from this blog
Step 1. Before you set up any materials, do any texturing or add lights, create a mia_exposure_simple node and connect it to your camera as a mentalray lens shader. In mia_exposure_simple set the following values: Pedestal=0, Gain=1, Knee=1, Compression=0, Gamma=2.2
Step 2. If you map a color value with an 8-bit image then connect either a mip_gamma_gain (preferred) or a gammaCorrect node between the fileTexture and the material. In mip_gamma_gain or gammaCorrect set Gamma=0.455
A few extra things to remember:
Lights: Use Decay Rate=quadratic or physically correct lights like the womArchlight shader.
Color Swatches: If they represent a material color, then like 8-bit images, they also need gamma correction=0.455, so either connect a mip_gamma_gain to the swatch or just guess and darken the color manually.
Non-color attributes: Things like diffuse weight, bump values, reflectivity etc are not colors. They do not need any gamma correction.
Floating point images: These are already linear. gamma=0.455 is NOT required.
Frame Buffer Gamma: Forget you ever heard about it and leave it set to 1.
Final output: Use the renderview window to adjust your lighting and texturing. Then for final renders that require no post processing, batch render and your done. For renders that will be processed further in a compositing application where you will also use a linear workflow, then before batch rendering, disconnect the lens shader.
