Seaspirit, So why do they say that the P900 camera has 83X optical zoom?
Well, the question is 83x of what? On its own 83 x is an impressive number, but essentially a meaningless marketing trick (like the "digital zoom, aka in camera image cropping)!
The over the thumb rule assumes that 50 mm focal length on a full-frame (24x36mm) camera shows approx. what our unaided eye sees. Thus a 500 mm lens on a full frame camera shows a 10x magnified image in the viewfinder/frame, whereas a 24 mm shows a wider field of view than we see with our eye (~0.5x, the reason we call those wide-angle lenses).
For the P900 the specs state: lens 4.3 mm -357 mm, i.e. an angle of view equivalent to that of 24 mm -2000 mm lens in 35mm (i.e. full frame) format).
So the marketing folks start their multiplication with the widest focal length of the zoom (83 x 24 mm), not with the 50 mm eye equivalent.
If usable a focal length of 2000 mm is pretty impressive, but "only" 40x.