RECOMMENDED: Made by Essilor, the world's leader in eyeglass lenses. Crystal Vision plastic CR39 lenses for bifocal vision distance viewing means that the reading area has a half moon shape with a "flat top", in the bottom area of the lens. The standard CR39, flat top 28 bifocal has a reading area that is 28mm wide at the top. This is a good choice for prescriptions up to +/-3.00 sphere; above that, we would recommend a higher index lens in order to reduce the thickness and weight of the lens. This lens is available in the following range: -5.00 to +3.00 sphere, -4.00 to +3.00 cylinder, and up to +3.00 add (prism and slab off excluded). This lens is also available in the Flat Top 35 style, and in the Executive of "Franklin" style. For the Flat Top 35 stye, the reading section is 35mm across thte top. For the Executive style, the line bisects the entire lens from one side to the other. Crystal Vision lenses are made by one of the world's largest lens company making top quality lenses. All Crystal Vision Lenses are made in the U.S., to exacting specifications, in an Essilor laboratory, and we are able to offer them at a great price point. This lens is a good choice for corrections up to +/-4.00, and it is available with a variety of options such as Transitions, UV protection, sunglasses or fashion tint, standard Anti-Reflective coating, and the top-quality Crizal Alize and Crizal Avance coatings.