For a confocal fluorescence microscope with annular pupils, the three-dimensional (3-D) image formation has been analyzed in terms of the three-dimensional optical transfer function (OTF). Based on the 3-D OTF, we have calculated the optical sectioning strength by considering the axial response. In addition, the effects of the size of the central obstruction of the lens and the detector have been investigated. In order to avoid the negative tail and the missing cone in the OTF, we have introduced optical fibers into the system. In this fiber- optical confocal scanning microscope, the illumination is from a fiber tip and the signal from the scan point is collected by another fiber and delivered to the detector. The optimum relationship of the central obstruction of the objective to the fiber spot size is presented.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.