Fraunhofer diffraction of unequal slits

Two equally illuminated slits of different widths have field amplitudes $A_j(\theta )\propto w_j\mathrm{sinc}(\pi w_j\sin \theta /\lambda )$. Their interference intensity is $A_1^2+A_2^2+2A_1{A}_2\cos (2\pi d\sin \theta /\lambda +\delta )$. Near the centre, $A_2/A_1=w_2/w_1$, so the local maximum-to-minimum ratio is $(w_1+w_2{)}^2/(w_2-w_1{)}^2$. At one slit envelope's zero the interference visibility vanishes even if the other slit still transmits.

The ratio of an envelope width to a fringe period counts spatial intervals. It need not equal the number of bright fringe centres strictly inside the envelope's zeros. For a symmetric overlap from phase index $-25$ to $25$, the width spans 50 periods, but the strictly interior integer centres are $-24$ through $24$, totaling 49. Endpoints where one aperture amplitude vanishes have no two-beam contrast. State the count convention rather than treating these as inconsistent answers.