The last few minutes of a setting sun’s rays can yield some incredible contrasts. The warm, hazy light of the sun contrasts with the cool shadows that overtake the retreating sun. The extreme differences in light between the dark shadows and the bright highlights also makes for a large dynamic range.
A camera’s sensor experiences little difficulty capturing detail in scenes with a lower dynamic range, such as that of an evenly lit room, for example. However, if the same room was lit only by a candle, the dynamic range would be huge, and the photographer would be left with a choice: either meter the exposure for the candle (thus leaving the rest of the room pitch black), or meter for the rest of the room and have an overblown white blob where the candle was.
Are the shadows or the highlights more important? After answering that question, the photographer can adjust the exposure accordingly and preserve one or the other. A third option, exists, taking multiple intermediate exposures and blending them together (à la HDR photography), but this is confined primarily to landscapes or still life scenes, as this method can’t accommodate frozen motion.
Had I metered for the men walking next to the building, they would have been exposed correctly, but the highlights in the metal and glass would have also been clipped and been overexposed. As the highlights were more important in this scene, I metered for them and lost some data in the shadow areas instead.