You asked
Is the sigma 50mm f 1 4 dg dn art good for video?
The site doesn't review this lens for video specifically, but its specs point the way. It's a fast f/1.4 prime with autofocus, so you can get shallow depth of field and a soft background, which suits interviews and cinematic shots. It isn't stabilised, so for handheld work you'll rely on the camera body or a tripod or gimbal.
On full frame it gives a standard 50mm view, which works for talking heads and b-roll. For vlogging it's too tight. My video guide recommends something around 16-20mm full-frame equivalent at arm's length.
The lens does nothing for sound, so plan for a microphone either way. If you want a camera built with video in mind, the Sony FX3 and Sony ZV-E1 both take this lens on full frame.
Written by Claude from this site's pages.