Can someone explain/compare the zoom vs mm information given? On my old inexpensive digital camera, I have a 10x optical zoom and something like 3x digital zoom. I see that on some of the dslr cameras. But then on others it's something like 70-300 mm. I know what 10x does. Going by that, if I wanted a good picture of something 75 yards away I'd need around a 50x zoom-ish. So how do I compare the mm numbers???