I have Tamron 17-50mm f2.8 and I found that the tamron has blurry on the top left corner and edges with aperture bigger than f4. So it can be a problem for somebody. For 50mm, I think pentax-A 50mm f1.7 is the best value followed by the pentax-M 50mm f2 for exposure point of view. For 50mm AF lens, the F/FA 50mm 1.7 is unrealistic high. I'd choose DA 50mm 1.8 over it. Anyway, better to stick with pentax brand lenses cause the characteristic of green will be the same for pentax brand lenses. That's one of the important reason for many people to use pentax lenses.
I'm a cheap hunter anyway
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