Wow, then you have quite a bit of material to cover :)

Exposure is a photographic concept, your best source is to look at an introductory photography book. In the short term, you can make sure that the Tonemapper in Lux is set to Linear and then click on the Estimate button, which acts as a Point-and-shoot camera, by calculating the exposure automatically. 

Like any automatic cameras, that doesn't work in all situations but it will give you a starting pooint. For anything else you will need to look at the Reality User's Guide, which has a chapter on how to adjust the exposure, with examples.

The rest is experience. You have a lot of ground to cover, in terms of 3D concepts and terms but that is the fun of it, there is so much to discover.

Good luck.

Paolo

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