Contrast transfer function ($\mathrm{CTF}(q)$)

The contrast transfer function describes how an imaging system transfers object information at different spatial frequencies. In phase-contrast electron microscopy, defocus and aberrations can suppress or reverse particular components, requiring calibrated correction and complementary data.

An underfocus setting that balances objective-lens spherical aberration against defocus to provide a broad first phase-contrast band for a weak phase object. A common approximate convention is $\Delta f_S\simeq -1.2\sqrt{C_S\lambda }$ for positive $C_S$; numerical optimization conventions differ slightly.