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David Attwood studies the use of diffraction for X-ray microscopy. With Janos Kirz, he authored the X-Ray Data Booklet's account of zone plates: concentric rings select contributions that interfere at a focus. Their treatment connects outer-zone width to spatial resolution and explains why an ideal opaque/transparent zone plate sends only $1/{\pi }^2$ of incident power into its first diffraction order. Resolving a small structure and collecting a useful fraction of the incident X-rays are therefore separate design constraints.^[ref]