disk of least confusion

Since the electron lens has spherical aberration, electron beams, which exit from a point at the optical axis on the object plane while traveling in various directions, do not come to one point at the optical axis on the ideal image plane (Gaussian plane). An exiting beam, whose angle to the optical axis is small, nearly comes to the optical axis on the ideal image plane. An exiting beam, whose angle with respect to the optical axis is large, intersects the optical axis above the ideal image plane, thus deviates from the optical axis on the ideal image plane. Adding these formed images produces a least circle (disk) image at a position shifted a little from the ideal image plane to the objective lens. This circle is called "disk of least confusion." The diameter of the disk of least confusion, d_s, is given by d_s = (1/2)Csα³, which is 1/4 of blur on the Gaussian plane. Here, Cs is spherical aberration coefficient, α is the angle between the electron beam and the optical axis.

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