You asked
Sigma 135mm f 1 8 dg hsm art vs samyang af 135mm f 1 8 fe?
Both are 135mm f/1.8 autofocus primes for full frame, so they frame and blur the same way. The difference is mounts. The Sigma 135mm f/1.8 DG HSM Art comes in Canon EF, Nikon F, Sony E and L-Mount, so it's the one to pick if you shoot Canon, Nikon or Leica. The Samyang AF 135mm f/1.8 FE is Sony E only.
If you shoot Sony, you have a real choice. Both fit full-frame bodies like the Sony a7 III and Sony a7 IV, and both work on APS-C bodies like the Sony a6400, where they act like about 202mm. Neither is stabilised. This page doesn't compare price or optical quality, so check those before you buy. For portraits, the lenses for portraits explains why 135mm works so well.
Written by Claude from this site's pages.