A very detailed explanation of the constants can be found in the tread "Is the K factor relevant to me or should I cancel it out?" Basically, the exposure constants K and C do two things. Hand held meters are separate from the camera's optical system. The light loss caused by the camera lens needs to be factored into the exposure calculation, and because the meter isn't reading through the camera lens, the actual value must be averaged. Second, the exposure calculation needs to factor out the influence of the physicality of the meter, ie spectral sensitivity. There is a rather involved equation for determining K. The following is a page from the K factor thread which shows the equation for K.
What's considered the average reflectance comes from K / C. 1.16*pi / 30 = 0.12.
Defining K, part 2.jpg
Last edited by Stephen Benskin; 02-23-2013 at 08:23 PM. Click to view previous post history.