SINGLE-VISION LENSES
MULTIFOCAL LENSES
Bifocal Lenses
Progressive Lenses

SINGLE-VISION LENSES
This is the most common kind of lens. It provides for only one focal distance at a time, i.e. a single prescription to correct one kind of ametropia or another:
• Myopia: with negative crescent-shaped single-vision lenses. High-power biconcave lenses can also be used for high powers.
• Hyperopia: with positive crescent-shaped single-vision lenses.
• Astigmatism: with toric single-vision lenses, i.e. a combination of a toric surface or cylinder and a spherical surface.

MULTIFOCAL LENSES
Multifocal lenses are used to correct presbyopia. There are several different kinds of multifocal lenses:

Bifocal Lenses
A bifocal is a conventional lens with a smaller inset lens or segment that enables the user to focus on near objects. This kind of lens has two different indices and two prescriptions: the upper part for far vision and the lower segment for near vision. The shape of this segment is what distinguishes one kind of bifocal lens from another.



Advantages and Disadvantages
The advantages of bifocal lenses are as follows:
• They enable the user to focus on both near and far objects.
• No need to have two separate pairs of glasses.

The main disadvantages of bifocal lenses are as follows:
• Image jumps due to the inset segment. The transition between the lenses is not smooth and images of objects on the separating line seem to jump up.
• Some distances are not covered by the addition. Given that there are only two focal points, only two distances are seen clearly. The other distances remain blurry and unfocused.
• They make you look older, because the inset lens is plainly visible.

Progressive Lenses
The surface on the inside of progressive lenses is designed so that the focal distance at each intersection point where the eye’s line of sight converges with the lens surface corresponds to the distance to the point of fixation. These lenses are made with all prescriptions required to enable wearers to see clearly at any distance.

TECHNICAL CHARACTERISTICS Progressive lenses are made up mainly of the following: • Main meridian: The line that defines the intersection of the lens plane with the “sight line”, given that it runs vertically from the far-vision zone to the near-vision zone and corresponds to the convergence of the visual axes. • Far-vision zone: Upper area of the lens for far vision. • Far-vision zone: Transition zone between far-vision and near-vision zones. Suitable for intermediate distances. • Near-vision zone: Lower area of the lens for near vision. • Periphery: Side areas of the lens where certain distortions appear. Not suitable for foveal vision, but perfectly valid for peripheral vision.