Originally published in "The" Dive Shop Jun/Jul 1992 Newsletter, © John Wall and "The" Dive Shop, all rights reserved.

Most people record above water activities with cameras that automatically adjust for a variety of light conditions. Similarly, amphibious cameras and housed cameras can operate underwater in an automatic exposure mode.

Before auto-exposure camera systems were available, underwater photographers (and land serious land photographers) calculated proper camera settings by using exposure guides. While doing so involves more work, it gives photographers greater creative control. You may want to achieve greater control over the images that you create underwater by doing the same thing.

Most underwater strobes come with an exposure guide. Unfortunately, these are often difficult to understand; it’s as though they are written in a foreign language. This article explains exposure guides in simpler terms and tells how to create one for your underwater camera system.

Several variables affect how you create images on film, regardless of whether you are on land or underwater. We frequently refer to photography instructors who throw these terms around without explaining them properly as using “photo macho”. Let’s examine these terms and try to de-mystify them:

• ISO (Film Speed) – Films have a specific light sensitivity rating called ISO (formerly ASA), or film speed. Higher ISO numbers (“faster” film) indicate a greater sensitivity to light. For example, ISO 100 film is twice as sensitive to light as ISO 50 film; in other words, ISO 50 film requires twice the light as ISO 100 film for the same exposure.

Without the artificial light that strobes provide, low light conditions, such as we experience underwater, require the use of faster (higher ISO) film. However, using faster film as a substitute for strobe light causes grainier images with less resolution, and loss of the rich color that slower film provides. Additionally, without strobes, we have no way of recapturing the colors the water filters out of the available light.

Because there is a very limited range of subjects that we can shoot using available light, underwater photographers usually use strobes. When doing so, ISO 100 or slower film gives the greatest color and clarity.

• Subject-to-Camera Distance – If the subject that we are taking a picture of appears to be three feet from the camera lens, we must be certain that we set the subject to camera distance on our amphibious camera to three feet. If we are using an SLR camera in a housing, we must be certain that the image in the viewfinder is sharp and clear. If we are using an auto-focus camera, we must be sure that there is sufficient light falling on the subject matter to provide the contrast that most auto-focus systems need to work. An out-of-focus image will not be pleasing to look at, even when properly lit.
• Aperture (f-stop) – Your camera’s lens mechanism contains a hole which light passes. On most amphibious cameras and SLR camera systems, you can change the diameter of this hole, thus controlling the amount of light to reach the film. This hole is the aperture. Each increment is an f-stop.
• The lower the f-stop number, the more light reaches the film. Conversely, higher numbered f-stops allow less light to reach the film. There is a complicated mathematical formula that is used to calculate f-stop numbers. You don’t need to pursue it; the important thing is to understand that each time you increase the f-stop by one increment, you cut that amount of light that reaches the film in half. Lowering the f-stop one increment doubles the amount of light that reaches the film. The following table shows the progression of common f-stops.

The aperture area of f/2.8 is twice the aperture area of f/4 so it allows twice as much light through

The aperture area of f/4 is twice the aperture area of f/5.6 so it allows twice as much light through

The aperture area of f/5.6 is twice the aperture area of f/8 so it allows twice as much light through

More light needed, use a larger aperture (smaller f/stop number)

Less light needed, use a smaller aperture (higher f/stop number)

• Shutter speed – Your amphibious camera or housed SLR camera will also give you control over shutter speed. This represents the time (expressed as a fraction of a second), that light will be allowed to pass through the aperture. A shutter speed setting means that the shutter remains open 1/60 of a second.

Because you plan to use an underwater strobe, you must set the shutter speed to the sync speed. Strobes fire for a very brief time, and must synchronize their firing to when the shutter in fully open. You can find your SLR camera’s sync speed by looking in the manual. On some cameras, the shutter speed setting appears in red. Most amphibious cameras sync at 1/90 sec and some automatically default to the sync speed as soon as you connect your strobe to the camera system.

So far, we’ve discussed four variables, all of which affect light sensitivity. By controlling these variables, we improve the odds of getting properly exposed photos. Here is a practical exercise that helps to establish better control of these factors: the creation of an exposure guide.

1. Select a film speed. ISO 100 is a good starting point. We also recommend using slide film for this experiment even if you regularly use print film for your underwater images. Slide film will more dramatically show the differences between varying exposure values.
2. Weight several bright-colored, medium sized objects you wish to use as photographic subjects. Place them at the bottom of a swimming pool.
3. If you are using an amphibious camera or housed SLR with a flat port, find a means of measuring a distance of 4 feet under water. A metal rod or wooden dowel works best. (Remember, lenses with flat ports – just like your mask – make things look closer and larger underwater. An actual distance of 4 feet appears to be three feet, to both you and the camera.) A precise means of measuring distance, such as a stick, helps prevent this distortion from fooling you. Not only will this help you to make a more accurate exposure guide, it will help you better estimate underwater distances.
4. Set the subject-to-camera distance to three feet. (Due to the distortion caused by refraction, this is the correct setting when the actual, measured distance is four feet).
5. Set the camera shutter speed to sync. Remember, many amphibious cameras sync at 1/90 sec. Check the instruction book for your SLR if you are uncertain of the shutter sync speed. If you are using an SLR with autofocus/TTL capability, you may have to also check the instruction booklet to be sure how to put the camera system into aperture-priority mode.
6. Set the f/stop to its widest aperture (lowest number).
7. Turn the strobe on, making certain that you set the strobe for a full power, manual exposure, and not TTL.
8. Carefully position the camera so that the actual, subject-to-camera distance is four feet, as measured by your stick.
9. Take your first exposure, making certain that the strobe is the same distance from the subject (four actual feet/three apparent feet) as the camera.
10. Set the aperture to the next smallest f/stop. Wait until the strobe has the opportunity to completely recycle (recharge its internal capacitor) and the indicator light comes back on. Take another exposure, remembering to keep the camera and strobe precisely four feet from your subject.
11. Repeat step 10 through all available f/stops.

With most camera systems, completing this exercise requires seven or eight exposures. With a roll of 24-exposure film, you should be able to repeat this exercise a total of three times. After you develop the film, you’ll be able to see which f/stop exposure that you like best.

 Most labs that develop slide film have a light table available for customer use. Arrange your slides on the table in rows. Assuming that you had seven aperture settings available, you’ll have three rows of seven exposures each, as in the illustration below:

 Examine your slides carefully. Select the aperture setting the you like best. Enter this information onto an exposure guide table such at shown below. The table will suggest to you how to vary f/stops based on distance to subject.

You may repeat this exercise with any combination of film speeds or strobe. You may also wish to repeat it from time to time to check your consistency.