An approach to the correction of distance-dependent defocus in electron microscopic reconstruction

As electron microscopy advances toward imaging larger specimens at higher resolutions, the effect of variable defocus in image formation becomes more pronounced. This variation in defocus along the direction of electron propagation results in each layer within the specimen (perpendicular to the direction of the electron beam) being blurred by a slightly different contrast transfer function (CTF). Using distance-dependent properties of the 3D Fourier coefficients of the projection data (in the case of 2D projections collected about a single axis of rotation perpendicular to the direction of electron travel), each coefficient can be corrected using a CTF which incorporates the appropriate defocus. This paper presents a simulation of image formation with variable blur, together with a Fourier-based correction of the blur in a distance-dependent fashion.