Great lens, the only alternative below 50mm and f1.4 would be the Samyang/Phoenix/Vivitar 35mm f1.4 but that lens is manual focus only and much bigger.

The center sharpness of the Sigma at f1.4 is very good and this is one of those lenses I won't hesitate to use wide open for most of my shots. Other lenses I tend to stop down one or two stops to get better sharpness.

If you don't need the range above 300mm you better sell the 150-500mm. I use the lens without tripod or monopod without any problems and can walk around for hours while I have the lens mounted but I can imagine a lot of users would run into problems when using this lens.

I would not want to miss the range above 300mm and would not sell the 150-500mm any time soon but when you don't use the range above 300mm a lot the 150-500mm is plain overkill. I think you would be better suited with the 55-300mm, better having a lens that you can use without too much problems than a lens that would end up being used less and less because it's too heavy.

I had the chance to compare the Tamron and the Sigma firsthand. The Tamron is somewhat better wide open but stopping down the Sigma to f3.2 already improves the IQ a lot. f2.8 is still quite usable but when I use this lens I tend to stop down to f3.2 at least.

Having HSM is a major plus for me, the Tamron is noisy when focussing while the Sigma is very quiet.

I sold the Tamron and kept the Sigma. Also because I find the handling of the Sigma better than the Tamron. Another point, the Sigma's tripod collar is aligned straight to a tripod mount while the Tamron is somewhat off center. So when aligning the lens on a tripod or monopod the body will be straight when using the Sigma and somehwat off-center when using the Tamron.

I would go with a 17-70mm, UWA is more a specialty lens and not everything landscape is suited for that kind of specialty. UWA on landscapes with a lot of mountains or even high hills gives the impression you're in a landscape with some small bumps. For architecture I would use an UWA inside some impressive building but outside I prefer a less wider lens for the same reason, you get a better sense of height and there is less distortion (most likely).

The 150-500mm has a quick shift function. I readjust focus when needed and you can really finetune your focus. I think the 50-500mm OS HSM has the same functionality.

I only have experience with the 150-500mm OS HSM but I consider it my best lens and I got some of my best results ever with that lens. Sure for critical sharpness my shorter primes are better but as a total package the 150-500mm is unbeatable. Won't take it on holiday this year but locally it's my walkaround lens.

Recently I have made the upgrade from K20 to K-7 and from K-x to K-r. I quickly sold my K-7 because I just could not live with the 'high' ISO-noise above ISO 800. Kept my K-r and intended to buy a K-5, in the end thought I would be better off with some new lenses and now I only have the K-r. Couldn't be happier with a new camera, sure sometimes I miss the double wheels and other controls on the body but most of the time I can get by with the controls the K-r offers.

If you like the viewfinder focuspoints the K-r would be a reasonable choice. Also you get a better LCD-screen at te back (bigger and higher resolution) and somewhat higher maximum frames per second.

Build quality of the K-7 on the other hand is very good but the K-r is well built too considering it's mostly plastic on the outside.

For me the choice was somewhat easier because I use my new 150-500mm quite a lot these days and every gram saved elsewhere is a plus for me.

Shifted quite a lot of lenses through my hands but never really encountered a 'duff' lens. Sure not all of them were top quality when it comes to sharpness or other IQ-factors but considering the amount of new lenses I have had I think it's not that common to encounter a misaligned lens. I test these lenses for sharpness in a controlled enviroment (tripod, indoors, static subject) and if they are not up to par it's more a due to the maximal optical quality of the itself not because the lens is bad.

SRS is very reliable, only dealt with them once when I bought my 150-500mm but have not heard any bad stories about them.

That 150-500mm stock of them is going fast so if you can find an opportunity to buy a new one soon your best option lies with SRS. There are some more options of course but not at this price. Around 800 GBP these lenses still would sell and if the old stock has been sold the price for a new lens could be close to 800 GBP.

I have the OS-version of the Sigma 17-70 and here the OS really works. Not every Sigma lens with OS built-in has the advantage over the SR of the camera itself but I have done some tests and could win 4 stops easily. I have a steady hand though and can win 3 stops and maybe 4 with SR on also, but with the Sigma I think I can push it above 4 stops without too much problems. Won't even think about it when I use SR.

I know this is not a scientific test but reviews also mention the 4 stop gain when using the OS on this lens.

I think the Tamron will edge out the 17-70mm when it comes to IQ but like you said you have more range.

PM sent for my Photobucket album.

I use a K-r and shoot without tripod or monopod. Have shot with monopod but did not like it because it makes me less mobile.