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@KurzonDax ...
I had some success eliminating the dark nostril and chinlines using the parameter "Backscattering Glossiness"....this is one of the most underused functions :-)... but it is very difficult to keep control over this effect on skin..but possible... i will try to replicate what i did some weeks ago....
Yeah, I played around with the backscattering also to try and eliminate or at least reduce the problem. Extraordinarily difficult to control it without having a noticeable negative impact on the overall look of the skin.
man I so agree with backscattering, been trying to get my head around for sometime now without success.
@Szark
I have found it useful for simulating fuzziness of certain fabrics on clothing items, but even then if you set the Backscattering weight much above 0.1, the effect rapidly becomes overpowering and details get obliterated.
exactly what I have found too.
@KurzonDax....
you can switch off shared glossiness and then set a different color for backscattering glossiness....
For learning what this paramater actually does i set two different colors.. Blue for Glossiness - Red for Backscattering....
You need to play with it on a marble -before trying on a skin (complex geometry)..... see my marble 2...bundled light leaves the surface in shadow areas...
@KurzonDax....
set backscatering weight ONLY to about 0.1 against glossiness with weight 1.. then you see absolut clearly what does what...
@AndyGrimm
Good advice! I so rarely change the shared glossy inputs setting, I never thought about that. Experiments must be run....
Yeah that is in the DAZ3D docs
Full control over F(0) and F(90) can be reached with a custom profile in top coat.
But that means you should not use glossiness in base or set the weight lower there..
then top coat - CUSTOM...you see two parameters....
marble 1.. only fresnel 90.. with 1, set this to linear color lower then skin or to zero and you cheat here physik but you will have lower rim fresnel..!
marble 2 only fresnel 0 -> the input is here the linear color... for skin, example: 0.028
And a exponent of 5 is there because F = Rs + (1 - Rs)(1 - Tv)^5)
- For a peferct skin you must use a touch backscattering and top coat with a fresnel custom profile....
That's where my skin tests stopped - because i figured out that something must be wrong with roughness/reflection amounts.... but it dosent mean that you can not get a perfect result - balancing weight by eye and reference photos...
Edit: Oh and i forgotten to mention that all effects such as specular, roughness, f90, are best to see on a BLACK marble.
For those which like to learn about custom curve fresnel settings...
(1) Rs = (n1 - n2)/(n1 + n2)
(2) F = Rs + (1 - Rs)(1 - Tv)^5
The first part, Rs, expresses the ratio of the IORs of the two mediums involved, where n1 represents the medium in which the ray of light starts, typically air, and n2 is the medium the light eventually hits, which would be the material we are interested in, perhaps composed of some sort of glass, liquid, plastic or metal.
The second part, F, calculates the final fresnel reflectivity based on the Rs and the Tv, which is our tangent view direction. As you can see, it is mainly the exponent (5) along with the difference between two IORs that affects the results. Thus, F is the value we ultimately want to get to, and n1, n2 together with the exponent will be the input variables that allow the user to tweak the final look.
http://www.terathon.com/wiki/index.php/Building_a_Fresnel_shader
Ok lets see if I get this for the first equaltion
the two mediums for example, (Air IOR 1.00 - say Plastic IOR 1.46) divided by (1.00 + 1.46) = - 0.19
here a example which should demonstrate where i think all skinmodels which i saw posted yet - failed to improve.
in this example.. i use backscattering (weight 0.5), and surpress to much fresnel rim reflections.. while still having a highligh....and roughness is somewhere close to skin/plastic.....
Everthing is there, translucency (weight 0.5), SSS amount 0.5 , backscattering weight 0.5) and Fresnel control...
The trick is to use a backscattering color close to the SSS transmitted color !
Yes - but you wont need to do this because .... everything is allready there in the custom fresnel curve in top coat...just use linear colors as input (aka IOR 1.5 = 0.04 )...
And play with the exponent (rim fresnel)- where you have some examples what you actually changing on the website i posted above...
Uh... almost™... Rs is identical to R0 (Fresnel reflectance at 0°). Your formula is missing the "²": Rs = ((n1 - n2)/(n1 + n2))² (https://en.wikipedia.org/wiki/Schlick%27s_approximation)
Quatsch... you guys start to confuse me. Sorry Pete, you're right, n1 is always the IOR of air, and n2 is always the IOR of the material in question.
well - i posted the website because it EXPLAINS ... what formulas dont explain! how to get control over the F90 rim reflections
.... the second formula is the one which openend my eyes (the exponent 5 in our shader 
...!
Why you did not tell me that 2 months ago - Arnold
By the way, is there some kind of explanation / turorial for all the surface variables for Iray. Right now I fell like I'm fumbling in the dark even upon trying to change something as simple as glossiness.
Tutorial..DAZ? hahahaha
...
Sory Hera - just joking... wish i had a good tutorial myself 6 months ago
The best one i know is for MAx 3d... (i try to find the link where it is....
Sure! You can start here: http://docs.daz3d.com/doku.php/public/software/dazstudio/4/referenceguide/interface/panes/surfaces/shaders/iray_uber_shader/start
i got it.... http://blog.irayrender.com/post/83815598089/iray-material-plugin-updated-for-3dsmax-2015
download the package for 2015... inside of the zip is a PDF manual.. which exaplains the parameters for Iray... and shows many renders... Just dont get confused when they show screenshots from 3dMax iray Gui.
This document together with the DAZ doc which Rafmer posted above - helped me most.
Umm, you didn't ask?
"you guys start to confuse me." ?
....
"Umm, you didn't ask" - Now i do
ok - because it is my intend to continue where i was before all this marble tests started - question.. backscattering... are there any datas which can be used and making sense in our shader?
Do you think this is what's causing the problem with too much specular for F(90)? Seems like that would be a pretty easy thing to fix. Making that exponent adjustable wouldn't be necessarily difficult either, me thinks.
Okay, I'm not Andy...
On the Top Coat in Custom Curve "Top Coat Layering Mode" you can manually adjust the exponent ("Top Coat Curve Exponent"), which is at 5.0 at default. Raising the exponent will make the F(90°) reflection less strong. Picture 1 (SPECULAR_10) shows the default plastic values (using the PBR SR mix). Picture 2 (SPECULAR_11) shows the F(0°) turned off, only the F(90°) is visible. Picture 3 (SPECULAR_15) shows the shows the F(0°) turned off, only the F(90°) is visible, and the exponent is set to 10.0.
Unfortunately you can do this only on the Top Coat, on the Base you can't turn the glossy layer into Custum Curve mode.
Sure, you can even go and lower the "Top Coat Curve 90" parameter... but that wouldn't be physically based anymore... this isn't 3Delight, and cheating has to stop!
Thanks Arnold. I was just rereading through the MDL to see if it was something that could be changed for the base there. I knew about setting up a custom curve in the top coat using the Uberbase, but I was trying to see if the shader itself could be altered to allow it to be modified for the base glossy as well. Or is the curve hardcoded in to the Iray engine? That wouldn't make sense in my mind.
ok....
with the exponent you can soften or harden the transition, Falloff - between F0 and F90... here you can cheat.... a lower value softens.. and higher gives a very sharper rim!
Setting both values to the same skinvalues on F90 and F0 (o.028 here - leaving exponent on 5 = that's how i expect skin to look.. Image one.
Using just the fresnel without custom curve... it looks like a breed between a steel ball and a orange but not like skin
Using anything else then fresnel custom curve.. does set the IOR only for F0 - but leaves F90 for metal! that was my problem since weeks - and that's why all skin renders which people posted looked ether really flat, powdered - whitout highlights - or suddenly render in another situation, backlight or sidelighted, wih mirrored sky and grass colors on skin....!
here the normal fresnel layer
now skin has also backscattering and a little bit anisotrophy...not the DARK border like on the marble above!
with backscattering glossiness and little bit backscattering anisotropy we get rid of the black dark border! light gets scattered in the other direction on skin to a part... that's why nostrils edges SHINE... where now dark shadows are!.
A litte bit overdone for demonstration - Image
Just now this marble starts to look like a organic material to me!
F(0°) and F(90°) at 0.028 for both? 0.028 would be an IOR of 1.4019177, looks a bit low, but okay... but having F(90°) at the same as F(0°) isn't physically based anymore.