Biotar 58mm corner sharpness Corner sharpness Biotar 58mm f2 vs. Helios 44M-4 58mm f2 vs. Canon EF 50mm f1.8 STM at aperture f2 Corner sharpness Biotar 58mm f2 vs. Helios 44M-4 58mm f2 vs. Canon EF 50mm f1.8 STM at aperture f4 Corner sharpness Biotar 58mm f2 vs. Helios 44M-4 58mm f2 vs. Canon EF 50mm f1.8 STM at aperture f8