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Enhanced contrast in electron microscopy of unstained biological material: II. Defocused contrast of large objects
Author(s) -
Johnson H. M.,
Parsons D. F.
Publication year - 1970
Publication title -
journal of microscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.569
H-Index - 111
eISSN - 1365-2818
pISSN - 0022-2720
DOI - 10.1111/j.1365-2818.1970.tb02220.x
Subject(s) - spherical aberration , contrast (vision) , optics , contrast transfer function , lens (geology) , aperture (computer memory) , scattering , high contrast , focus (optics) , chromatic aberration , materials science , physics , chromatic scale , acoustics
SUMMARY A contrast minimum is observed when 88 nm diameter polystyrene latex spheres are underfocused, which is related to the wide‐angle scattering peaks. Images due to scattered and non‐scattered wave components are displaced due to objective‐lens spherical aberration and defocus. Maximum overlap of these components produces a contrast minimum at underfocus, related to the spherical aberration of the particular lens used. Similarly, a high‐contrast band at carbon‐film edges arises from spherical aberration and defocus separation of non‐scattered and wide‐angle scattered waves. This band increases in contrast with film thickness and in width with lens defocus. These geometrical effects account for the well‐known ‘blinking’ of contrast of large biological objects upon swinging through focus without an objective aperture, and for the general contrast increase of defocused large objects. Fresnel fringes account for only a narrow band of enhanced contrast at distinct edges and cannot account for contrast enhancement of large objects lacking distinct edges.