A confocal microscope can be considered as a 3-D sampling instrument to collect data from spatial structures, especially biological ones. Optimal performance requires the adaptation of the dimensions of the sampling volume to the lateral and axial raster parameters employed during data collection. It will be shown that the shape of the sampling volume can be controlled through optical means by a combination of specific illumination and detection parameters. The use of these techniques in computer controlled instruments is discussed, especially in relation to operation in fluorescence.