Quote Originally Posted by kintatsu View Post
But they are calibrated to produce the same 12-18% depending on manufacturer, which leads to different readings on the meter, and between manufacturers.

Gossen uses K=11.37, so if C=250 for a flat diffusor, we end up with a reflected gray value of about 14.3%. Other values for C exist for hemisphere diffusor, 320 is common. So using C=320, k=11.37, gray= 11.36%. The reflected percentage formaula is % reflectance for grey= pi(3.1416) x k/c.
That is my understanding too. I wish I understood more about the designer INTENT of the engineering values they use in their design models.

No matter, I really believe that Bill Burk's bottom line (post #57) is correct. The design intent was for a photographer to be able to meter, use the provided exposure recommendation, and have a reasonably successful exposure MOST OF THE TIME. Knowing when that will and won't work is the difference between a good photographer and a photographer who is risking failure.