= Axial resolution
<Axial resolution> describes the ability to distinguish features separated along an imaging system's axis. In <optical microscopy>, the three-dimensional <point spread function> is usually longer along that axis than across it, so two layers can overlap even when comparable sideways separations are distinguishable.
A small fluorescent bead moved through focus provides an experimental response curve: record the detected <intensity> at successive axial positions and measure a stated width, such as its <full width at half maximum>. The result depends on <numerical aperture>, <wavelength>, specimen <refractive index> and the detection method. <Confocal microscopy> can reject much of the signal from outside the focal plane, improving <optical sectioning>.
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