Exposure compensation. Not everyone's cup of tea, but handy for situations like shooting in the snow or at the beach. It's available on the M7 via the rear dial, in the SET menu of the M8 and configurable for both/either on the M9. But what about older bodies? Easy! Change the film ISO. Remember, each doubling or halving of the ISO (e.g. 200 vs. 100) results in a one-stop adjustment (just like full apertures or shutter speeds). Of course, this only applies to metered bodies - and a limited few at that (mainly the M6, M6 TTL, M5)... Also works on light meters. Don't forget to change it back!