Putting an uncoated or single-coated filter on an SMC lens is like putting all-season tires on a Ferrari.
Or, to put it another way, putting any filter in front of the lens will cause _some_ image degradation (loss of contrast or sharpness). An SMC filter will cause the least degradation. In some instances, the difference is hardly visible, but for people to whom it matters, the few extra dollars are money well spent. Pat White
