Binary lens: investigation of local focuses

Within the limits of the scalar theory the analysis of the axial distribution formed by a binary lens (zone plate) is made. It is analytically shown, that for lenses with the low numerical aperture, some local focuses close to the main focus have the intensity close to intensity of the main focus due to proportional reduction of the transverse size of local focuses. In area close to an optical element where paraxial approach is not valid, the transverse size of focal spots ceases to decrease and, accordingly, intensity falls proportionally to a square of focus's number. Numerical and experimental results show accordance to analytical calculations