Exposure Value (EV for short) is a way of adjusting your exposure. Exposure, which is to say how bright the photo is. By default, EV compensation is set to zero: don’t make it brighter, don’t make it darker. Just take it at what the sensor thinks is the correct exposure.
Notice that I’m set at -1/3. Each full number can be considered a “stop”, and (gross oversimplification, I know) each photo will have roughly 5 stops. So, by shifting it to -1/3 (that’s 1/3 of a stop darker than normal), I’m slightly skewing the end results to be darker. Why?
The main reason it to retain detail. Sometimes, when there are extremely bright highlights, will bump it down to -2/3 or -1. A full stop darker will help keep those highlights from being blown out.
Digital photography basically works like this: if something gets too bright, it stops registering info. Or rather, the sensor keeps reading light, but it won’t go past 255 (for red, green, and blue channels) – too much light and it stops there. There I go, getting lost in the technology again. Ignore it, just accept: if too much light hits the sensor, no detail gets recorded.
Stepping down the EV will cut down the amount of light hitting the sensor. Fewer pixels hit the maximum, and more detail gets saved. The tradeoff? More noise – darker photos have less time to write valid data to the shadows, so shadows get noisier.
So, I take slightly darker photos, and massage them in Photoshop after the fact to get back to where I want.