The most important photographic technique is exposure; exposure is governed by metering. These two parts must be understood altogether.

Measuring light with light meter or the metering system in camera are the same. Meter can be used freely, using Gray Card and other means. Metering system in camera is intended for automatic exposure.

Gray Card for metering must have a fixed reflection rate, normally 18% (Kodak's Gray Card is the best choice), which will meet the basic assumption of light meter. Using metering system in camera with average reading, the result may be very close if the measuring area includes various bright and dark subject almost mixed to an average reflection rate of 18%. With the high latitude of modern films, we can have good photo for average scenic with automatic exposure. If the scene has a high balance to light or dark color (high key or low key), we can use Exposure Compensation to correct, this is the way of control by those Exposure Programs, e.g. when the scene has high portion of light color, the exposure will be based on metering with additional of one or two stop to compensate the metering error.

Meter, classified by the measuring angle, has two major type, spot (1 to 5 degree) and average (15 degree); metering system in camera usually has spot and average, and the angle will be varied according to the focal length of lens. Gray card is the best, but we can use our palm which is about half to one stop brighter, that can be adjusted by half to one stop higher exposure. Average metering has very close result, providing that there is no interfering from other light source, direct, diffused or reflected.