to increasing or decreasing only the flash's output.

Many cameras also have a program mode. In this mode the camera will set the flash to act only as a supplement to the existing light source. This mode can provide a nice, natural look to the images, with the downside being the decreased control over depth of field. In manual exposure mode the flash acts as the main light source.

As with all types of photography, decreasing the size of the aperture (by increasing the f stop value) increases the depth of field. This knowledge can be used to manipulate the depth of field to the desired goal. Just remember that there's always a compromise between ideal depth of field and adequate shutter speeds - it's up to the photographer to determine the best compromise. Generally speaking, portraits (people, fish or whatever) are shot with large apertures to minimize depth of field and focus the viewer's attention on the subject's eyes. For portraits of fish, however, it is usually a good idea to have the subject's entire body in focus, and this frequently requires the use of smaller apertures.

To get great images… experiment. Practice, practice, practice! Trial and error is the best learning tool; the more you practice, the better you'll get - I promise!