Thon Rings: rings visible in the power spectrum of micrographs obtained by bright-field electron microscopy. These rings can be explained as the effect of the contrast transfer function, which modulates the Fourier transform of the object in a defocus-dependent way. Presence of axial astigmatism is evident from deviations of the Thon rings from perfect circular symmetry, defocus is gauged from the placement of the rings, and resolution can be gauged from the range of the rings. For these reasons, electron micrographs are routinely inspected by optical diffraction, prior to scanning and digital analysis.

 

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