Yes, it is proably more petzval field curvature than diffraction limiting: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Petzval_field_curvature" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Petzval_field_curvature</a>