Trifocals are eyeglasses where the lenses have 3 regions to correct for distance, intermediate (arm's length), and near vision. They are mostly used by people with advanced presbyopia who have been prescribed 2 diopters or more of reading addition. The intermediate addition is normally half the reading addition. So, for someone with a distance prescription of -4 diopters and a reading addition of +3, the reading portion of their trifocals would have a net power of -1, and the intermediate segment would be -2.5 diopters.