Really? That's something I've always just accepted as true myself.
Well, now you know how things like "prime lens" happen, then.
Now you've piqued my curiosity: how is the 18% tale wrong?
It's just wrong; light meters are not, nor have they ever been, calibrated
to 18% gray as a standard. What they really read can vary slightly from
one manufacturer's opinion to another, but it's closer to 12.5%. ISO
sensitivity is of course not based on middle gray at all, which accounts
for the variation.
The 18% myth seems to be based on what Ansel Adams wanted rather than what
actually came to be. Adams lobbied for 18% gray to be the standard. He
was not successful.
Isn't an 18% gray card really 18% gray? (I have one around here somewhere
but never thought to test its eighteen-percentness. :-) )
It really is, yes. Unfortunately, 18% gray is about a half-stop up from
middle gray, and if you meter reflected light from it without somehow
accounting for that (by angling the card, for example) you will be
underexposing by a half stop from what you thought, more or less. With
a Nikon it will be a half stop, but I'm given to understand that Canons
meter a bit higher, so it may be less than that (I have no Canon, so this
is secondhand information).
But the 18% gray myth is so pervasive that even Kodak makes 18% gray cards,
when they most certainly know better.
By now, of course, lots of photographers have over the years been consciously
or unconsciously working around the error in their workflows, to the point
that "18% gray plus workarounds" can actually work. If someone came out
with a 12.5% gray card right now, people wouldn't know what to do with it!