> I mean may be filters have nothing to do with dust inside lenses... Indeed. I think most internal dust enters between moving parts (when focusing (and zooming, with zooms) (and perhaps also through the mount end). I think front filters have very little to do with internal dust (but a lot to do with dust on the front element - <g>).
Furthermore, I think that any lens that focuses without changing length (e.g., IF lenses) will gather less internal dust than those lenses that focus (and/or zoom) by acting as a bellows, sucking in and blowing out air every time their helicoids are exercised. Even in the case of the above "bellows-action" lenses, keeping their outsides as free of dust as possible helps - why have dust sitting on them just waiting to be sucked in the next time they are focused? One interesting way of coping with this "bellows-action" ability to collect internal dust is in the VS1 600/8 and 800/11 Solid Cats - there are four little air filters over four openings at the back of the lens, intended to filter air on the way in when the lenses are focused closer (which is when air would be sucked in). Presumably the internal construction of these lenses includes some "gasketry" that slows down dust entering at the edge of the focus ring, or the filters would not be very effective. Fred
