You asked
Is the panasonic lumix s 40mm f 2 worth it?
I think it's worth it if you want a small, light standard prime on a full frame L-Mount body. At 40mm it sees roughly the way your eye does, which suits street, travel and everyday shooting. The f/2 aperture gives you some background separation and room to work in dim light, and it autofocuses.
It isn't stabilised, so you'll rely on your body's stabilisation, if it has any, or a faster shutter speed. On APS-C it frames like a 60mm, so it becomes more of a short telephoto there.
For a sensible pairing, try the Panasonic Lumix S5 II, or the Panasonic Lumix S1R II if you want the most resolution. If you're unsure whether full frame is worth the cost at all, Sensor sizes walks through it.
Written by Claude from this site's pages.