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Yup, among other things, at that level of detail the bump and normal start to do much of what we approximate with roughness to (since roughness is caused by microsurface detail). Of course a map with enough detail to simulate the different microdetails needed for all over the body would also cause my computer to choke and die, So I compromise and use a relatively realistic tiling map and then using roughness to fudge the parts that look off.
I'm also using myself as my main reference, Its rather fun. For instance, while my hand has much deeper microdetails than my shoulder , my shoulder has much smaller details, thus while my hand has much deeper "bump" my shoulder has a higher "roughness".
(also my one hand is way bumbier than the other yay asymmetry)
Nice render.
But you can only do such thing if you're single, your girlfriend/wife is out for "Ladies Night" or herself a DAZaddict.
Speaking about "roughness"... in contrary to someone's suggestion I still did some research on how to convert a PBR M/R "Glossy Roughness" parameter value over to PBR S/G "Glossiness". (You'll all hate me, again, 'cause it will include some math... again.
).
On PBR M/R, the "Glossy Roughness" value is "secretly" squared (behind our backs. The "Roughness Squared" brick at the Shader Mixer does that). That means, that a value of 0.50 for Glossy Roughness is actually only a 0.25. Not that rough. But if you like how it looks, and want to use the Specular/Glossiness Mix instead of the Metallicity/Roughness, you just have to subtract the squared value from 1, and you'll get the apropriate value for "Glossiness". [1 - 0.25 = 0.75] As a formula:
Glossiness = 1 - square(Glossy Roughness).
Now, if you have found a real-world measured value for roughness (some BRDF measurements include one, mostly named "m"), which you want to use for your PBR M/R "Glossy Roughness", you can't simply type it in. ' Cause it will be squared, remember? You have to calculate it's square root first, and then type that value in. The "Roughness Squared" brick of the Metallicity/Roughness Mix will then convert it back to the correct value.
F.e.: you want to use a Glossy Roughness of 0.30. The square root for that is 0.5477225575051661. Type that into the Glossy Roughness parameter, and it will be converted back to 0.30 by the shader.
Glossy Roughness = square root(m)
If you want to use it on your Specular/Glossiness Mix, you simply subtract the value for "m" you got from 1. [Glossiness = 1 - m]
I Did some test renders (using the DS 4.9 RC) to prove my theory; the Victoria 6 on the left is at PBR M/R, the one at the right at PBR S/G. On the first a value of 0.5477225575051661 (square root of 0.30) is used for Glossy Roughness, and a plain 0.70 for Glossiness. On the second render, Glossy Roughness is at 0.7071067811865475 (square root of 0.50), and Glossiness is set at 0.50. Just the material surface of the face (Template 1).
Slight differences rely on the fact that both mixes use a different method to determine specular reflection.
Updated vers2-0 : Thx to Arnold C. and KurzonDax
Do we need one more formula? Or are 5 enough ?
The 5 simple formulas you must know for mastering IRAY
We know a sRGB color and need the linear color
linear color = (sRGB / 255) ^ 2.2
example: sRGB = 128 (50% sRGB grey)
(128/255)^ 2.2 = 0.22 (22% slider)
(187/255)^ 2.2 = 0.50 (50% slider)
We know a slider value (linear color grey) and need the sRGB grey color
sRGB = (slider value ^ 0.4545) * 256
example: 0.5 slider (linear grey color)
(0.5 ^ 0.4545) * 256 = 187 sRGB (50% slider)
(0.1 ^ 0.4545) * 256 = 90 sRGB (10% slider)
We know the IOR (index of refraction) and need the specular color
Specular color linear = (sqrt(IOR-1))/(sqrt(IOR+1))
example: IOR = 1.33 Water
(sqrt(1.33-1))/(sqrt(1.33+1))
0.1089/5.4289 = 0.02
example: IOR = 1.5 Plastic
0.25/6.25 = 0.04
We know the specular color and need the IOR (Index of refraction):
IOR = (sqrt(linear color)+1)/(1-sqrt(linear color))
We know a IOR and need to set the parameter reflectivity in the MR workflow for dielectric materials (monochrom specular, non- metal)
Physically correct formula or how it is calculated in the IrayUbershader is:
Reflectivity = sqrt((sqrt(linear color)/(sqrt0.08))
where linear color is albdedo F(0) calculated from the IOR..
We know roughness (PBR metalicity/roughness) and looking for the matching glossines value (PBR specular/glossines)
Glossiness = 1 - square(Glossy Roughness).
No dont do that - we will only find that Iray is limited
...
The uploaded images show.. NO i dont tell what it is - but we can take the translucency color from Image 2
Too late, Andy.
You almost needed to.
Hadn't them any smaller? 
that's big enough or are you colorblind?
Naah, not that I know of... but of what good would be a green translucence for skin?!?
i dont have time for small talk - need to prepare me with graphics and wavelength-filter results for the next translucency color attack
Image 1.. ???
not only is the a great learning thread but you two make me laugh out loud wih your banter.
Couple of things
Arnold C you made a derogatory remark about using "pink" as a transmitted color for SSS (or so I remember). What colors are people using for that? Seems to me the pink/red range ought to be right, but I am often wrong. Certainly not the blue that I see in some products, I hope because that would just really knock me down.
Do you treat the face differently from the rest of the skin? Seems like I can get some nice glossiness for the face, but... that really gives the rest of the body what I like to call the "male porn star gloss", which can look good under some circumstances, but is a little exagerrated for most uses.
Has anyone looked at the specular maps that Sickleyield and Fusling have done for thier Beautiful Skin products? For me, the face maps seem pretty good... nice bright areas covering the T zone, with everything else muted. I don't always get the results I expect, though. Where ought one to place this map? Glossy Color? Or Glossiness? I'd think it would work well in Glossiness, but maybe I don't understand how that works.
Glossiness
Based on my photoreferecence settings tests a human skin has glossiness in the range from 0.6 - 0.8...
Translucency
Based on wavelength pentration charts - the average translucency color is a dark brown (that's what i simulated above....
Skin (Albedo - BASE color
Skin is yellowish greiyish........and gets the red mostly from SSS and very small bloodfessels which can react on stress, emotion, and environment -> A very pale face goes more reddish in the cold as example.....
BUT - we can not put our Albedo map in the SSS channel (there would be the right place)... because IRAY has just ONE color single scatter.... So we must SWITCH that.. aka a Albedo which includes all the color details (but flat) is in the Base Color
SSS
and the single scatter which would be the greyish/yellowish map is now our SSS transmitted color.... (you set that to lighten or darken the face ...
Now everybody and depending on the ALbedo - does their own MIX... and balance this 3 things with variations... BUT you will note that very pale skin is extrem difficult because of to much saturation and other mistakes....(its close to impossible to make a pale skin with 100% red in the translucency -> in nature the translucency amount on skin is everywhere about 0.4 - 0.5.
Backscatter
Is a not to be overlooked factor on a skin - in nature it is one of the factors which makes a skin silky - no real data here.. but you can set it very LOW and with a bright color (IRAY is limited here too).. which comes close to the bright transmitted sss single color (lightened skin).. it brings light in shadowed parts..
The Image shows a very pale skin : just a test render which uses EXACTLY (in the face) - what i posted here... It is the TRY to follow research in a limited Iray.
Andy, to my untrained eye, that is looking much more natural than many of the other attempts at a pale skin that I've seen.
I've been taking a break the last few days from human skin to focus on grapes. If you want a SSS challenge, they are absurdly difficult to get close because they are highly scattering. So much so, that I'm now starting to think since Iray only supports a single scattering color that they may be impossible to do with a high degree of accuracy and not look like molded plastic props.
Yes - the image above is the best natural simulation whitout a lot of cheating which i can do yet...the rest (i still can imporve a lot!) is in perfect maps which i will try to create soon... all this tests and research was needed to me to understand how i can build a as close as possible skinmodel in a limited iray. The next what must be done is : perfect maps.
Kurzon: Maybe try nested geoshells and make several highly transparent overlapping objects?
@KurzonDaxo
i need a break from skin too... why not grapes.... can you post a link or upload the reference photos which you try to replicate?
For whatever reason, I never think about using geoshells. I think I have the reg and green varietals dialed in pretty close. The dark purple/blue ones are driving me insane, though. In the research I've read, photons in grapes bounce around 600+ times on average before either being absorbed or reflecting back out. This is apparently more than milk which is highly scattering as well. This is why, I suspect, grapes seem to have a sort of internal glow that almost appears too bright relative to the amount of light being shined on them. With the darker violet grapes, it's much less noticeable though, so using a geoshell(s) might be a way to still get the depth without the glow.
@KurzonDax
out of my stomache - i would try to "abuse" backscattering... scroll back and look on my backscattering marble using two colors.... you can simulate the "glowing" effect with it.
here.. this is only backscattering.. looks allready close to grapes without any textures...
@AndyGrimm
For references, I've just been using Google image search. I've been focusing on table grapes (as opposed to wine making grapes). So far, I've not really messed with trying to imitate the white powdery yeast/wax substance, just the grapes themselves (as if they've been washed and dried).
I've been using backscattering, but now I'm thinking maybe not enough. I've focused more on SSS so far. Might need to switch my thinking.
Bingo!! @AndyGrimm, if you were within driving distance, I would be purchasing you vast quantities of your preferred adult beverage. Jacking up the backscattering and adjusting the backscattering color seems to be the trick for the violet grapes. Going to make some adjustments to the green and red varietals and then run a full render. Will take a while to finish because of all the translucency bogging Iray down.
Nice! Glad when a plan comes together.
what i see looking on green grapes is....
base color = yellow
backscattering = green
transmittet color is yellowish green
translucency more yellow then green
refreaction weight about 0.8 - 0.9
refraction color yellowish
and if they are very ripe or depending on the sort...
.
- 1m grape in the forest 
backscattering goes to a yellowish green too...
And there is also anisotropy
Funny - how my VIEW on images changed since i study human skin
testing now - where i was wrong with my observation/guessing... refraction is difficult.. that's what i see on the first tries
Kurzon...the powdery 'bloom' would be another geoshell use, and probably pretty easy to do.
hmm.. i want to see the kernels - there is one small ball in my marble... but still have a good to see surface... how to do that ?
Agreed. I was just focusing on getting a shader setup for the skin/flesh first.
Must be some amazing growth hormones to make a 1m grape
I have played around some with the refraction weight, but felt it was a little too difficult to control for the look I was after. I'm using refraction indicies around 1.37 based on what I could find on the net. Tons of information out there on grape juice and wine IOR, but not as much on just the grape, much less for different strains.
lol, I chose seedless. However, there are veins somewhat near the surface that I'd like to add in for more of a macro shot.
The seeds might best be done by inserting some blocking geometry...
Veins...probably easiest with a control map somewhere...not sure where would be the best to insert a vein map, though.
got it....... one must refraction weight 0.8 = /grape goes darker.). bring in balance with opacity.(grape goes brigther) .(just about 0.9)
.. But now i must adjust everything else again.
And my kernel ball in my record sized grape is to see
The veins are really only visible with strong backlighting, so will probably require a separete shader setup for that stiuation.