In addition to aduncanson's excellent point, there is something called coma, where instead of a nice Gaussian circle of blur, we see a cone-shaped blur, or an elliptical blur.

My 150mm APO Sironar-S my be my sharpest lens when in focus, but shows cone-shaped coma otherwise. A vintage Tessar blows it away, in that regard. Shh.. Don't tell the others, it will drive up the price.