Ophthalmic lenses are use 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 basic categories are:
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 can also be 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 for objects between 30cm to 40cm 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 distance. It is therefore necessary to chronically put on or remove the glasses depending on the viewing distance.
Office reading lenses are also known as intermediate-to-near-progressive lenses. They were specifically developed for the administrative or office worker. This reading lens will focus clearly for objects between 30cm to 90cm. It is important to note that this lens is still a reading lens and will not focus clearly for 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 normally has the distance prescription is allowing a person to focus clearly on objects further than 6m away. The bottom section of the lens is normally focused for near work around the 30cm to 40cm mark. There a few different designs available in the bifocal lens range:
Types of bifocal designs
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 are normally 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.
Advantage of bifocal lenses
Focus clearly for both distance and near objects
Provide wide peripheral vision without any distortion
Disadvantage of bifocal lenses
Do not focus clearly for intermediate distances like computer screens and other activities done at arms length
Severe image jump when walking and looking down into reading portion.
Trifocal lenses look exactly 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 largely become in disuse to a 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 normal 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 must be added on the surface of the lens and all these curvature are then blended together 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 basically be divided into the following categories:
Types of multifocal designs
This is an entry level multifocal. The distance and near zones are equally balanced but relatively narrow compared to the softer multifocals. The transition from the clear to the unclear areas are a very hard blend, hence the name, hard design multifocal. This type of design is prone to creating vertigo problems with many people when they are mobile and normally takes longer to adjust to when they are worn for the first time.
Soft design multifocal are more complex designed progressive lenses. They have wider distance, middle and near zones compared to hard design multifocals. The transition from the clear to the unclear areas are much more blended creating a more 'softer' feel to the lens with less vertigo problems. The adoption period to these lenses are also much less than hard designs.
Short corridor designs
Short corridor multifocals have 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 other intermediate near work more difficult.
Freeform or individual designs
Freeform and individual design lenses represent the most advance technology available in ophthalmic progressive lenses. The lenses are individually designed based on the patients prescription, frame design and accommodative system and then manufactured using a micro lath technology. These lenses provide maximum distance, middle and near zones with very soft distortion on the periphery of the lens.
Advantage of multifocal lenses
No visible lines of demarcation hiding the fact that a reading zone is added to the lens
The intermediate zone allows clear vision for all activities in the intermediate distance
No image jump, which reduces the risk of loosing your balance when looking down at steps and sidewalks
Disadvantage of multifocal lenses
Distortion or unclear vision when a person looks into the periphery of the lens
An adaption period of about 2 weeks are necessary for the mind to adjust to the optics of the lens when the lens is worn for the 1st time. It is however important to note that normally the softer the design of the multifocal, the shorter the time it is necessary to adapt to the lens.