400mm prime options

Not at all John. The thread may be nearly 5 years old but we've still got to keep it accurate.

All M42 screw mount lenses can be mounted, as can T2 lenses, and some M39 lenses with appropriate adapters.
However many screw mount lenses are for rangefinder cameras and don't have enough rear focal distance to use for anything other than macro on a any SLR.
The situation gets even worse for C mount, CS mount, D mount, RMS... which are all screw mount lenses!
The last three of these are difficult/impossible to get infinity focus with on mirrorless cameras.

If we're being pedantic, we might have to say "almost all M42...etc." but of course you're quite right.

Incidentally, those smaller lenses can be used with some success, although quality is very variable, on the lovely little Pentax Q cameras. I found CCTV lenses very poor in this respect, but as ken pointed out their resolution is low. he suggested C mount cinematography lenses, and when I find one I'll try that thought out.

Thank you petro chemist and others for helpful advice. I know you are all experts on pentax.
my lenses specifically are
SMC Takumar 1:2.8/105
SMC Takumar 1:3.5/35
SMC Takumar 1:1.8/55
SMC Takumar 1:4/200
Would love to put them back in use and would buy second hand modern pentax dslr if I thought it would work.
Would be grateful for any further advice/comments. Thanks

I have a beautiful Tamron 300mm F 5.6 Adapt-a-matic in mint condition (Model 670AU), this would work well with a TC, but focussing manually would be a challenge. Also, to perform at a decent level it needs to be stopped down to F8, which would make it even more of a challenge. It is capable of producing images that I think are superior to the 55-300, but some may disagree. The images have a different sort of quality, not bitingly sharp, but very pleasing to look at with lovely warm colours and great bokeh. I paid £20 for it, so I can live with its shortcomings.

I use it with a K-mount adaptor permanently fitted, but clearly it doesn't have the advantages of a proper K mount and is manual only.

I'm not suggesting it's a viable alternative to a decent 400mm lens, but it's an interesting lens to use and one I enjoy owning.

Regards
David

The longer telephotos get, the harder they are to handle (bigger, heavier, slower etc.) and 400mm is significantly longer than a xx-300 zoom (especially on APS-C). CA also gets progressively worse unless you go for a good APO or a mirror lens. There's a point where a shorter lens cropped is likely to give better IQ, not necessarily because the lens is better but because it's easier to handle. Technique plays a much bigger part of the equation as the lenses get longer/heavier.

I got through all the sigma 400mm primes and didn't like any of them, also tried the tokina 400mm didn't like that.

Eventually I bought a tamron Adaptall 400mm f/4 it might be manual focus and bloody heavy but find one and you will never sell it :-

Here's a shot with the tamron with the tamron 2x tele attached

Oh and it's bloody big!

Lol and I used to think it's little brother I once owned the 60B was huge, I sold mine because most of the photos seemed to have a bluish caste to them.