What filter adapter do I need for the old chrome Planar lens? It has a bayonet fitting, so is this what's referred to as a bay 50 or bay 60? The lens cap is around 60mm across, does this make it a bay60? If so, what adapters to be able to use screw-in filters do I need? Ideally I would like a 58mm or 67mm adapter so I can use what I've already got - do they exist, do they still make them, and where can I get them from? Failing that, can I go the filter holder route (eg Cokin) - what do I need?