In Fresnel Diffraction at a circular aperture the central image according to 'Optics' by brij lal and subramanyam will be bright if odd number of full half-period zones can be constructed.But(according to me) an odd number of half-period zones would correspond to 0 amplitude i.e. the central spot will be dark as each half-period zone differs from the next by a phase of pi.

I'd like to know if I'm wrong and why!