The 'rounding' of f-numbers follows tradition and are not necessarily mathematically correct, that is why some EXIf readers insist f/5.6 is 5.7 and f/3.5 is 3.6. Likewise, f/11 should be more correctly given as f/11.3 (or f/11.5 if you round to nearest 0.5). Etc.
The 1/3 stops are f/4, f/3.5, f/3.2, f/2.8. The half stops are f/4, f/3.4, f/2.8. (f/3.3 is the true 1/2 stop but one often sees 3.4 in stead, depending on the way round is conducted. f/3.4 is actually +7/12 over f/2.8 ). Like +1/3 over f/5.6 is usually taken as f/6.3, but sometimes the figure f/6.4 is used. The list of ambiguous f-numbers go on and on.
I see more than my allotted share of 1/12 and 1/6 steps when I program CPUs for the Nikkors. Some EXIF readers manage 1/6 steps, nearly none 1/12, and the cameras usually go by 1/3, or by the traditionally used numbers.
