Lenses can be less sharp around the edges for a number of reasons:
- Chromatic aberrations. Most old lenses are not achromat or apochromat, so different wavelengths of light get refracted with different indices of refraction. In black-and-white photography, the colour fringes near the edges simply show up as being slightly blurry, so it's not as big of a deal as in modern colour photography.
- The plane of focus might not in fact planar, but may be curved due to the lens having a simple optical design with few elements. So when one focuses on the centre of the frame, the edges of the scene do not coincide with the plane of focus.
As for the claim that old lenses are sharper than modern lenses in the centre... perhaps it is true when comparing that lens to compact camera lenses and the such. But from my casual perusal of DSLR lens benchmarks, modern prime lenses are very, very sharp in the center and should certainly beat this lens - especially when stopped down. Modern DSLR prime lenses are often faster than f/2 wide open, whereas that old lens is f/6.8, yielding a deeper depth of field. But stopping the modern lens down to f/6.8 should result in superior resolution. Anyway, small cameras and cell phones can be less sharp due to diffraction effects on such a small scale.
Thank you for posting this. I still think of lenses as just a tuple of (focal length, f-number), and can't understand why two lenses of the same "spec" can be different. Probably a bad mode of thinking to be in.
