Originally posted by WPRESTO I think most of the options have been covered, but here is my list:
560mm Pentax
500mm Sigma
150-450 Pentax
Old lenses that are good.
400mm f4 Tamron, works well with 1.4XL or 2XL Pentax converters, a massive lens, built to hold up small buildings.
400mm f5.6 SMCA Pentax, apparently works well with several Pentax 1.4X converters and also the 1.7X AF. I think the Tamron has the edge in IQ, but this lens is beautifully made, lighter, but not significantly shorter than the Tamron.
500mm f4.5 Pentax, sometimes available at a bargain price, but very big and has a very poor minimum focus distance. Early versions have rack-and-pinion focusing, later versions (all K-mount versions SFAIK) have double helical
600mm f4 Pentax, an absolutely outstanding big telephoto, if you can find one, can afford it, have a really sturdy tripod + head, and have a porter to transport it (unless you don't mind almost 16lb before adding any other equipment)
600mm f5.6 SMCA Pentax, much less expensive than the f4, manual focus, smaller, lighter, cheaper. Apparently has quite a bit of CA.
Nikon 600mm f5.6 EDIF, relatively small & light for 600mm, can be Leitaxed or use a no-glass N to K mount = give up infinity but get somewhat better minimum focus distance (will it be used @ infinity or nearby for wildlife & small birds?). Set @ f8 and use as you would a no-diaphragm mirror lens.The focus is one finger silky-smooth. Better, cheaper and more available than the 600mm f5.6 Pentax.
Nikon 600mm f4, older manual focus versions, MUCH bigger heavier and at least three times as expensive as the f5.6, but is thought to have better IQ. Same options for mounting on a Pentax.
500mm f8 Zeiss Mirotar, pretty much every evaluation of mirror lenses rates this the very best mechanically and optically. No Leitax available, but again, a no-lens Contax-Pentax adapter would make it usable for wildlife, especially birding. This is a really beautiful, compact "elegant" lens, but it has the major drawback of all mirror lenses: doughnut highlights.
Tamron 500mm f8 mirror. Second best to the Zeiss and much cheaper. Just need an Adaptall K-mount and be willing to accept doughnut-highlights or spending time to PP them out
560mm f6.8 Leica, has "trombone" focusing, comes apart in two sections for easier traveling, if complete, it comes with a very nice shoulder brace with pistol grip for hand-holding. This is a simple long-focus lens, not a "true telephoto" so it is VERY long, but not especially heavy (it looks like a metal wrapping-paper tube). Very easy to Leitax because the diaphragm is manual. All reviews I've found indicate that the earlier f5.6 version of this lens is significantly inferior. I've been skeptical about the ease & precision of push-pull focusing, but it may be a little quicker than helical or rack-and-pinion.
That's a great list....
And while he'll never find one, I'm convinced my Tamron SP AF 300mm 2.8 LD [IF] with the F 1.7x AF Adapter on it, for 510mm ƒ4.5 is the cats meow. I can even shoot it hand held for 10 minutes before my strength gives out.
There's also the Sigma 300 2.8
Sigma 300mm f/2.8 EX DG Lens for Pentax 195109 B&H Photo Video that might work with the Sigma 2x adapter. ( I've never investigated that.)
Then to go super light, the DA*200 ƒ2.8 with stacked 1.7 and 1.4 TCs give you 476mm ƒ6.3 on APS-c, although that 1.4 might cause problems with vignetting on a K-1. But on a K-3 that 476mm is about 745mm of FF reach.
And last but probably the least functional my A-400 5.6 with F 1.7x AF Adapter for 680mm ƒ 9.3. Not the sharpest and needs bright conditions to focus, but, you can get results with it. (shot on a K-1) That close to 700mm for $700- $800. Anytime your creeping up on $1 a mm you're doing good.