I've read a couple articles, but do not claim to be an expert. I'd think I'd not get an SLR lens if it could ONLY be used on a digital back, (though if the cost was really low, it still may not be a bad deal). It'd be smaller, lighter, faster. Not sure we will ever see cheap sensors approaching film size, particularly if the digi-optimized lenses prove to be a lot better. Otherwise, digital optimized lenses in theory bend the light so it hits the sensor at a more perpendicular angle than is needed for film. Supposed to more evenly illuminate the frame. Also, have anti-reflective coating on INTERNAL metal parts to reduce reflections.