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