Do you know how to calculate stops with ISO and Shutter Speeds?

Okay, finally I am blogging about last week. I am so exited!! The first day, Jon talked about film and the different kind and how to shoot it. I have known for a long time that if you want to give yourself one stop more light, you can double the ISO setting. So twice the ISO (or ASA) is one stop difference. Well, did you know that doubling the shutter speed also gives you a stop less light? Which would mean that doubling the shutter speed gives you one stop less light. WOW! So that means that f2.0 1/500 and ISO 400 is the same exposure as f2.0 1/1000 and ISO 800. I never had to know that with digital, but here is why it is important to know: When you shoot film, obviously you can't just change your ISO setting if you need more light. Let's say you are at a photography workshop and the instructor calls out the proper exposure for the image, "I am shooting this at 2.0 1/500" Lets just say that earlier in the day he instructed everyone to use 400 speed film and lets just say (hypothetically) that you have 3200 speed film in your camera because you were shooting low light the night before. Whatever would you do? Well, 3200 speed film is how many stops higher than 400? That's right, 3 because you would say in your head 400, 800 (double that's one), 1600 (double again that's two), 3200 (that's three!). Then you would say in your head, "I can't shoot this at 1/500 it will be too overexposed, so I need to figure out how to get 3 stops less into my shutter speed!" 1/500, 1/1000 (that's half the time, one), 1/2000 (that's half again, two), 1/4000! Then you would say to yourself, "Wow, I feel like a genius! Tomorrow, Aperture calculations!