400mm prime options

What are the best value Pentax-k mount options available for a 400mm prime lens? I would consider 3rd party or legacy lenses provided the quality was good.

When you say "best value", do you mean cheap? If so, you might be best off with one of old the Sigma f/5.6 400mm's that crop up here and on ebay from time to time (the APO version is meant to be the best). Alternatively, you could go Manual Focus, but such older lenses tend to suffer in terms of CA and the like.

I should have been a bit more precise. I guess I mean the cheapest lenses that will give an improved IQ over a zoom. I currently have the pretty good 55-300 Pentax DAL but doesn't seem too good with a tele-converter so would like a straight 400mm that might give at least as good or better quality than the 55-300mm. I should add that since writing this I have become more aware of good earlier 300mm primes that could achieve this focal length with a tele-converter (Tamron/Kenko) and still give as good as quality as the 55-300mm.

There's a sigma APO 400mm on fleabay and don't tell a soul but there's a Tokina 80-400mm (I know you don't want a zoom).

I thought I read somewhere about a 400mm from Pentax themselves in the roadmap.

Mine is not the APO version but the 55-300 is far better IQ than my Sigma 400 f5.6

At 400mm, the cheaper options are Tokina and Sigma MF lenses, both F5.6.
But as said, the 55-300mm will likely give you better quality.

Other options are Takumar (cheaper of the bunch?) and Pentax and Tamron but they'll be more expensive.

Takumar is a brand name of pentax and all the lenses were originally marked Takumar. A screw thread Takumar, Super Takumar or SMC Takumar lens is a top class lens.

Later on a few "budget" Takumar (Bayonet) lenses were made by Pentax, but not in very long telephotos.

I have both of these too, and I'd agree with this. Especially wide open, where the Sigma suffers from a lot of CA.

I think my writing was not clear, when I said "cheaper of the bunch", I meant cheaper of the second bunch.
The Asahi S-M-C Tak F5.6 is considerably cheaper (AFAIK so is the Tele-Tak) than the Asahi Pentax SMC F5.6, SMC-A F5.6, SMC-A* F2.8 or SMC-FA* F5.6.
Same goes for the Tamron Adaptall-2 400mm F4 (which I never found for sale).

I would expect so. Although the earlier screw thread lenses are in fact a very good option being of the highest quality of their day. The concept of different qualities of lens (consumer/professional) came much later, probably as more and more less experienced users came into the SLR fold.

But were there not store brands at those times? 60's onwards?
There are many many brands from Japan in M42 mount and most are clearly on a lower league (dunno about original price though).

But certainly with regards to OEM, there were good and not so good, but not so much a consumer versus a professional lens.

Am I correct?

When comparing different marques yes, absolutely. Within the same manufacturer, most lenses would be expected to be of a certain standard.

The exception would be "cutting edge" designs, such as the first Pentax Fish-eye, where it was revolutionary to have such a lens without needing to lock the mirror up to use it. Some compromises would be happily accepted.

I'm looking for some long prime lens. I was wondering about Sigma 400mm/5.6, does anyone has some personal review about this lens? Unfortunately Pentax 600mm is not very popular on eBay and has unusual price. So thinking about Sigma 400/5.6 or something about that focal length.
thanks

hi i have just traded my sigma 400mm 5.6 and got a 55-300dal far better iq
i found my sigma ok at 6mil pix but not at 10 mill on my k200d either my ability or the lens not u to it so went for pentax glass

One of the unicorn lenses much like the supposed DA* 28mm f2