In the majority of implementations of light sheet microscopy, such as ultramicroscopy, the laser beam illuminating the specimen is truncated by a slit aperture before it is focused to a light sheet by a single cylindrical lens. A light sheet generated in this way can be made very thin near to the focal point, but unfortunately its Rayleigh range is severely limited. This problem can be partially solved by using a smaller slit aperture. However, this also causes a major loss in power, a severe broadening of the beam waist, and thus a significant loss of resolution along the detection axis. We developed improved light-sheet-generation optics, which provide longer Raleigh ranges, whilst retaining beam waists comparable to our standard system with one cylindrical lens. Using the modified system we achieved a marked improvement in the resolution of ultramicroscopy reconstructions of representative biological specimens.