Crystal Vision plastic CR39 lenses for bi-focal vision distance viewing means that the reading area has a half moon shape with a "flat top", in the bottow 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. After you have selected this lens, you will see the Flat Top 35 Upgrade option and the Executive Bifocal Upgrade option that you can then select.