Well the filter/hood interference problem is solved. At Stan Tamarkin in CT I tried a Heliopan "Slimline" filter on the 24mm. The filter is far less thick or tall however you want to describe it and the outside diameter is slightly less than the lens barrell itself so the filter fits perfectly. <p> Only glitch was they didn't have the UV I was looking for but they did have the orange I like to have around anyway. But at least I know what to shop for. This may help users of the 21's (maybe new 28 'crons?) also when looking for a good fit.