In a tight competition, the Sigma was sometimes sharper, and the Tamron’s chromatic aberration was a concern. Stabilization and weather sealing were important considerations in Tamron’s favor.

The Tamron 35mm f/1.8 Di VC USD was announced years after (and probably as a response to) Sigma’s ART series lens,  but it’s not a perfect answer to the lauded Sigma lens. Tamron did not match the Sigma’s large, f/1.4 aperture with their product, but they did add stabilization, which more than makes up for it in certain circumstances. Which one is better, ultimately? It probably depends on what you’re using it for, but I’ll cover some of the basics of the lenses below (and of course, in the video above).
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https://www.lightandmatter.org/2017/videos/lens-comparison-tamron-35mm-f1-8-vc-vs-sigma-35mm-f1-4-art/