The process of object reconstruction from its projections is widely used in many fields. One of the applications of reconstruction from projections is in electron microscopy. Various methods have been developed for correction of blurring that occurs when the projections are obtained by a real instrument. As the attainable resolution increases, new issues become apparent and need to be taken into account in the imaging model. In this review we concentrate on the contrast transfer function and its impact on the quality and usefulness of the reconstructions from images obtained by today's electron microscopes.