Definition: Measure of maximum spread of light being taken received by an optical device e.g. exposure meter, wave guide. * The field of view of a detector or guide. (1) For handheldexposure meters It is typically approximately 30º; and 1-3º for spot meters. For cameras with TTL meters, it varies with field of view of lens in use. (2) In fibre optics, it is measured as the semi-vertex angle of the cone of optical power capable of being coupled into bound modes: hence also called acceptance cone.
Less but not too little
For today's photography, a 30º acceptance angle for exposure metering is rather too wide, too indiscriminate - which rules out most camera meters. But a 1º angle is too fine: it is too easy to make errors by choosing the wrong area as the key tone. In practice, I find about 5º (when using a standard lens) is ideal: this is approximately 2%-3% of the picture area or a 5mm diameter spot on a 35mm focusing screen.
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