Complex sentence: Trifocal lenses are similar to bifocals, except that the two focal areas are separated by a third middle area with intermediate focus correction, used for intermediate vision, roughly at arms' length, "e.g." computer distance.
Split into simpler sentences: Trifocal lenses are similar to bifocals, except that the two focal areas are separated by a third in the middle. This segment corrects the wearer's vision for intermediate distances roughly at arms' length, "e.g." computer distance.