Fourier transformation of such BG beams using a lens of focal length, f, results perfect vortex at the back focal plane with field amplitude distribution given as Formula （2）.

According to the Fourier transformation theory20, any object can be Fourier transformed by placing the object behind the lens at any arbitrary distance D from the focal plane. The basic principle of the technique is pictorially represented in Fig. 1(b), where the object (axicon) placed at a distance (f–D) behind the lens of focal length, f , is Fourier transformed into perfect vortex at the back focal plane for input vortex beams. The amplitude distribution of the perfect vortex at the back focal plane is given by Formula （4）.

I don't understand how to get Formula (4) from Formula (2).