I've wondered about this for awhile: say you are using a 100mm lens for a portrait on a full-frame sensor. You have a nice perspective flattening effect that compliments a person's face and makes the portrait look especially pleasing. Now suppose you take a 50mm lens and stock it on a micro four thirds camera. The focal length is the same due to crop factor; 100mm. But is the perspective flattening roughly equivalent to that of the 100mm full-frame lens, or is it the same as it would be on a 50mm lens on full-frame? In other words, does sensor crop factor contribute at all to perspective flattening, or do the lens characteristics stay the same based on optical focal length regardless of the camera's crop factor or lack thereof?