Multifocal literally means a lens has multiple focusing capabilities in a single lens. A trifocal means a single lens has three different prescriptions located within the lens; a distance corridor, an intermediate corridor, and a near vision corridor.
The distance Rx of the lens is at the top portion of the lens and brings images far off into focus.
The intermediate Rx is located just below the optical center of the lens and brings images at a distance of about an arms length into focus.
The near vision Rx of the lens is located in the lower portion of the lens and brings close up images into focus. This is also good for viewing tiny detail.
A trifocal lens is constructed by placing enough additional diopters on top of the distance vision Rx to achieve the intermediate Rx. The near vision Rx is achieved by placing enough diopters over the intermediate Rx to reach the needed reading prescription.
Trifocals work well in different lens materials:
- Glass – works well with very low prescriptions
- 1.53 Index of Refraction
- Heaviest of all lens materials (higher Rx’s can be quite heavy on the face)
- Hardest of all material
- Safety Issues – most eye doctors will not put into a child’s eyeglass frame
- Most scratch resistant of all lens materials
- Has the sharpest vision acuity
- Slightly more expensive due to not in demand
- CR-39 – works well for low, medium, and slightly high prescriptions
- 1.50 Index of Refraction
- Most commonly used lens material
- Made of a plastic resin material
- Standard thickness
- Thicker than glass lenses of the same strength
- Light weight material
- Soft – scratches easily
- Durable – doesn’t break easily
- Polycarbonate – works well with low, medium, and high prescriptions
- 1.586 Index of Refraction
- Very popular
- Thinner than plastic
- Light weight
- Safer for eyes’ protection
- 3 different prescriptions can make a lens thicker, polycarbonate holds a higher Rx in less material
- Holds a higher Rx in less lens material as compared to plastic and glass
- Hi-Index – works well with higher prescriptions
- 1.54 – 1.8 and up Index of Refraction
- Most light weight
- Thinnest of all lens materials
- Expensive (the higher the Index of Refraction number, the thinner the lens, the higher the cost)
- 3 different prescriptions can make a lens thicker, a high index holds a higher Rx in less material
- Soft – scratches easily
Trifocals are very useful for older eyeglasses wearers who are presbyopic needing help seeing in all three ranges. People around the age of 50 years old and up are particularly fond of these types of eyeglasses. Trifocal lenses are very convenient in that they accommodate a wearer without the need to switch to a different pair of eyeglasses to bring images into focus.