You asked
Best budget lens for the sigma bf?
The Sigma BF is a full frame mirrorless body, so any of its lenses will frame as marked. I don't list prices on the site, so I can't rank these by cost. Going by what the site says about budget kit, I'd start with a cheap fast 50mm prime, and these are the ones I'd look at first:
A prime like these is the best way to spend a small budget. As Your first camera explains, a good lens matters more than a flagship body. The BF has no in-body stabilisation, so a fast prime is a good match for low light.
Written by Claude from this site's pages.