Using External Underwater Strobes

When using a slave strobe that cannot produce pre-flashes, a major drawback will appear: fully automated shooting is out of the question. Many photos are overexposed as the the camera does not incorporate the amount of light that is produced by the external strobe.

Manual correction of the camera and reducing the strobe's strength is the only way to go.

A number of tips to get better results:

External strobe

When shooting macro, close-up or reflecting subjects (sand, silver fish etc.) the maximum strength of the average strobe is many times more than required. Decreasing the output to the minimum setting is usually the best approach. Further reduction of the strength can be obtained by placing a diffuser cap.

Internal Strobe

When an external strobe is connected the light of the camera's internal strobe is no longer needed to illuminate the subject. In fact, it only further increases the overexposure and the backscatter also remains. Shielding the internal strobe with a diffuser or cap is therefore the best approach. Of course the light of the internal strobe still needs to be directed to the slave for triggering purposes. Sea&Sea fiber-optic cable, for example, use a Velcro solution on the housing that achieves this.

Bracketing

Most cameras provide a Bracketing mode that takes a series of photos from underexposed to overexposed in multiple steps. This is an excellent feature to find the ideal exposure setting for the camera and strobe without too much hassle.

Exposure compensation

Besides the Bracketing function, the exposure can also be controlled manually through the Exposure Value (EV) Compensation facility. Usually it allows for selecting from a range of between -2.0 EV to +2.0 EV in steps of a 1/2 stop. -1 EV in this setting indicates an underexposure of 1 step, which means that only half the light (as compared to the correct exposure, 0 EV) is used for exposure.

This option allows the user to manually prepare the camera for the additional light from the external strobe, without having to worry about aperture and shutter speed.

ISO values

Due to the dark conditions under water the camera may select a higher ISO value (200, 400 or more) as compensation. Manually locking the ISO value at a lower sensitivity like 50 or 100, decreases the sensitivity of the camera.

The resulting exposures are of course related to the specifications of the camera: a more light-sensitive model will be able to compensate for the lower ISO by further opening the aperture for example, thereby leaving the actual exposure unchanged.

Shutter time and Aperture

A strobe's flash only last for a very short period of time: in general less than 1/250th of a second. A camera's shutter time typically varies between 1/30th to 1/125th of a second. As this exceeds the flash duration by far, all light produced by the strobe is completely used in the photo. This makes that the variation of the shutter time has little effect on the resulting exposure.

The aperture controls the size of the lens opening. It is indicate with the letter "F" followed by a number, like F2.8, F5.6, F11. A low number indicates a big opening, a high number a smaller one. For example, an aperture of F2.8 will allow twice as much light to pass through as an aperture of F5.6. Because the opening does not change during the time the lens is opened, the aperture has direct influence on reducing the light of the strobe in the photo.

In the Aperture Priority Mode, usually indicated with "A" or "Av", the aperture is selected manually but leaves the other settings up to the camera. This function enables users to pick the smallest opening (if possible F8 or higher) to prevent overexposure.

Experimentation

In the end it still comes down to a lot of experimentation when an external slave strobe is used. The influence of reflection, ambient light, particles and colors varies in each photo and demand a different approach each time. The above mentioned settings will help the underwater photographer in getting getting better results, although the effectiveness of the external strobe will largely depend on the camera's capabilities.

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