Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2019 / Paper 1 / Section B / 8 / iii

For a weak phase object, the contrast transfer function gives the signed transfer of an object's spatial Fourier components into intensity contrast. In a common convention,where $u$ is a cyclic spatial frequency, $\Delta f$ the defocus, $C_S$ the spherical aberration coefficient, and $E(u)$ a coherence envelope. A zero suppresses that frequency; crossing a zero reverses contrast. The overall sign depends on the image convention, but the zeros do not. Single-image division by $H$ cannot recover information exactly at its zeros. The microscope manufacturer's discussion explicitly connects the first zero with interpretable resolution and the higher-frequency sign changes.^[ref]

The plotted first nontrivial zero is at approximately $u_0=4.0\mathrm{n}{\mathrm{m}}^{-1}$. Hence the smallest spacing in the first uninterrupted contrast band is about $d_0=1/u_0=0.25\mathrm{nm}$. This axis is cycles per length, not an angular wave number requiring a factor of $2\pi$. Smaller spacings can contribute in later bands, but their contrast has reversed and requires appropriate interpretation or multiple-defocus reconstruction.