I’m testing some older Nikon lenses, and one of the lens flaws that pops up is spherical aberration, such as with the Nikon 50mm f/1.2 AIS (still in production!).
Spherical aberration is one of the few aberrations that affects the entire frame, usually but not always causing a veiling haze to “glow” around details, a “soft focus” effect that can be a positive quality for some types of photography eg classic portraiture. I like the Nikon 50mm f/1.2 for this quality, and it’s far cheaper than the hard to find Nikon 58mm f/1.2 NOCT-Nikkor.
Spherical aberration is usually found with fast lenses, but it can be found even in f/2.8 lenses, such as the Nikon 14-24mm f/2.8G (see case study in DAP). For 35mm cameras, that means f/1.2 and f/1.4, though it can also be found with f/2 and f/2.8 lenses. Stopping down reduces and then eliminates SA, with associated focus shift, sometimes quite strong, which is one reason why a lens might be sharper at f/2 than at f/4 where it was thought to be focused.
Strong under-corrected spherical aberration causes quite low contrast wide open; sharpness might actually be quite high, but the contrast is so low as to give the perceptual impression of little detail.
The examples below show spherical aberration at work. Certain color aberrations are also present in the top example, probably spherochromaticism, which is in essence a color-specific spherical aberration.