My stance on the matter of (protection, UV, etc.) filters is that you introduce more air/glass // glass/air surfaces which increases the risk of unwanted reflections, but also makes you lose light, more so with cheap filters than with expensive ones. The question is whether the light you lose that way is wanted or unwanted (glare, CA, ...). And I'd guess that even with the more expensive filters, even when some of the lost light is unwanted, you'll always also lose at least a little bit of wanted light. I've got no data to back this up, those are just my feelings.
On the matter of the "HD" designation, even with the coincidences of the filter being from Hoya and their past with Pentax, it would be possible but unlikely that it is the same coating. I don't think using "HD" in reference to lens coatings is protected or even protectable in patent/trademark law.
I think this feeling is further reinforced by reading and comparing the descriptions from Pentax for their HD coating and Hoya for the coating of their filters. What I found: Hoya claims
Quote: "HD HIGH-TRANSMITTANCE COATING
16-layers anti-reflective multi-coating on both sides of the filter glass"
"comes with HOYA’s top quality HD glass, which is a special, chemically enhanced optical glass"
, which seems to call both the glass and coating "HD", while Pentax says
here about the HD coating:
Quote: Compared with a conventional seven-layer coating applied by a vacuum evaporation process, the new HD Coating forms an eight-layer coating with a uniform layer thickness, created by a high-precision, ion-assisted evaporation process. This is further reinforced by a ninth coating added to the outermost layer by a vacuum evaporation process.
When you're not in an environment that you don't want your front lens to be exposed to, like on a windy beach or in a dusty room, you can certainly use a filter for protection, but otherwise I would trust the lens hood to prevent mechanical damage and leave filters other than ND or polarizing filters off if you don't want them for the effect they have on your pictures.
Last edited by ehrwien; 11-17-2022 at 01:02 PM.