Learn to shoot with the Nikonos V
Getting the Exposure Right

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One of the most frustrating problems encountered by beginning underwater photographers is inability to consistently get good exposures. Often, the diver is overwhelmed with the beauty of the subjects and the sea around him, only to be disappointed with the results when the rolls of film are subsequently developed.

There are a number of factors which make underwater photography difficult:

• Varying visibility
• Light levels
• Fast moving subjects
• Color absorption at depth
• Other hazards to deal with...etc

In this article, we’ll show you a simple method for gaining better exposures and increasing the number of “good” shots you get with each roll of film.

One of the first pieces of equipment that an underwater photographer buys is an underwater strobe. This artificial light source is absolutely necessary for most types of underwater photography. Remember, as the depth of water increases, colors are absorbed and most scenes tend to appear a dull gray, blue or brown. Also, as you know, the deeper you go, the less light is available for your camera to work with. In order to compensate for these conditions, an artificial light source is required. However, it is the proper use of this piece of equipment that escapes many beginning photographers.

Let’s first examine what happens when a strobe is used underwater. When your camera triggers the strobe to fire, light will be sent from the strobe to the subject. This light, in turn, is then reflected back to the camera and to your film. The amount of light that returns to the film is critical in determining if the image is properly exposed or not. 

Most photographers know that the lens aperture controls how much light is permitted to enter into the camera and onto the film. A smaller aperture (higher f/stop),  means less light gets in but a larger depth of field is created. Conversely, a larger aperture (smaller f/stop) means more light enters but a smaller depth of field is created. The question is, how do you know how much light from an artificial light source (strobe) is needed to properly expose the film and what aperture should you use? Well, a simple formula holds the key.

All strobes come with documentation which, among other things, lists the Guide Number for that particular model strobe. This Guide Number (GN) indicates the strobe’s relative power. It is important to read and understand how the manufacturer is choosing to express these numbers. Generally, these numbers are given for various ASA  films and for distances measured in feet. However, some manufacturers differ.

Once you have determined the GN for the strobe you are using, simply divide this number by the distance to the subject from the strobe (not the camera!) and the result will be the correct aperture (f/stop) for that photograph. As an example, suppose the GN for your strobe is 48 and the distance from the strobe to the subject is 3 feet. An f/stop of  f/16 should give you a properly exposed photograph. 

Sounds easy, right? Well, there are a few catches! First, the Guide Numbers for your strobe may have been calculated in clear water under bright conditions with highly reflective subjects. If you are diving in dark conditions or your subject is very dark, you may need to adjust the f/stop by one or more stops to compensate. We also highly recommend bracketing your photographs to further ensure proper exposures. Whenever possible, expose one shot using the formula we just described and then repeat the shot with the aperture set one f/stop higher and one f/stop lower. 

What about cameras and strobe systems that use TTL metering? Many photographers make the mistake of assuming that the camera will automatically adjust the strobe output correctly for all conditions. While there are some definite advantages to TTL systems, they are not perfect. In order to use TTL correctly, you need to make sure that the strobe can deliver the amount of light required for the situation. In other words, if you close the lens down too much and you are too far from the subject and your strobe is not powerful enough to generate the light required to expose the film properly in those conditions, your photograph will be under-exposed. Once again, the strobe’s Guide Number is important.

In order to determine if you are within the strobe’s TTL capabilities, you must determine if you are close enough to your subject. Suppose your strobe’s GN is 32 for the film speed you are using. If the strobe to subject distance is 4 feet, your f/stop must be f/8. If you set your aperture to this f/stop, under TTL conditions, the strobe will generate all the light it can to properly expose the image. If you open the lens up wider (such as f/5.6), you will require less light from the strobe and the camera will automatically shut the strobe off before it generates too much light for the situation. However, if you use a smaller aperture (such as f/16 or f/22) your strobe will not have enough power to generate enough light and the photograph will be underexposed. 

The advantage to TTL metering is that you have a range of distance to choose from and you don’t have to be as exact as in manual metering. In the example above, if your strobe to subject distance was CLOSER than 4 feet, your calculations would still come out correctly since the camera would compensate by shutting the strobe down before it overexposes the shot. With manual systems, you would have to make sure your strobe to subject distance is exactly 4 feet since any variation from that would not be adjusted for by the camera. Again, even when using TTL metering, bracketing your shots is the best way to ensure good exposures. 

The pursuit of excellent underwater photographs continues to be an exciting journey for all of us. No two conditions are exactly the same and each piece of equipment behaves slightly differently. This article presented a simple method for determining proper exposure when using a strobe as a primary source of light. Remember, divide the strobe’s Guide Number by the strobe to subject distance to get your proper f/stop. And remember to bracket!

Finally, experiment and enjoy the journey towards better photographs.

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