The ability of an imaging system to resolve detail is ultimately limited by diffraction. This is because a plane wave incident on a circular lens or mirror is diffracted as described above. The light is not focused to a point but forms an Airy disk having a central spot in the focal plane whose radius (as measured to the first null) is Δ Δ x = 1.22 λ λ N , where λ λ is the wavelength of the light and N is the f-number (focal length f divided by aperture diameter D ) of the imaging optics; this is strictly accurate for N ≫ ≫ 1 (paraxial case). In object space, the corresponding angular resolution is θ θ ≈ ≈ sin ⁡ ⁡ θ θ = 1.22 λ λ D , where D is the diameter of the entrance pupil of the imaging lens (e.g., of a telescope's main mirror).