Meter Calibration Constant
The factor K in the exposure equation that ties a measured luminance to a recommended camera setting.
Formula / rule
N² / t = L × S / K
Measured in
cd·s/m²
In depth
The ISO standard permits K to fall anywhere between 10.6 and 13.4, and manufacturers choose differently — Canon and Nikon have historically used 12.5, Sekonic 12.5, Minolta 14. This is why two correctly working meters can disagree by a third of a stop, and why photographers who mix brands learn their own offsets.
Examples in practice
A handheld meter reading f/8 and a camera reading f/8⅓ on the same grey card are both within specification.