Dale, the only way a single meter can be used for exposure with both negative and positive materials is through the speed constant. The exposure meter only wants to make one exposure at 8 lux * 1/ISO. The speed constant is necessary to determine where the exposure range falls. For black and white negative film, it's 0.8. With color transparency film, it's 10.
Originally Posted by ic-racer
There are a lot of variables that combine to make K. The K factor thread really goes into detail on this. In a nut shell, many of the factors have to do with the physicality of the meter and the camera optical system. But no matter what the value of K is, the exposure is intended to be 8 / ISO.
Here's a page that introduces the equation for K and if you're wondering about where the value of 8 in 8 / ISO comes from, check out the value of K1.
Defining K, part 2.jpg
Last edited by Stephen Benskin; 11-01-2012 at 11:16 PM. Click to view previous post history.