Contrast reversal because of defocus. (Left) A bright-field image of a diatom taken through a conventional microscope using a 60× (NA = 1.4) objective. (Right) An enlargement of a small portion of the image on the left, showing contrast reversals because of changing amounts of defocus. The diatom shell is curved, being thinner at the edges. As a result, the distance from the lens to the surface of the diatom varies; that is, the view includes a range of defocus values. Over this range, the CTF changes sign several times: From left to right, the holes change from black to white, to black again, and finally to white on the right-hand edge. The white rectangle indicates a narrow band midway between a black and a white hole region where the contrast of the holes is low; that is, the CTF is nearly 0 for structures of this size at this value of defocus.