Optical serial sectioning is a technique by which the 3-D structure of a microscopic specimen is observed by incrementing the plane of focus of a light microscope through the specimen. Ideally, if the depth of field of the microscope is sufficiently shallow, the image at each focusing plane is an in-focus rendition of the specimen containing structural information from that plane only. Unfortunately, the limited aperture of any practical light microscope makes this unfeasible; at each focusing plane, the 2-D image obtained contains unfocused information from planes above and below the focusing plane. In this paper, the nature of the distortion of the light microscope is analyzed using principles of geometric optics, where it is assumed that the absorption of the specimen is linear and nondiffractive. It is found that the limited aperture of the microscope results in the loss of a biconic region of frequencies in the Fourier spectrum of the specimen along the optical axis, resulting in a severe loss of resolution along the axis; outside the missing cone of frequencies, the spectrum is distorted by a strong low-pass effect, further reducing the resolution of the image observed at each plane of focus.