Your professor isn't as dumb as he seems
Your camera can easily be fooled by light and dark backgrounds, backlit subjects, and highly reflective surfaces. A gray card is a great way to get perfect exposure. Since the camera has a reflected meter that is looking to average out the scene to an 18% gray, the gray card just makes it's work easy. It represents the happy medium the camera is trying to find. See if you can average the following set of numbers quickly in your head: 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18 18, 18, 18, 18. That's what a gray card does.
If you have the camera in Av or Tv mode, point it at the gray card, fill the entire frame, and hit the AE lock button. If you are in M mode, again fill the fame with the gray card, you should see a similar scale to the one shown on the Nikon. Turn your thumb wheel until the indicator is in the center, then hit AE lock. Once you've done this you can adjust either the shutter speed or the aperture up or down and the other setting (the one you are not adjusting) will move in the correct direction to maintain perfect exposure.
Edit: I think on the K20d, I had to turn on the "display sensitivity" setting in the custom menu to get the little scale to show up. Unfortunately you have to chose between this and the number of shots remaining.
Last edited by maxfield_photo; 11-16-2011 at 10:05 AM.