the specular value controls the IOR for reflection, the IOR value is indeed just used for refraction.
The principled BSDF-Shader is based off of disneys principled BRDF.
It's called principled because of these 5 principles:
- Intuitive rather than physical parameters should be used.
- There should be as few parameters as possible.
- Parameters should be zero to one over their plausible range.
- Parameters should be allowed to be pushed beyond their plausible range where it makes sense.
- All combinations of parameters should be as robust and plausible as
Because of their first principle (easy instead of scientific) Disney decided to cloak the fresnel-IOR in a specular reflection value. This is of course a more artistic approach.
from the disney docs: specular - incident specular amount. This is in lieu of an explicit index-of-refraction
and from the blender docs: Specular
Amount of dielectric specular reflection. Specifies facing (along normal) reflectivity in the most common 0 - 8% range.
You can use the formula 𝑠𝑝𝑒𝑐𝑢𝑙𝑎𝑟=((𝑖𝑜𝑟−1)/(𝑖𝑜𝑟+1))2/0.08 to create a node group so you can control both IORs with one slider.
Note that the specular value accepts values above 1.