= Diffraction bandwidth and detector pitch
{title2=$p\le\lambda f/(2D)$}
For incoherent imaging with an unobstructed circular <aperture> of diameter $D$, the intensity-image spatial-frequency cutoff is $D/(\lambda f)$ in the focal plane. The <Nyquist–Shannon sampling theorem> therefore gives the displayed pitch limit. The <Rayleigh criterion> instead gives a point-image separation $1.22\lambda f/D$. Consequently two samples per Rayleigh separation and sampling the full optical <bandwidth> differ by the factor $1.22$. Neither sampling estimate guarantees useful contrast at arbitrarily low signal-to-noise ratio.
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