= Objective aperture
An <objective aperture> selects outgoing <Electron> directions in the objective lens's back <focal plane>. Passing the central transmitted beam gives <bright-field transmission electron microscopy>; selecting a separated diffracted beam gives <dark-field transmission electron microscopy>.<Objective aperture by JEOL>[Definition: back focal plane and beam selection]{ref}
In a <paraxial approximation>, an outgoing <angle> $\theta$ maps approximately to position $r=f\theta$ in this <focal plane>, where $f$ is the objective <focal length>. A centred opening of <radius> $a$ therefore admits $|\theta|\lesssim a/f$. Selecting a direction selects a component of the outgoing <wavefunction>; the subsequent image still assigns intensity to specimen positions. The opening is not a hole cut into a real-space specimen image. Its angular selection must be distinguished from selecting a specimen region for <electron diffraction>.
\Image[/tem-objective-aperture-bright-dark-paths.png]
{title=<Objective aperture> selection for <bright-field transmission electron microscopy> and <dark-field transmission electron microscopy>}
{description=Original functional diagram. Lower relay boxes denote image formation, not literal ray paths. Scales and beam angles are illustrative.}
{source=tem-objective-aperture-bright-dark-paths.py}
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