What are Bifocal & Trifocal Glasses?
Bifocal and trifocal lenses on glasses, have separate sections that allow for near and far vision (trifocals have an extra middle section used for vision over middle distances.
What are bifocal and trifocal lenses used for?
Bi- and trifocal lenses are typically used to treat a condition called presbyopia. This is a natural consequence of aging and can affect anyone at the age of 40 or above. Your eye has a lens which relies on its elasticity to function correctly. Faced with objects at variable distances, your eye needs to adjust the shape of the lens by means of muscles in and around the eye. As you go older the lens can lose its elasticity, meaning that it also loses its ability to focus on objects closer to your face.
Why bifocals and trifocals?
Bifocal lenses have two separate sections each with a different lens power, meaning that one part, usually the lower half, is designed to help you focus on objects closer to you, while the other segment is to help you see objects further away. Trifocals have these same two segments, but also have an additional middle section to help you see objects at a middle distance. The section designed to improve your near sight can be in any one of a number shapes, it can be a narrow rectangle called a ribbon, a half moon, circle, or in fact the entire lower portion of your lens. On trifocals the middle section is usually right above the near sight segment and used to view objects at roughly arm’s length.
Fitting bifocals and trifocals
Because of the different sections involved in bifocal and trifocal lenses, it is important to have them fitted so that they sit comfortably on your eye and you can adjust which segment you are looking through easily. Bifocals are fitted so the segment partition lines up with your lower eyelid, meaning that when you read your eyes naturally look through the appropriate part of the lens as they drop down to look at the text. Trifocals are fitted higher up to accommodate the third section, the idea being to place the relevant segment in a position which the eye normally looks through to allow for ease of use and minimal adjustment on the part of the wearer.
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