Sixty eyes of 30 patients undergoing bilateral cataract surgery were enrolled and randomly assigned to one of two groups : the bifocal group , including 30 eyes implanted with the bifocal diffractive IOL AT LISA 801 ( Carl Zeiss Meditec , Jena , Germany ) , and the trifocal group , including eyes implanted with the trifocal diffractive IOL AT LISA tri 839 MP ( Carl Zeiss Meditec ) .