I'm not following any of this, adp. Help me understand.

You said "this is a well known error" - but what is a well known error? We haven't even seen the render. It would be better to analyze the render, and then ask questions.

Whatthe: Could you post a render,  please? I have various suspicions, but I need to see the issue before I make too many wild guesses. For example, if you can see through the iris, it may be because you mistakenly made the iris transparent - thus we see into the back of the head. Or perhaps you are using refraction, and the ray bias is too low. There are many possible issues.

adp:

Whatthe has said that the shadow intensity is 1.0 (100%) so that is not the issue, but I still don't understand what you say about it being buggy.

Shadow "softness" (shadow blur radius) has nothing to do with intensity. The shadow intensity is specifically to allow some light to leak through. The amount that leaks through exactly follows the shadow intensity value. If you set it to .8 (80%) then 20% leaks through. I'm not suggesting that shadow intensity is something you want to use. It is an old leftover technique from before the availability of IBL. Nevertheless, it does work correctly, if not realistically. (Trying to use it to create some "ambient" light will still allow specular reflections, which is not realistic.) It is much better to use IBL for ambience, to decrease the intensity of shadows.

I will now demonstrate. I have 5 renders to show you. Please bear with me.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)