[Jonathan Mac]

I briefly owned an 18-50mm/2.8 but it was very soft wide open, negating the usefulness of the speed, though of course this could have been just my copy. I returned it and got the Tamron 17-50/2.8 instead and that served me very well for a number of years. The Sigma 18-50/2.8 didn't last long on the market (suggesting that it wasn't just my copy that had problems) and was replaced with a 17-50/2.8 which is vastly superior, though bigger and heavier. I've had one for a few years and sold the Tamron shortly after buying it.

The 17-50mm is a lens I'd heartily recommend, the 18-50mm is not.

[Gene D]

I'm thinking of getting the above-mentioned Sigma as a one-lens solution for most of the time on the KP. Does anyone use this lens particularly on the KP? How is the handling? How's the IQ?

The 20-40mm limited seems like the fancy one-lens solution for the KP, but it's both more expensive and a little slower - but if IQ or rendering is particularly better I might be convinced to save up for it.

Also looked at the Sigma 17-70mm f2.8-4, it's in the Contemporary line which has a bit nicer looks and finish from what I've seen, but how does it compare to the above?

I currently have the 17-50mm f2.8 and the 20-40mm lenses and am using them on my KP, so a bit about qualities and tradeoffs for what its worth. Overall, it depends on the kind of shooting you do and of course looks like maybe budget?

My path to having both the 17-50mm f2.8 Sigma EX HSM (not the 18-50mm) and the 20-40mm Pentax started with the Sigma initially used on my old K-S2 as a replacement for the Pentax kit. A tremendous upgrade in sharpness, IQ, low light capability, auto focus is dead-on. That lens became my go-to for travel and a lot of hiking. The pro's of flexibility and IQ overcame the con of no WR and a bit of bulkiness, so I used a rain hood in my travels to rainy locations in England and Europe along with the sling bag to carry. The 20-40mm I purchased on a whim to try and get the "limited" output (colors, contrast) and the WR. It's a great lens. To me the colors are a bit richer than the Sigma, which feels a bit more clinical when shooting landscapes, and it was easier for my hikes albeit a bit less range.

Now on the KP, both handle well--the 20-40mm feels great especially. I use the larger grip and for my longer hikes along with the battery grip accessory. So in the end I am keeping both, just could not part with either. The Sigma is my lower light (usually no flash attached), inside, portrait go-to and still a lens that I love and hike with on nice days. 90% of my shots are keepers but I do find myself enhancing contrast and color a bit more in post. The 20-40mm ends up on the camera most of the time for general use and where the color range of my shooting with be high. I am lucky that I can have both and options to use them in these different conditions.

If someone were to say I could only have one on a budget, I would go with the Sigma. As a newbie several years ago, it was the lens I learned on the most in all conditions. Got mine used so probably $200 less than the 20-40mm, which makes it a financial no-brainer from that standpoint given the pros/cons as I now look back. Some have mentioned the size/balance as an issue. Is not for me, even on the KP, and I have big hands. I got used to it and would mention that I have a clear filter on it to prevent bangs when its on my shoulder and I am on a train or in narrow areas.

With no budget constraints, those Pentax colors/contrast and WR, then the Pentax, again conditionally given my shooting preferences. By the way, that lens then took me to start looking at acquire other limited lenses, 70mm etc., so it also had that aspect to it.

Good luck. Your needs may be different, this is just my experience. For me it was bit of a journey to get to use and decide to keep both.

[AgentL]

I currently have the 17-50mm f2.8 and the 20-40mm lenses and am using them on my KP, so a bit about qualities and tradeoffs for what its worth. Overall, it depends on the kind of shooting you do and of course looks like maybe budget?

My path to having both the 17-50mm f2.8 Sigma EX HSM (not the 18-50mm) and the 20-40mm Pentax started with the Sigma initially used on my old K-S2 as a replacement for the Pentax kit. A tremendous upgrade in sharpness, IQ, low light capability, auto focus is dead-on. That lens became my go-to for travel and a lot of hiking. The pro's of flexibility and IQ overcame the con of no WR and a bit of bulkiness, so I used a rain hood in my travels to rainy locations in England and Europe along with the sling bag to carry. The 20-40mm I purchased on a whim to try and get the "limited" output (colors, contrast) and the WR. It's a great lens. To me the colors are a bit richer than the Sigma, which feels a bit more clinical when shooting landscapes, and it was easier for my hikes albeit a bit less range.

Now on the KP, both handle well--the 20-40mm feels great especially. I use the larger grip and for my longer hikes along with the battery grip accessory. So in the end I am keeping both, just could not part with either. The Sigma is my lower light (usually no flash attached), inside, portrait go-to and still a lens that I love and hike with on nice days. 90% of my shots are keepers but I do find myself enhancing contrast and color a bit more in post. The 20-40mm ends up on the camera most of the time for general use and where the color range of my shooting with be high. I am lucky that I can have both and options to use them in these different conditions.

If someone were to say I could only have one on a budget, I would go with the Sigma. As a newbie several years ago, it was the lens I learned on the most in all conditions. Got mine used so probably $200 less than the 20-40mm, which makes it a financial no-brainer from that standpoint given the pros/cons as I now look back. Some have mentioned the size/balance as an issue. Is not for me, even on the KP, and I have big hands. I got used to it and would mention that I have a clear filter on it to prevent bangs when its on my shoulder and I am on a train or in narrow areas.

With no budget constraints, those Pentax colors/contrast and WR, then the Pentax, again conditionally given my shooting preferences. By the way, that lens then took me to start looking at acquire other limited lenses, 70mm etc., so it also had that aspect to it.

Good luck. Your needs may be different, this is just my experience. For me it was bit of a journey to get to use and decide to keep both.

Thanks for the detailed comparison, that's just the kind of thing I was looking for. I think I will end up using the 17-50 indoors quite a bit, so it's probably the best option. Not that there's any reason to worry about low light with the KP, but the combination of shallower DoF and maybe softer contrast might be a look I could go for.