Model of image formation by defocus contrast. A, a projection of the experimental density map of PaaZ. B, projection of the map at the best-match orientation determined from a particle image. C, the same projection, but filtered by the CTF. The substantial defocus (2.6μm) results in considerable delocalization of information, resulting in fringes surrounding the particle. D, the corresponding experimental image of a particle. The large size of the particle image (440 × 440Å) is necessary to include the delocalized information up to a spatial frequency of about 0.4Å−1.