I know that the Fresnel and Layer Weight nodes do much the same thing, but with one accepting a proper Index of Refraction, and the other a value between 0 and 1. I have always used the Fresnel node as it allowed me to simply look up the proper IOR of any specific material online and plug it in. However, if I wanted to use Layer weight, how would I go about translating an IOR, which I've seen as low as .18 and as high as 2.5, into a value between 0 and 1 to get the same effect out of layer weight? Is there a conversion formula or ratio?
By comparing visual results from both LayerWeight's Fresnel and Fresnel Node on a test sphere I concluded that these settings give similar results:
Blend: 0.00 0.10 0.50 0.70 0.80 0.90 0.95 0.98 1.00 IOR: 1.00 1.10 1.70 3.30 5.10 10.0 20.0 50.0 1000
The Y scale is logarithmic
Confirmed by Brecht (thanks) the conversion formula is:
IOR = 1/(1 - Blend)
Blend = 1 - 1/IOR
Layer Weight node:
Layer Weight Facing output mixes the two shaders based of angle of incidence.
The Fresnel output mixes them based on fresnel formula (Dielectric fresnel weight), which is a function of angle of incidence. The exact math is here: Wiki
The Blend slider adds additional blending between the two shaders: at 0 it will be green and at 1 it will be red.
On the other hand the Fresnel node allows for proper IOR input:
I use Fresnel node when dealing with glass-like materials, where the light goes through - where it bends at the interface. I can set proper IOR for the interface this way.
I use Layer Weight node when dealing with reflectivity, it allows for better control over the transition on the surface (how smooth it is) and over how the materials are mixed (with that blend slider). This suits artistic approach better when you are eye-balling stuff.