Single vision lenses are simple lenses to which we are all accustomed, they maybe for distance, near or both if you are under 40. After 40 years of age people begin to have difficulty changing focus from distant objects to near ones and back. This is because the lens of the eye becomes less pliable with age.
To overcome this it is possible to have a lens made with two seperate focuses, these are bifocal lenses and will be recognised as those having a small segment at the bottom of the glasses lens. they are either round or shaped like a letter D on its side, flat side up.
Varifocal lenses are blended bifocal lenses and give a variable focus effect.