You asked
Is the sigma 135mm f 1 8 dg hsm art good on aps c?
On APS-C, the Sigma 135mm f/1.8 Art becomes a long lens. It frames like about 202mm on Nikon F, Sony E and L-Mount, and about 216mm on Canon EF. That's useful reach for tight headshots from across a room, distant subjects, or sport, but it's a lot of lens for close-up or indoor work. Its f/1.8 still gives shallow depth of field, though the blur is less dramatic than on full frame.
If you want a compact pairing, try the Sony a6700. For sport and wildlife, the Nikon D500 or the Canon EOS 90D are the sturdier choices. For how focal lengths translate between sensors, see focal length, and for portrait framing, see portraits.
Written by Claude from this site's pages.