Originally posted by netuser Between this lenses, which one would you buy and why ?
Or do you advise other ? (budget is not an issue)
Observation: I have the 1.4 tamron adapter (works with sdm)
Sigma 150-500mm f/5-6.3 DG OS HSM Pentax AF
Pentax smc DA 300mm f/4,0 ED IF
Sigma 120-400mm f/4.5-5.6 DG OS HSM Pentax AF
Sigma EX 50-500mm f/4,0-6,3 DG Pentax AF
I have bought the Sigma 50-500 for mainly the same application. And I would buy it again, even if it is a bit soft at the longest end. But softness in this lens usually is an indicator of a slightly misfocused image - the moment the focus is right, the contrast improves very obviously. The only drawback of that lens is its limited max. aperture, but that it has in common with all other zooms you list.
I would very seriously think about getting a used 300/2.8. The old FA is out of that price range, but a Tamron 300/2.8 (+ Adaptall PKA adapter) is easily within that range and you will be able to throw the excellent Pentax L tcs into that package, which would give you a max. reach of a 600/5.6. This is a combination that is really great and very versatile, though in that respect not quite as versatile as a zoom lens. The Tamron + the Pentax L tele converters give excellent image quality and serve me more often as for example the old Pentax 500/4.5. Though this latter lens is also a good choice and within your price range - just not a really flexible option.
The drawback of either fixed fl would be, that they are not AF. But depending on the birds, this is in reality not a big issue, as the AF will anyway not lock reliably on a bird, if there are branches or other smaller obstackles around it. Instead you'll get sharp foilage and a out of focus bird.
On the other hand, you could easily combine the Tamron 300/2.8 with the Pentax 1.7 AF-adapter, to make it a fast AF lens. With such a fast lens to start with, the AF adapter locks on fast and reliably.
I only mentioned combos I have and use - there might be others and even better suited ones (like the old FA 600/4 or the FA zoom 250-600)
Ben