While it may not necessarily be a bug, the current solution in corona is rather clumsy. Vray's model is a lot better in this regard. Let's say you have an object made of two components based on a bitmap. You know you want white part of the bitmap to have IOR of exactly 1.25, and black part to have IOR of exactly 3.1. How do you do it?
With current solution, it will be quite difficult to achieve. With Vray on the other hand, it's quite easy. You plug a Mix map in the fresnel map slot. As a mix texture, you use your IOR bitmap. As a white color slot of Mix map, you plug in Output map with output amount of 1.25, and as a black color slot of Mix map, you plug in Output map with output amount of 3.1. And there you have it... exact fresnel map :)
So then, if you have more complicated map with say 10 different exact IOR numbers, you can just paint shades of gray using floating point values, and then use that as a bitmap with output amount of 100, therefore making it in 0-100 range, and each shade of gray on the map will then interpret the exact same IOR. That's the way Vray works and it makes a sense.
