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For three decades from film through digital, I have never considered or placed a permanent protective filter on any lens.  Digital does not even have a UV issue.  But after all these years, this old topic has resurfaced in my head for one reason.  After only a few years of good care, my Canon EF 100-400 Mk2 has a significant amount of dust inside the front element.  Perhaps not impacting image capture but certainly enough to be a resale concern.  Having just acquired the RF 100-500, certainly don't want a repeat. Yet this rather slow lens should probably not even get a 0.5% light reduction from the best of UV filters.  Any thoughts on using the highest quality Hoya, B&W, or Zeiss UV for protection or is the golden rule still the same, don't stick clear glass on optics?

I'm not sure the dust actually enters around the front lens element. Zooming pumps air in and out through multiple inlets I would think.

According to this page from Canon, the dust and weather sealing looks pretty good on the 100-500:

https://www.usa.canon.com/internet/port ... 1-l-is-usm

The info on the 100-400 is not as detailed. I have lenses that have gotten a lot of dust inside and, as you stated, it doesn’t seem to affect IQ, but I wouldn’t want to buy a lens with dust in it. I always feel that a lens is made to be used and if a filter degrades any IQ, I don’t want to use one. Beware especially of the B&W filters, as they have a reputation for getting stuck and being hard to remove, as this thread and many online mention:

viewtopic.php?f=57&t=296795

Joe

A UV filter won’t prevent dust from getting under the front element. It is not coming in from the front of the lens, it is coming in from the rear and from any of the moving surfaces. In a lot of cases it isn’t actually external dust getting into the lens but stuff that was already inside the lens finding its way to a lens element.