Testing should be done very carefully. For instance, just using the Box prop, in front of the camera, lit with an RGB=1,1,1 @ 100% infinite light or a spot with flaps full open or anything.

a) Set material to: diffuse color White, value 0,5, no specular. Render with and without gamma, and you'll find a difference on the box surface, as can be expected according to GC.

b) Set material to: diffuse color 50%Grey, value 1, no specular. Should be the same material, right? Render with and without gamma, and you'll find NO difference between those two. At least I don't. This is because color swatches (and not: Diffuse channels as a whole) are affected by the "image gamma" step before rendering.

Poser is fun, isn't it? Also try 75%grey and value 0,67, as well as 25%grey and value 2. All same material in theory, different render respons in practise. This is why just lighting a box and drawing conclusions is not good enough for me. Neither for you I guess, so please share your details, your process and your results.

c) replace the box by a ball. Since diffuse is not homogeneous in all directions but falls off at grazing angles, the ball gets darker towards the sides. This is what I called "shading" in the previous posts. Diffuse (=Lambert), Clay etc are different shaders. Set diffuse to 0/black and plug just a color into Alt_Diffuse, and you'll have homegeneous diffusion. The ball will render as a flat disk, shading gives form like shadow gives depth. 

Shading is affected by GC, it's not shadow because you'll have the effect too when Cast Shadows is switched off when rendering. When an object or part of it blockes the light onto another object or part of it, that's (what I call) shadow. It's affected by CG too. In rendering it's affected by Cast Shadows or Render Shadows Only. As a result, both effects can be seperated and recombined in post. This way, one can soften the shadows more than the shading fall-off, if desired.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though