The variation of W₁₃₃ = C₁₃₃h₀ρ³/λ (where h₀ = 17 mm, ρ = 21 mm, and λ = 550 µm) versus the separation of image plane V_image for the optical system of [12]. The Gaussian image plane of this system is located at V_image = 92.088474 mm when the object is placed at P_0z = −200 mm. This figure shows that C₁₃₃h₀ρ³(cosϕ)³ has non-zero value when the image plane is not the Gaussian image plane. It also shows that C₁₃₃ = 0 when V_image = 92.088474 mm, indicating C₁₃₃h₀ρ³(cosϕ)³ is the defocus component of primary aberrations.