The 50/1.2 is often about twice the price of the 55/1.2. Probably because the edge performance is improved. The 50/1.2 is 7 elements in 6 groups, the 58/1.2 Noct-Nikkor is 7 elements in 6 groups, and the 55/1.2 is 7 elements in 5 groups. That means the T-Stop of the 55/1.2 is "ever so slightly faster"....probably the T-Stop is faster than the 9 element in 7 group single coated 5cm F1.1 Nikkor-N. That makes the 55/1.2 the fastest regular production Nikkor lens ever.