PSF -- Point Spread Function The visible and measurable smear of photons from a point source.
They might be trying to say that dispersion (by which they may mean the scattering of light in the emulsion?) is minimized when the image point spread function (smallest possible spot) is minimized - but that happens at wide apertures (f5.6-f8), ...
 See also: Shape, Stops, Photograph, In focus, Dimmer
  
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