Originally posted by zapp Sigma does not consider itself a low cost alternative anymore. The Art series lenses are currently unmatched, even Leica and Zeiss have tears in their eyes and cannot keep up. The huge aspherical lenses in the f/1.4 Art series are rather unique. In this regard Sigma is ahead of all the others at the moment. They still go for larger volumes, not for exotic designs. Sigma strategy has shifted significantly in recent years. Pentax does not offer a complete or superior range of lenses in all categories. And lenses should not subsidise for camera R&D, each product should have its own business plan. From a user perspective third party lenses like Sigma Art series are very welcome for Pentax K-mount.
I need to provide more details.
Lets imagine you buy a Canon ILC (I take a brand that represent the market), then you need lenses to mount on your new Canon camera. Beside the Camera, Canon offers you a variety of high quality premium glass.
1) Ten years ago, if you could not afford the Canon glass, you had a reason to buy cheaper glass from Tamron or Sigma, the build quality and optical quality was less, but if you were not a pro and on a budget, you'd get the Sigma or Tamron lens.
2) In 2017, Sigma and Tamron designed new lenses with quality at the same level as Canon L lenses, and they offer them at a price almost as expensive at the Canon L glass. But, if you mount the Canon L glass on the Canon DSLR , it works best because the Canon camera was tested with the lens, the coating of the lens and the color filter array and white balance automation were designed for Canon lens coatings, and whenever you'd develop in camera jpeg, you'd have lens correction for the Canon L lenses but no lens correction for the Sigma or Tamron lens of the similar price. What do you do? Do you buy the Canon L lens or do you buy the Sigma lens.
Personally, in year 2000 , if I wanted a cheap lens, I'd buy a Sigma or Tamron lens. However, in 2017, if I have the choice between a premium Canon L USM II lens and a Sigma premium lens, I get the Canon L USM II because the price difference is small, but the Canon L offer me a lot more (faster AF, better coatings, etc).
If Sigma / Tamron, do premium lenses, they sell less because customer have less good reasons to buy them.
Regarding Sigma Art optical quality, there are too components to it:
1) The user placebo perception , call it "Art" all of a sudden make it higher quality. For example, some of the Art series lenses are actual "C" grade lenses which are optically low cost...
2) The true optical quality which is, as good as OEM lenses in best cases, otherwise still slightly inferior.
If you take a Art lens, it is good, but it is not true that it is unmatched. Take Zeiss and Leica lenses, they are better than the best of Sigma. And take the best of Canon or Nikon, they are as good as the best of Sigma.
One thing that appeals to me is the spirit of the "Art" lenses, the name itself inspire me, and especially as opposed to non "Art" lens, you immediately feel that you belong to another class. So , if you test a "Art" lens, you gonna give it the best conditions to deliver the maximum MTF because you are already biased by knowing it's a Art lens.