Originally posted by panonski
I have a wonderfull copy of Sigma 17-50 Dc HSM 2.8 ,
and recently I have driven to buy Pentax Limited 20-40 mm; 2.8-4.0.
If anybody here have (or had ) both of these lenses, I would love to hear, read from the first hand - how can this two can compare in terms of overall pic quality - and specially I'm interested in bokeh,
because Pentax 20-40 have only 4.0 at 40 mm - and Sigma have 2.8 at 50 mm - so for the bokeh - it's obvious Sigma should be better.
Then again, I'm reading about Pentax 20-40 mm limited - and bokeh - as many reviewers mentioned is also extra beatifull.
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Sigma is wonderfull lens, but a bit heavy, and for portraiture I have Pentax 50 mm FA 1.7 (wonderfull at 1.7 ) - which came later in my collection - and squeezed out Sigma 2.8 because I have now two bodies - to avoid lens change with more serious work.
On the other hand - Sigma 17-50 2.8 - is more capable when you need ONe Lens - for shoots. Is Pentax 20-40 mm capable with 4.0 to beat 2.8 in portraiture ?

thank you all !
I also have both. The DA 20-40mm is a great little lens for all-around use. I bought mine specifically for its fine imaging properties, its small build but with excellent construction including WR, for use on my smaller camera bodies- the K-S2 and KP. It is good in low light also being faster than average in aperture. I agree with clackers and Big Mack, although I've never yet noticed the effects of field curvature in the shots I've taken with it.
The Sigma 17-50mm f/2.8 is an excellent choice. Just as good as clackers and Big Mack indicate. Its main lack is in not having WR. But it has better zoom range, and 50mm is a better FL than 40mm for closer portraits. Its f/2.8 will produce more background blur than f/4 can do, but its sharpness wide open at its long end of 50mm is not going to be at its sharpest. This is typical, a slight loss of sharpness at the longest FL of a zoom lens when wide open. Yet ultimate sharpness is not necessarily the goal for portrait. But we do like to see sharpness in the eyes. The DA 20-40mm LTD shows better sharpness at its long end in the Imaging Resource still-life test shot, but I think the Sigma 17-50mm could do as well at f/4 and 50mm.
For use with larger body cameras like the K-3, I would just keep using the very fine Sigma 17-50mm f/2.8. It is a great choice.
For portrait and other uses, and for very good bokeh, I would suggest you consider the wonderful DA 70mm f/2.4, which in its current HD version has upgraded to rounded blades for enhanced bokeh, and well as the superior HD coatings. That is an excellent portrait lens. You might also consider the DA 18-135mm for all around use when you don't need the speed of f/2.8 availability, and for having WR construction.
Last edited by mikesbike; 12-09-2018 at 09:54 PM.