Different dark field techniques in transmission electron microscopy are investigated. In particular, a displaced aperture technique is developed, which produces high quality off-axis dark field images. The essential feature of the method is that the chromatic aberration of the objective and intermediate lenses is compensated for with the aid of a one-stage beam deflector in the object plane of the intermediate lens. In addition, the image distortion and the effect of the curvature of the image field are minimized by reducing the coefficient of spherical aberration of the intermediate lens. Diffracted beams, forming angles α up to 2.5° with the optical axis, can be used for dark field imaging. Resolutions better than 30 Å for α = 2°, and 17 Å for α = 1°, are obtained. © 1975 North-Holland Publishing Company.
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