Stored for 2 years due to the pandemic, my Yongnuo 50mm was throwing errors unless aperture is set to 1.8. On rare times that a shot happens, the result is the left side of the featured photo (over exposed, high bokeh f/4). As further confirmation, while on movie mode, the aperture blades were not closing even if the aperture is changed.
[Will update disassembly instructions/steps the next time I need to open the lens]
I disassembled the lens until I got to the aperture group:
Using a pencil/toothpick, I nudged the exposed lever so that the blades would close. Not sure if it was problematic as it was or due to my manual rotation, but one of the blades didn’t follow the pattern. Gently opening/closing the blades and coercing the rogue one to align/synchronize, the aperture seemed to be fixed. Some blades had blotches of oil/lubricant, so just to be safe, I carefully wiped those with tissue.
After reassembly, I re-did the tests above with the exposure correct now and aperture blades moving during movie mode.
p.s. Writing this, I just now remembered that cams have aperture/DoF test button, so I didn’t need to do movie mode. doh.