Photo Tip #5

Programming Your Light Meter

Part 2.

Learning the fundamentals of ambient light  photography forms the foundation of all nearly all aspects of underwater photography. Knowledge of and proper use of an ambient light meter is essential to this understanding.

Contrary to what many believe, pointing a light
meter at an object will not necessarily produce the correct exposure. Your light meter needs to be calibrated to your chosen film speed (ISO / ASA), modifying this setting to account for individual light meter inaccuracies plus  personal preferences regarding a specific film. (See Photo Tip #3 for further details on calibrating your light meter.)
Once your light meter is calibrated to your film speed with these two possible modifications in mind, "correct" exposure will result from aiming the light meter at a subject which is equivalent in average tonal value to 18% gray, the standard to which all light meters are calibrated. Subjects which are lighter in value will be rendered underexposed
unless appropriate compensation is made, and
subjects which are darker in tonal value will be
rendered overexposed unless compensation is
made.

The photographer must make the counterintuitive
compensation by increasing exposure when
pointing a light meter at light toned subjects and
decreasing exposure when pointing the light meter at dark toned subjects.

This is the key to understanding light meters and
perhaps the most poorly understood aspect in all
photography. In underwater photography this
concept is critical, as the tonal gradient in the water column varies to such a large degree. It therefore becomes especially important that the light meter is directed at the elevation of the water column present within a given composition which approximates the 18% gray tonal level. Cut to the chase: aim the light meter into the sun (too bright) and the photo comes out underexposed. Aim the meter too low... or into the reef... (excessively dark) and the blue water comes out overexposed.

About This Photo

The above photo utilizes combination lighting. It is
largely an ambient light shot, with a very slight
amount of strobe fill. It was taken in Kavieng,
 Papua New Guinea, with a Canon F-1 camera,
14mm lens, Aquatica 3 housing, 8" dome port, two
Oceanic 2003 strobes, TLC strobe arms and
Professional Kodachrome 64 film.

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