The f1.8 is 1/3 of a stop faster than the standard sequence of f 4.0, f2.8, f2.0, f1.4, so a stop at f2.0 is the normal position, there just isn't room to mark in on the lens. Nikon originally made 50mm lenses in f2.0 and f1.4 but as part of a marketing move in 1978 they started making the lower priced lens with a maximum aperture of f1.8 to make it seem that you were getting more.