Why simple metering gives an average gray image

[Ted Cousins]

I've often wondered exactly how it does that.

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Hope the link works ... scroll up to see beginning ...

After much formula manipulation, it says:

"Camera manufacturers build light meters adhering to international standards (like ISO 2720:1974). For a standard handheld meter or camera sensor matrix, typical calibration constants are:

Camera manufacturers build light meters adhering to international standards (like ISO 2720:1974). For a standard handheld meter or camera sensor matrix, typical calibration constants are:

K (Reflected Light Constant) approx 12.5
C (Incident Light Constant) approx 250

Plugging these exact industrial constants into our equation:

R=pi x 12.5/250= approx 0.157 This proves that the system is hardwired to expect a surface reflectance R of roughly 15.7% (mid-gray)."

Not 18%, not 12.5% ...

 

[Doug Kerr]

Using reflected light metering with K=12.5, and if the relevant ISO standards have been observed (and assuming that the ISO speed of the digital camera is the "saturation-based" ISO speed rating), the average phtometric exposure on the sensor of a digital camera for a metered shot should be very nearly 12.7% of the saturation phtometric exposure.

Matters relating to incident light metering play no role in reckoning that.

A corollary is that if for the scene being photographed the average luminance is 18% of the maximum luminance, the maximum photometric exposure on the sensor for a metered shot should be 1/2 stop below the saturation photometric exposure.

Best regards,

Doug