The imaging of biological macromolecules at high resolution is predicated on the use of imaging modalities that combine a sufficiently high signal-to-noise ratio (S/N) to observe the desired detail with a sufficiently low dose not to have perturbed the structure at the resolution of that detail. For 3D structure determination the S/N has to be sufficiently high to permit the accurate position and orientation determination for individual molecules or assemblies.

For 2D crystal specimens the position is determined a priori by the lattice, orientation by the externally chosen tilt of the specimen, and signal-to-noise in a single unit cell enhanced by the translocational redundancy of the many unit cells of the crystal. This is close to an ideal specimen, which has permitted imaging with resolutions of 3.4 À at doses of 20-35 e/Â2[l]. Specimens with high internal symmetry, icosahedral or helical structures, are examples which confer intermediate structural redundancies to assist in reducing the requires electron dose.