Exposure lock. Available on the M7, M8 and M9. With a half-press of the shutter button, it's much faster to use for one-off shots than fiddling with exposure compensation. If your scene meters too dark/bright, just frame an appropriately brighter/darker area of the scene and lock the exposure; keep the shutter button depressed halfway as you recompose and shoot. Grass makes for a convenient 18% grey card equivalent for metering. You can also use your hand, but add about a stop to the exposure.