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Forum: Pentax SLR Lens Discussion
07-30-2018, 12:45 AM
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Hello and welcome to the forums :)
I have both lenses, and they're quite different propositions.
The Sigma 17-50 f/2.8 has a constant aperture and larger focal length range, both of which make it the more versatile lens of the two in terms of field of view, subject-to-background separation and light gathering. It's sharp right across the frame, even wide open, but I find the rendering to be lacking a little in character. There's nothing wrong with it - it just doesn't have a lot of "soul", in my opinion (I've previously described it as "clinical"). But it's a great walk-around lens and capable of excellent results. I really like mine.
The HD DA20-40 is smaller and lighter, due to the variable aperture and narrower focal length range. It's weather-resistant, beautifully built, lovely to use, sharp at the wide end and renders very nicely. As you get towards the long end, the performance in the borders and corners is a bit disappointing - largely due to what I believe is strong field curvature, most noticeable at longer-distance focusing (another member here described it as "runway-style" field curvature, and I think that's quite fitting). In the past, I haven't been a big fan of the HD DA20-40, but I've been forcing myself to use it quite a bit recently, and I'm warming to it. Like any lens, you just need to get used to its quirks and use them to your advantage or work around them.
And yes... 17-50 and 20-40mm are actual focal lengths. Using these lenses with your K3II (with its 1.5x crop factor APS-C sensor), they're equivalent to 25.5-75 and 30-60mm on full frame respectively.
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