[StiffLegged]

I guess if you are using catch in focus that could make sense. Otherwise why would the autofocus system matter on manual focus lenses?

Quite right, I should have said “focussing method”. Post has been edited to correct this

[Kendra59]

I will move forward with this understanding of filters:

1) Use camera lenses without filters, unless I cannot create a desired effect without them

2) Use a circular polarizer when needed on my DSLR camera body, regardless of what my lens type is (AF/MF)

3) Get an IR filter (I already have a neutral density filter)

4) If I need a new filter, I will buy the best quality version I can afford

5) Have a step down ring or two in my camera bag, so my 52mm filters can work on my 49mm lenses

6) Avoid using ANY filters when shooting under low light conditions, even if the lens is a newer AF type to ensure as much light gets through the lens as possible (and with an AF lens and DSLR camera, doing so will improve AF tracking)

7) Put the multitude of 49mm Haze filters I got when I bought my old lenses away, as my Think Tank Sling-O-Matic 20 camera bag does a great job of protecting them

[othar]

3) Get an IR filter (I already have a neutral density filter)

While I agree with your other points, this one is not essential for everyone. Only if you are intersted in IR photopraphy, but be aware that without an IR converted camera you will get very long(ish) exposure times and you are basically limited to photographing on a tripod.

[Kendra59]

While I agree with your other points, this one is not essential for everyone. Only if you are intersted in IR photopraphy, but be aware that without an IR converted camera you will get very long(ish) exposure times and you are basically limited to photographing on a tripod.

I appreciate the added feedback!

[Wheatfield]

That is what I hoped would be the case. The circular polarizer... is it needed because of the digital sensor, or because of digital specific lenses, or...?

Circular polarizer are needed to avoid cross polarization in AF and exposure systems that get their information via a partially silvered reflex mirror.
Using a linear polarizer with those systems can blind the AF, making it non functional, and play havoc with exposure.
The only Pentax whose exposure can be thrown out of whack by a linear polarizer is the LX. All other Pentax bodies take exposure readings off the focusing screen.
If you are using manual focus lenses, or are not using AF, a linear polarizer is fine. Most of the time with Pentax, a linear polarizer is fine anyway.
However, they are far less common and far more expensive these days than circular polarizers, so unless you have a bunch lying around from 4 decades ago that are good quality and haven't delaminated, circulars are the way to go.

---------- Post added Jun 20th, 2021 at 11:49 AM ----------

I have tried linear polarizers on my Pentax bodies (k100d super, k-50, k-3, kp) with autofocus lenses and they seemed to work but I wasn’t sure if that was a fully repeatable event. I would love to hear from Pentax if there’s a technical reason for it.

Light going through the partially silvered mirror gets a dose of polarization. Consequently, the light hitting the AF sensor is polarized. Putting a linear polarizer filter on can cross polarize this light. If the offset is great enough, no light reaches the AF sensor, and the AF stops working as it is blind.

Try taking a pair of linear polarizers and rotate them against each other. You will see what is happening to the AF sensor in your camera.

[DWS1]

Never had a focus problem with a circular polarizer.

[clackers]

I will move forward with this understanding of filters:

1) Use camera lenses without filters, unless I cannot create a desired effect without them

2) Use a circular polarizer when needed on my DSLR camera body, regardless of what my lens type is (AF/MF)

3) Get an IR filter (I already have a neutral density filter)

4) If I need a new filter, I will buy the best quality version I can afford

5) Have a step down ring or two in my camera bag, so my 52mm filters can work on my 49mm lenses

6) Avoid using ANY filters when shooting under low light conditions, even if the lens is a newer AF type to ensure as much light gets through the lens as possible (and with an AF lens and DSLR camera, doing so will improve AF tracking)

7) Put the multitude of 49mm Haze filters I got when I bought my old lenses away, as my Think Tank Sling-O-Matic 20 camera bag does a great job of protecting them

Well, ND filters are still the way to go to reduce shutter speeds, Kendra!

[othar]

Well, ND filters are still the way to go to reduce shutter speeds, Kendra!

I think ND filters are covert with point 1 in the list

[clackers]

I think ND filters are covert with point 1 in the list

Not necessarily concluded, Othar, or there wouldn't have been points 2-7 in the list.

[Kendra59]

3) Get an IR filter (I already have a neutral density filter)

Like the man from the old spaghetti sauce ad said to his son "its in there."

[clackers]

Like the man from the old spaghetti sauce ad said to his son "its in there."

Ah, right you are, sorry, Kendra! You're ready to shoot epic landscapes.