Originally posted by mikesbike With the Pentax lens you get f/2.8 maximum aperture even at 50mm, (although I believe it was tested closer to 47mm FL zoomed all the way by Pop Photo), and you get superior construction including WR. But you also get less FL range. So the main consideration is how much do you need WR, and how often do you need to increase your ISO due to your f/4 limitation? If you have the earlier Sigma 17-70mm f/2.8-4 before it was upgraded to having image stabilization for Nikon and Canon, I read it tested a little better optically than the current version. It is a highly-regarded lens.
It would be my first WR lens, and while I don't often necessarily need the WR, it would be nice to have something like this for travel/walk around and not constantly worry about bringing a cover (or grocery bag). Heading to the Azores in the spring, and I want something where (if it's misty/rainy) that can get a little wet.
I also have the 15mm and 40mm Limiteds for tripod and street photography, respectively.
Originally posted by BarryE I used the 16-50 a lot with my K5/3 - I've since moved to FF. I know some will say it has too many weaknesses - when assessing the lens from test charts it does seem to be a fact - but in use things are a little different. I found it a tricky lens to start with, but after learning its foibles it became a useful backup lens when I was not using my preferred primes. I was surprised, when I recently checked, that some of my most successful sales came from images made with this lens. Understand it and it's a good lens in actual use...
Tricky in which way?
As an aside, one of the other reasons I was thinking the 16-50 might be an upgrade is due to the wacky flare on the Sigma 17-70. Not sure if this is just my copy, or if it's something more, but I often get a very nasty green flare in the centre of the frame. See the attached.
Thanks!