The electromagnetic lenses in electron microscopes can be used to focus them into very small probe diameter (1 nm or even 0.1 nm) or to produce a electron image of the specimen, with a spatial resolution down to atomic dimensions in TEM systems. The focusing properties of a lens may be given in terms of a parameter k²,
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where,
        B_m -- The maximum value of the magnetic induction between the polepieces.
        l -- A length characteristic of the lens, which can be the half-width of the axial magnetic field distribution, or the polepiece gap S or the radius R of the bore through polepieces.
        K -- A numerical factor incorporating the charge and mass of the electron.
        V_r -- The relativistically corrected value of the electron beam voltage.