so I have been told ...
VIS is measured in eight directions of compass from observer's station by evaluating contrast elements seen by naked eye (can you see the window on the church tower?) compared to a chart.
Unless great variations are observed, the minimum value is given as reported visibility. In the opposite case, minimum and/or maximum values are provided as well. The logic (incomplete, as my memory is fading) being if a single direction is twice as bad or worse than the lowest other direction you'll see it coded as
4000 1800SE in METAR. Decode: reported(!) VIS 4000m significantly worse to south-east 1800m.
On the other hand, RVR is measured by state of the art electronic equipment parallel to RWY edge at prescribed height (1,5m ???, anyone?). Maximum value encoded to METAR would be Rnn 1500P meaning more than 1500m.
The requirement to provide RVR is set by VIS limit which I cannot remember now. (2000m or less?).
Another important part, what fog is. FG stands for visible moisture that degrades VIS below 1000m!
Thus I conlude that VIS and RVR have no relation, with regard to their values. On the other hand, there is some logic and rules for encoding them.
800 R24/1200D FG R31/500N - possible
4000 1600SE BR - possible
1200 R24/500 BR MIFG - possible
It had been my experinece on ATPL that one must read the question very thoroughly and think VERY hard about the TRUE meaning of every bloody SINGLE word. Seem actually harder than learning the stuff as such. Witnessed also on PPRuNe, yes, at maximum speed, you indeed have maximum drag.
Cheers, FD.