This article describes a fiber optic system contrived to perform spectral imaging of extended objects. It consists of two fiber bundles mounted simultaneously on the spectrograph, which allow one to change the spatial and spectral resolution without any modification in the spectrograph's configuration. At the focal plane, the 61 fibers of each bundle are arranged in a hexagonal lattice, which permits regular and symmetrical mapping. When attached to the Nasmyth focus of the 4.2 m William Herschel telescope (WHT), the diameters of the projected areas on the sky of each bundle are ˜13" and 60", respectively. This implies a spatial sampling (distance between the centers of two adjacent fibers) of 1".5 and 7".3 with a spatial resolution (fiber core diameter) of 0".9 and 1".4, respectively. Technical details of the design and building processes, as well as its peculiarities when acquiring and reducing data, are reported. Special attention is given to the problem of guiding and centering the object, which is of importance for such systems. Several tests to illustrate the performance and possibilities of this system were carried out in combination with the FLEX spectrograph and the 4.2 m WHT. They indicate good efficiency and, for some astronomical applications, clear advantages of the fiber systems with respect to other techniques. The Hα flux intensity map and the two-dimensional velocity field in the central region of the active galaxy NGC 4151 are presented to show explicitly the possibilities of the instrument.