Kummer surface and focal error

World point images and Minkowski coordinates are calculated within a quadrifocal configuration. A Kummer surface detail K(X) of a general Riemann surface X_g is highly composite. Accessible with lowest complexity spinor matter is defined as a square root of a Kummer surface subjected to Hermite substitutions for a cubic invariant polynomial. Whereas coordinates ds² are norms of cubic number field a spinor is a sextic number field having a cubic subfield. Despite a linear least squares algorithm K(X) yield a systematic focal error ε_focal.