In practice and contrary to what that pure geometric analysis predicts, the shadow does not have hard and fast edges, instead, this boundary is characterized by an area of penumbra where light and shadow blends together. In photography that happens when light skims the edges of the aperture. The smaller the aperture the greater the diffraction will be. The way light waves advance is very similar to the way a water wave advances when we drop a stone in a pool of calm water, like a circle that keep getting bigger and bigger. It is Fresnel diffraction that interest us for Zone Plate imaging.
Light waves behave in the same way.
We can generalize and say that the light diffracted by EVEN ring numbers will cancel out the light diffracted by ODD ring numbers. If we could eliminate all the light diffracted by EVEN fresnel zone rings, the cancellation of light of EVEN with ODD zone rings would not exist and the total light reaching the receiver would be the sum of light diffracted by ALL the ODD fresnel zones. Zone plates used for photographic purposes are made by drawing a magnified zone plate on paper, for instance a 100 times larger than the actual zone plate we need and then photographing that drawing at a distance such that its size on film will be 100 times smaller, this will give us a zone plate of the right size we need. Use of a very high contrast film is mandatory to make this zone plates. Once the film is developed, we use the zone plate in the same fashion we use a pinhole.