The 1.8 STM is very sharp. This is what opticallimits/photozone writes:The Canon **EF** 50mm's are all pretty bad for sharpness. I've had all but the STM, which I think is the same as the MkI 50/1.8 that I have had. Also the 50/1.8 has a 5-bladed aperture whie the 1.4 might be 6-bladed, both pretty old-fashioned.
"Classic 50mm primes tend to be rather soft at max. aperture setting but they get extremely sharp when stopped down to around f/4 or at least f/5.6. This is mostly true for EF 50mm f/1.8 STM as well. However, unlike its predecessor, it is actually very sharp in the image center at f/1.8. The corners/borders have a decent resolution but the contrast falls apart here. The quality increases continuously towards medium aperture settings. The center is already excellent at f/2.8 and there's also a lift in contrast in the outer image regions. The real boost occurs at f/4 with very good borders/corners. The sharpness is outstanding across the image frame at f/5.6. As usual diffraction has am impact beyond f/8 but f/11 remains perfectly usable."
Canon EF 50mm f/1.8 STM - Review / Test Report - Analysis
Canon EF 50mm f/1.8 STM - Review / Test Report