Portrait lenses are getting more and more ridiculous these days with their oversized glass elements and their super wide apertures. It's like every year a company will come out and try and outdo everyone by producing the most over the top lens they can and honestly, I couldn't be happier.
It this type of competition that really benefits the photography industry. The kind of performance we're now seeing in full-frame lenses is pretty much unprecedented. Lenses like the 135mm f/1.8 Art from Sigma is one of the sharpest lenses on the market. In my testing I found that it performed incredibly well and on par with the Zeiss 135mm f/2.0. This is saying a lot because the Zeiss lens comfortably beats almost any medium format portrait lens when it comes to sheer detail and sharpness. In a recent comparison by Tony and Chelsea Northrup, they put the Sigma 135mm lens up against the new Sony 135mm f/1.8 GM lens. When it comes to sharpness this lens may not be quite as good as the Sigma or the Zeiss version, but, it does have a few tricks up its proverbial sleeve. The Sigma is a much more affordable lens between the two but Northrup does discuss several reasons as to why you may want to pay extra for the Sony.
Check out the full comparison to see how both lenses perform.