You need good PBR mats to do PBR, and there is basically no way around that. Many material sets are not PBR, they are just converted from the diffuse as D mentioned.

You could use generated maps like EzSkin can set up, but that is very limited in what will get out of it simply because all you get is the same thing everywhere on the map. That never looks right on skin that has many different textures in different areas of the bump on the skin. Think of the differences from your nose to under you eyes. Those areas are completely different, and right next to each other on the same maps.

You will also run into issues of maps being saved at a certain gamma setting, and you really need to use them at another setting. I have all the grayscale maps in that shader set to 2.2 versus at 1 as they load in the render I posted. You can easily adjust grayscale maps with the gamma setting. Gain, lift, and center contrast are a function of gamma and you can use that to your advantage in Poser without adding any additional nodes. Just keep in mind that the gamma setting is map usage wide. If you need to use different gamma settings with the same map on a different material, you will have to resort to a gamma node to change it on one material and have it be a different gamma setting on one material.

I almost always scrap the specular map on skin as well, because it is rarely needed. Yes you can assign different areas of the skin to reflect more or less with a specular map. But doing that can make the shader incredibly light and environment sensitive right off the bat because that is exactly what the roughness map (combined with the bump effect) is already doing. There is usually no need for a specular map on a skin shader. There are certain exceptions to this, like metallic makeup, etc. But that is a bit different thing to address.

The biggest problem people will run into with content is the burned in specular/bump/shadows/etc and grayscale maps that are not PBR at all....

And sadly that describes a large portion of Poser content.....

Close enough, and just fake it, doesn't work well in PBR....

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