There must be a certain amount of refraction taking place for translucency to work, otherwise adjusting it does nothing. Typically, I will use Hard (wax) model for translucency. For the Thickness, the default is around 80’ but I lowered it closer to 30’. For the back-side color, it’s driven by an image map that is a lighter version of the diffuse map.
For this effect to correctly work, the subject (in this case a leaf) needs to have volume or thickness. If this material is applied to a plane, then translucency will not work correctly as it is based on refraction.
The downside to this method, especially if you are applying it to vegetation, is that you will likely have hundreds of thousands of leaves in your scene. Having your leaves on boxes instead of planes will increase your polycount by six times! Let’s look at another solution that favors planes rather than volumes.
*To speed rendering times, especially for scenes with many trees and objects with opacity maps, set the Opacity mode to “Clip” under Options for the VrayMtl. This is an often-overlooked setting but can dramatically speed up rendering time.
This material is used more often to create translucent effects through thin objects such as paper, leaves, curtains, etc.