Nope friend.. Nikon don't have ISO 50..
Nikon chooses to declare their main ISO range, 200 to 3200 in the D90's case, and then offer one more stop on each end as an extended range that they call Lo.
200 is the base ISO here..
L0.3 minus 0.3 of a stop from base, and L0.7 minus 2/3 of a stop, L1.0 one ful stop
HTH