The UNITRON ZSB and FSB stereo microscopes offered the best resolution, quality and flatness of field in its time. The ZSB and FSB were convergent objective systems. Although these models were discontinued completely in 2009 after more than 35 years of continuous production, UNITRON still maintains a limited number of replacement parts and accessories for these models.
The Unitron ZSB Zoom Stereo Binocular and ZST Zoom Stereo Trinocular microscopes featured a parfocal twin-zooming objective system; 0.7x through 4.5x magnification that provided a large flat field of view with high resolution.
Magnifications were variable by rotating a comfortably located, smooth-operating knurled ring, graduated to show different magnifications at a glance. The zoom ratio of 6.5:1 gave a total magnification capability of 3.5x to 180x using depending on the eyepieces and auxiliary lenses selected.