Least confusion, plane

This is the transverse aberration at the paraxial focus. In simple geometrical terms the circle of least confusion is at a quarter of the distance from where the outermost ray crosses the axis to the paraxial focus. But when diffraction effects are comparable with geometrical aberrations, the relevant focal plane is half way between the paraxial focus and where the outermost ray crosses. Thus the minimum geometrical aberration is... 

It is apparent from the ray diagram of Fig. 3.3 that there is a plane, A-A, nearer to the lens than the geometrical or Gaussian focus plane, where the resolution is improved. It is called the plane of least confusion or Scherzer focus [3, 4, 7], and it is close to where the rays from the outermost parts of the lens intersect the axis. From Fig. 3.3 the distance 6Z between this plane and the Gaussian image plane is approximately the radius of the image disc in the Gaussian plane divided by a. The radius is Ma C and a = ajM, so... 

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