Originally posted by Wombat In the case of the 35mm to 175mm, for example, how can you get an apparent increased reach of 210mm from a 175mm lens?
That's just it -- you DON'T get an increased reach. You just get an image with the edges chopped off. Forget about crap.factor! It's bogus, a marketing term. Just learn what each focal length means ON YOUR CAMERA! Not on some 135/FF camera you don't have! The "apparent reach" is just image cropping, nothing more.
Your calculations are confusing because 1) if you're not a long-time 35mm (135/FF) photographer, you don't have the FOV's of various FL's imprinted in your brain, and 2) they're unnecessary. Sure, if you multiply two numbers by the same factor, their difference will multiply by the same factor too. That's trivial algebra. Look at the table Adam posted above. It tells you all you need to know about what a lens sees.