Ophthalmic lenses are used to manipulate or bend light to focus the incoming light rays correctly on the retina of the eye. There are different categories of ophthalmic lenses on the market, with each category having unique features and benefits. The primary categories are:
- Single vision lenses
- Office reading lenses
- Bifocal lenses
- Trifocal lenses
- Multifocal lenses
Single vision lenses
Single vision lenses as the name implies will only focus light for a single distance. These lenses are typically used to correct refractive errors like myopia, hyperopia, and astigmatism. They are also used for reading lenses in people older than 42 for the correction of Presbyopia.
As reading lenses, they provide a simplistic solution for near vision problems (presbyopia) as they will clearly focus on objects between 30 cm to 40 cm with no peripheral distortion in the lens.
They do however have the limitation of blurring distant objects when a person looks further away than the 30cm to 40cm reading distance. It is, therefore, necessary to chronically put on or remove the glasses depending on the viewing distance.
Office reading lenses
Office reading lenses are also known as intermediate-to-near-progressive lenses. They are specifically developed for the administrative or office worker. This reading lens will focus clearly on objects between 30cm to 90cm.
It is important to note that this lens is still a reading lens and will not focus clearly on objects further than the 90cm away. Due to the progressive nature of the lens (gradual power change from top to bottom), there is a small amount of distortion noticeable in the periphery of the lens.
As the name implies, Bifocal lenses have two (bi) focal points. These lenses have a visible line dividing the optic zones into two sections.
The top section has the distance prescription allowing a person to focus clearly on objects further than 6m away. The bottom part of the lens is focused on near work around the 30cm to 40cm mark. There a few different designs available in the bifocal lens range:
The flattop bifocal has a reading segment that looks like a capital D turned sideways. The length of the D segment can vary between 25mm, 28mm, and 35mm. The industry norm for bifocals is usually the Flattop bifocal with a 28mm segment length.
The fused bifocal looks very similar to the flattop bifocal except the near zone has a rounded or curved top instead of a straight line as is found with the other bifocal types
The executive bifocal has a straight line running across the whole surface of the lens providing maximum reading space.
Benefits of bifocal lenses:
- Focus clear for both distance and near objects
- Provide wide peripheral vision without any distortion
Disadvantages of bifocal lenses:
- Do not focus on intermediate distances like computer screens and arms lengths activities
- Severe image jump when walking and looking down
Trifocal lenses look the same as bifocal lenses accept that a third intermediate zone in added above the reading segment.This lens has the same advantages and disadvantages as bifocal lenses except that the intermediate viewing zone is now also in focus.
With the birth of multifocal lenses, trifocal lenses have mostly become in disuse to the point that most lens manufacturers don’t offer trifocals anymore.
Multifocal lenses are also known as distance-to-near-progressive-, progressive-, varifocal- or progressive additional (PAL) lenses. The top section of the multifocal lens corrects for the distant refractive error of the patient and then gradually adds the near addition power until the maximum reading power is achieved at the bottom of the lens.
For every power change in the lens, a different curvature is added to the surface of the lens, and all these curvatures are blended in the periphery of the lens. It is this blending effect that creates the noticeable non-focus area in the periphery of the multifocal lens.
There are many different designs of multifocal lenses available on the market. They can be divided into the following categories:
Hard design multifocal lenses are an entry level multifocal. The distance and near zones are equally balanced but relatively narrow compared to the softer multifocal. The transition from the clear to the unclear areas are a very hard blend, hence the name, hard design multifocal. When worn for the first time, this type of design is prone to creating vertigo problems with many people when they are mobile and frequently takes longer to adjust.
Soft design multifocal are more complex designed progressive lenses. They have wider distance, middle and near zones compared to hard design multifocal. The transition from the clear to the unclear areas are much more blended creating a more ‘softer’ feel to the lens with fewer vertigo problems. The adoption period to these lenses is also much less than hard designs.
Short corridor designs
Short corridor multifocal has a reduced or shortened intermediate zone allowing the eyes to reach the near zone quicker when looking down. This option allows the lenses to be more compatible with the small modern fashion frames available today. The disadvantage, however, is a smaller intermediate viewing zone making computer screen viewing and another intermediate near work more difficult.
Freeform or individual designs
Freeform or individual design lenses represent the most advanced technology available in progressive ophthalmic lenses. The lenses are individually designed based on the patients’ prescription, frame design, and accommodative system and then manufactured using a micro lathe technology. These lenses provide maximum distance, middle and near zones with very soft distortion on the periphery of the lens.
Benefits of multifocal lenses:
- No visible lines of demarcation, hiding the fact that there is a reading zone in the lens.
- The intermediate zone allows clear vision for all activities in the middle distance.
- No image jump, which reduces the risk of losing your balance when looking down at steps and sidewalks.
Disadvantages of multifocal lenses
- Distortion or unclear vision when a person looks into the periphery of the lens.
- Wearing the progressive lens for the 1st time requires an adaption period of about two weeks for the visual mind to adjust to the optics of the lens.
It is, however, important to note that the softer the design of the multifocal, the shorter the time it is necessary to adapt to the lens.