Bifocal lenses are available with two different segment widths: 25 mm and 28 mm. The number after the "CT" in the product name indicates the segment width in millimetres. The segment width of 25 mm is adapted to standard conditions of use, providing a sufficiently large visual field for reading purposes. The diameter 28 mm provides correspondingly wider fields of view.
For the segments of glass bifocals from ZEISS, materials with different refractive indices are selected deoending on the addition in order to ensure that, even with higher additions, all bifocal lenses are as thin and flat as possible.