Just a note ... don't want to throw confusion into the mix; so if the 18% gray card works for you then don't worry about reading any further.

If, however, you use the 18% gray card method of metering and find that your images are a little underexposed for your tastes, try metering with the grey card and then adding +1/3 to +1/2 stop to the exposure.

This use to bug the crap out of me when I first started using gray and white cards. One day I researched into it and found out that almost all camera and meter manufacturers use the ANSI standard to calibrate. The ANSI stardard is an ~12% luminance reflectance. Of course, then I had to find out where the hell 18% came from. No one seems to want to say for sure, but it may be one of those numbers like 72-dpi for monitors. A holdover from another segment of imaging (possibly the print industry).

To really throw garbage into the mix, it appears that some vendors have chosen to define their own interpretation of the ANSI standard (what good is an interpreted standard?) and can be using anywhere from 12%-15% ... ah, progress.

Again, if you're noticing a little underexposure, don't assume you're doing it wrong. Try setting your exposure compensation to +1/3 or +1/2 and see if that suits you better.

-=>Donald