You asked
What is the pentax smc d fa macro 50mm f 2 8 good for?
The Pentax smc D FA Macro 50mm f/2.8 is a macro lens, so its main job is close-up work: small subjects, details, textures and anything you want to fill the frame with. It's a full frame lens, and on a full frame Pentax K body such as the Pentax K-1 or Pentax K-1 Mark II it frames as a true 50mm. On APS-C bodies like the Pentax KP or Pentax K-3 Mark III it behaves like a 75mm, which is a comfortable distance for small subjects where you don't want to crowd them.
It's autofocus, but it isn't stabilised, so use a decent shutter speed or a tripod for close work. If you want more working distance for insects or flowers, the Pentax smc D FA Macro 100mm f/2.8 WR is the longer option. For general people and everyday shooting, the portraits page is a better place to start than a macro.
Written by Claude from this site's pages.