Real world photography is a fantastic and easy source of data in computer graphics and visual effects, be it for textures, backgrounds, or light maps. There is one complication that is very easy to overlook, and tricky to get right: linearity.
In real world light transport (and the simulated version of it in our various renderers) the math of light operates in a linear manner. That is, light source A with intensity 1 and light source B with intensity 1 will combine to intensity 2 (in whatever units those numbers are).
However, image capture and display devices do not work in a linear space. This is mostly historical, and maintained for backward compatibility for the content that has been produced in the past, but we must still deal with it to this day. In order to be immediately useful for 99% of cases, non-RAW image formats (as produced by your camera or the RAW converter) have the inverse curve already baked into the image so that they appear similar to the real world when viewed on a display.
It is this curve that must not be applied in order for our photography to be representative of the world (as far as the math is concerned).
I won't get into what "gamma" means, or scene-referred vs. display-referred imagery, so for an in-depth look at linear workflows see Sony Pictures Imageworks' 2012 SIGGRAPH course nodes (PDF) and this FXGuide article.
So how do we get linear output from our DSLR?