OK - I might be behind on my history here, but why the change away from 18% gray that was used for years?
No change...even Kodak has said to take the 18% card reading and adjust it by upping the reading by 1/2EV!
When we look at printed tones from black to white, where black is at 0% and white is at 100%, one might think that middle grey would be in the center at 50% reflected brightness. But middle grey falls at 18%. When grey is printed at this value we see it as being in the exact middle between black and white. It is from this perception of middle grey that we get the value of 18%. We have the print world to thank for this "standardization" of what value middle grey should be.
The published standards for correct reflectance values of middle grey (ANSI PH3.49-1971) actually put it at 12%-13%, which is about 1/2 stop under 18%. And it seems that most meter and camera manufacturers agree with this standard. Even Kodak, maker of 18% grey cards agrees, as Kodak's earlier instructions on using their grey card read: "Meter readings of the gray card should be adjusted as follows: 1) For subjects of normal reflectance increase the indicated exposure by 1/2 stop." For a while, though, Kodak omitted these instructions, and then added them back...
The instructions have since been updated to: "Place the card close to and in front of the subject, aimed halfway between the main light and the camera." Angling the card like this has the effect of reducing the reflectance of the card, which in effect gets the meter to increase the exposure by about a 1/2 stop.