To use exposure compensation, just take the shot and then evaluate the image using the histogram. It is important to use the histogram for this and not just the picture as it appears on the back of your camera. The LCD is not all that accurate when it comes to exposure. Remember that if you shoot in RAW, what you are seeing on the back of your camera is a compressed version of the image and not an accurate representation of the RAW file. Also, your LCD’s brightness can be adjusted, so if you turn the brightness down when you are photographing at night all your images will appear darker during the day.