Single Vision lenses are the standard eyeglass prescription. These glasses have only one viewing area throughout the entire lens and the correctional area can be for far distance, mid distance or reading. Far distance- If you wear glasses all day, but don't have bi/varifocals or mostly need these glasses to see things in the distance.E.g. For driving. Mid distance- If you mostly need these glasses for working on the computer or something else at arms length or you are a musician and need these glasses for reading music. Reading- If you need to read things close to you.Papers, document, etc.
BIFOCAL LENSES WITH A LINE
A bifocal lens means a lens with two points of focus. A small portion of the eyeglass lens is reserved for the near-vision correction. The rest of the lens is usually a distance correction. Generally, you look up and through the distance portion of the lens when focusing on points farther away, and you look down and through the bifocal segment of the lens when focusing on reading material or detail work up to about 18 inches away.
PROGRESSIVE LENSES - NO LINE
Progressive lenses are the closest to how natural vision is that you can get in a pair of prescription eyeglasses. They are more than just a defined near and distance correction in one lens. Rather, progressives provide a smooth transition from distance through intermediate to near, with all the in-between corrections included as well. This constant graduation of the prescription means that you can look up to see in the distance, look ahead to view your computer in the intermediate zone, and drop your gaze downward to read and do fine work comfortably close up.