You asked
Samyang af 135mm f 1 8 fe vs sigma 135mm f 1 4 dg art?
Both are autofocus 135mm primes for full frame, and neither has stabilisation. The difference is the aperture. The Sigma 135mm f/1.4 DG Art gathers a bit more light and gives more background blur, and it also fits Leica SL and Panasonic S-series bodies. The Samyang AF 135mm f/1.8 FE is made for Sony E only.
If you shoot Sony and want the most blur and low-light room, I'd pick the Sigma. If f/1.8 is enough for your portraits, the Samyang covers the same job. For why a 135mm suits headshots, see portraits.
Written by Claude from this site's pages.