Publisher Summary The unique properties of gas field ionization source (GFIS)—small source size and the ability to produce light-weight noble gas ions—make it a valuable candidate for use in focused ion beam (FIB) systems for certain applications that supplement those of FIBs based on the liquid–metal ion source (LMIS). This chapter investigates the GFIS source optical properties based on the sphere‐on‐orthogonal cone (SOC) model that well represents thermally annealed field emitter geometry. The chapter presents two alternative spherical and chromatic aberration integrals. They reduce to the usual forms under special circumstances. In addition, the derived spherical aberration involves only up to the second‐order derivative of the potential in the integrand, which, thus, eases the numerical calculation of the potential distribution for general emitter topography. One important feature of field ionization is that the critical distance of ionization, though very small in magnitude, plays an essential role in determining the aberration coefficients so that the results of field electron emission cannot be simply borrowed in deriving source optical properties. Two methods to evaluate the GFIS virtual source size are reviewed in the chapter—the addition in quadrature and direct ray tracing.