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I'm looking to get a tele for my k-5. I shoot landscape/nature and this would be somewhat experimental, so I'm hoping to get something cheap.
So far, the best options seem to be:
Pentax DA 55-300mm f/4-5.6
Pentax F 70-210mm f/4-5.6
Sigma APO DG 70-300mm f/4-5.6
Sigma DC 55-200mm f/4-5.6
Photozone's reviews on the modern zooms seem to describe the 55-200mm as being unusually sharp. The unscientific consensus on the F 70-210mm seems to be that it has ridiculously good IQ for the price, not just in sharpness but in contrast / color rendering as well. I'm willing to sacrifice the extra 100mm on crop for better performance at the same focal lengths.
What would you guys recommend? At the moment, I'm leaning toward the F 70-210. This is not only because of its price and user reviews, but the 49mm filter thread would mean that I could use the same CPL I use on my prime lenses and not have to use an annoying step up ring to my 77mm filter (which i use for my primary zooms) or step down ring and risk vignetting.