The concept of forwards and backwards compatibility is only intelligible if everyone uses the terms in a similar manner. Backwards compatibility is generally understood to mean being able to use new accessories (lenses, flashes, etc.) on old cameras. Forwards compatibility is old accessories on new cameras. Then there is mechanical and electrical information interface compatibility. There can be good mechanical compatibility (mount the thing), but crummy functionality (flash will no longer work in TTL mode, lens supports limited metering, etc.) Pentax has good mechanical forward compatibility of lenses: screw to K (with adaptor), but poor backwards - K to screw. The flashes have good forwards compatibility, but poor backwards. Aperture coupling issues in newer cameras is the same can of worms that Nikon has (aside from G lenses which are much worse). Nikon lens mount, mechanical, backwards compatibility (one has to be very specific here) is perfect: the latest Nikon lens can be physically mounted (without damage) to their first SLR. Forwards and functional lens compatibility is much messier (but I do manage to use a single set of lenses, with full functionality, across MF and AF bodies). Nikon flash compatibility (both ways) is excellent: their latest TTL flash can be used on the first body that had TTL flash, retain TTL flash and visa versa.
From: "Christopher Lillja" ....(and severely limited backwards compatibility in the Nikon system, and now forward compatibility, with the debut of the "G" series lenses, unuseable on MF bodies).
