Single vision lenses have only only one correction across the entire lens.
Single Vision lenses are ideal for simple prescriptions. For seeing things at a distance, if you have trouble reading close up or, at arm's length - relieving eyestrain when you're using your computer and other electronic devices.
Reading - Single Vision Lenses
Choose the Reading -Single Vision option if you have difficulty reading without your glasses, or for seeing things close to your face.
Your prescription may have an Near ADD (addition) reading, or more commonly just ADD value (eg ADD +1.50) which is only relevant if you wish to buy multifocal lenses.
Freeform Progressive Lenses
Progressive (or varifocal) lenses provide an alternative to traditional bifocal lenses. Progressive lenses provide a smooth progression from (upper) Distance to (lower) Reading corrections without the need for distinct viewing areas within the lens. The central area of the lens permits clear vision at arms length, ideal for computer screens, with less peripheral distortion than experienced with traditional bifocal or trifocal lenses.
There may be a period of adaptation for someone used to wearing traditional bifocal or trifocal lenses. Ideally customers buying progressive lenses online should already have worn similar lenses before.
Not all frames are suitable for varifocal lenses so if you wish to select Progressive then make sure that the frame you select is suitable for multifocal lenses.
Readers - OTC
Readers are non-prescription "Over The Counter" glasses with a simple POWER correction added (for example, +1.00). Ideal for people who, typically over the age of 40, need help reading their newspaper or book but don't need prescription glasses for other activities.
If you just want to order the frame - perhaps because you want to go to your local optician to get lenses fitted - then choose Frame Only.
Bifocal lenses contain two distinct corrections within the same lens. Typically, the wearer has difficulties both focusing on distant objects as well as difficulties reading newspapers or small-print, hence the need for more than one correction - essentially combining the Distance-Single Vision and Reading-Single Vision prescriptions into one lens.
Generally the upper part of the lens is used for distance viewing and the lower part for reading up close.
Not all frames are suitable for bifocal lenses so if you wish to select Bifocal then make sure that the frame you select is suitable for bifocal lenses.