Originally posted by mecrox I can see no reason why bringing on board a Tamron 150-600mm, say, would necessarily also mean bringing on board a Sigma Art 50mm.
Unfortunately Sigma can't see their way clear to bring the 150-600 for K-mount. And in any case, I've never seen a chart that suggests the 150-600s produce more line widths per picture height than the DFA 150-450 at full reach. The question has been asked many times. There's never been a definitive answer.
The fact that an image is taken with a longer lens in no way predicts that the image will be sharper than the same image taken with a shorter lens if it's a better lens. Lw/ph is related to a combination of lens quality and sensor quality. I'd venture that my Tamron 300 2.8 gets me better resolution than my A-400 5.6, on the same camera and same distance.
Sometimes people get fooled. They think they are getting something better for cheaper because of more MP or a longer focal length. It isn't always true. Third party manufacturers have in the past been skilled at undercutting price by undercutting specs and build quality. It's only recently companies like Tamron and Sigma have started turning the engineers loose. And when you look at the DFA 70-200 where production was held up by a year while Pentax got it right, and the focusing issues on the Sigma 18-35 etc. I still think the third party companies are less likely to keep going until they get it right for Pentax. We've seen Pentax delay a lens for almost a year while they got it right. Sigma and tamron can just get it right for Canon and Nikon and leave Pentax users to fend for themselves, like they did with the 18-35.
They design a lens, it's selling on Canon and Nikon, there is no way when they get around to the Pentax mount if it doesn't quite work they are going to go back and change the lens design so it's optimized for Pentax. They are going to try and make the same design that worked on Canon and Nikon work on K-mount, whether it works or doesn't. Just slap the mount on and that's it. It doesn't matter how much Pentax users note there are problems. They won't be addressed. The Sigma 18-35 proved that. But Pentax will keep going until they get it right for K-mount.
I'm unclear why folks would even suspect that a lens designed to fit multiple systems (of which K-mount was probably the smallest) would be better than a lens designed from the ground up for that system.
When guys like digitalis test a lens like he did with the Sigma 18-35 and says "there's a problem with this lens." , there's not chance in the world Sigma doesn't know that. They don't care. They don't make lenses for Pentax, they make lenses for Canon and Nikon and then try to adapt them for Pentax. At least they used to. Apparently with the electronics in modern lenses that's a losing strategy. Amazing how many people seem to blame that on Pentax.
I like my Sigma 70 macro, it's great lens and it's screw drive, and I actually paid less in real money for my DFA 100 macro, which is just as good, lighter smaller and WR,. But these days, the 55-300 is better than my Sigma 70-300 faster and more accurate focussing, and it's about the same price adjusted for inflation. Every time some one comes on and says something like 'The Sigma 30 Art is better than the 31 ltd, someone with more knowledge comes on and says "no it isn't" and posts images to show why. Now there's a song and dance I'm really tired of. If Sigma didn't exist as a company looking for shortcuts that make their lenses cheaper than OEM lenses, it would make more sense.
IMHO the only reason for buying a sigma lens is you're giving up on something that's on the equivalent Pentax, and saving the money that Pentax advantage is worth. That's not readily apparent. My Sigma 70-300 died when components failed during ordinary use. I have much older Pentax lenses that hasn't happened to. Sometimes they are immediately apparent, like the difference in IQ between 70-300 type lenses and the Pentax 55-300. There just is no free lunch.