You will have perhaps noticed that in the EXIF Wizard post the other day, theere was a value called “EV”, or exposure value. That is the absolute “exposure value” that corresponds to a certain “light value”. The exposure value is basically a camera setting for exposure, where:
(N = aperture number, t= exposure time).
0EV corresponds to “1 second at f/1.0”. A typical exposure value for a full daylight scene at mid-day in full sunlight at 100 ISO is 15.
And this is useful, why?
First, because it allows you to realize that there is such a thing as absolute exposure values. Second, more practically, because using this, if you use a tool like Exif Wizard, you can see what the light value was when you took a certain picture. And that is always useful for a photographer. And third, because light is predictable (the sun is always equally bright, for instance – rememer the “Sunny Sixteen Rule”?), so you can use the theory behind this to calculate camera settings.
This article on Wikipedia is good reading, if you feel like a little light academic liftwork.