I have one quick question: on a zoom lens - let's say an 18-55mm - you won't get the same framing zoomed out as you would with an 18mm prime lens. Why is that? Shouldn't they be the same focal length, therefore the same framing when your position is kept the same?
The answer is "marketing." A zoom lens might be 17.5mm, or 18.7mm, or your prime could be 18.2mm. Also, most still lenses "breathe" when focused to different lengths--i.e. the image expands or contracts a bit when the focus is closer or farther away. But how much of a pain in the butt would it be for their marketing departments to distinguish their lens line when the zoom is slightly wider at 17.7mm than your prime at 18.1mm? Or whatever. But that's the answer--there is small variation that for convenience of marketing purposes, they round to the nearest standard millimeter.