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A light meter is a "computer" of sorts (either analog or digital) which measures an amount of light, and, from a practical standpoint, provides the photographer with any of a variety of possible shutter speed and aperture combinations which all allow the same amount of light to strike the film. When used in the proper fashion, the knowledgeable photographer can use this information to produce predictable and correct exposures under a wide variety of lighting conditions, both normal and extreme.
However, this "computer" needs to be programmed prior to use, otherwise, like all computers, the information it provides has no value. The information which has to be programmed into the light meter is the film's sensitivity to light, referred to as the ISO or ASA number.
In a perfect world, the photographer would set the film speed dial on the camera to the manufacturer's printed film speed for that particular film. (Some cameras calibrate the light meter automatically, using the "DX coding" on the film canister.)
Alas, our world is not perfect. Two factors can individually or in combination affect the actual ISO / ASA number set on the camera. These can be additive, or they can be offsetting.
1. The camera's light meter may not be accurate, and may require a "fudge factor" on the ISO / ASA setting to correct the inaccuracies and cause it to output exact readings. Testing one's own camera's light meter will be covered in another Photo Tip.
2. The film manufacturer may rate a particular film at a speed that produces less than optimum results for your subjective tastes. For instance, many chose to overexpose Fuji Velvia by 1/3 stop, and many prefer to underexpose Kodachrome 64 by 1/3 stop (as compared to the manufacturer's published rating) in order to achieve a more pleasing result. It is important to understand that the actual film speed rating which will produce this result may be modified by any compensations required by individual light meter deviations (see #1 above). Thus, it would be misleading to say that Velvia should be rated at 40 instead of 50, or Kodachrome 64 at 80 instead of 64, though this might be true with a camera with a perfectly accurate light meter. |
