Single Vision
A single vision lens emphasises natural vision and corrects the following:
Near-sightedness: Faraway things are blurry while close images are reasonably in focus; correction with ‘minus-concave-lenses
Far-sightedness: Faraway things are reasonable in focus and close ones blurry; correction with ‘plus-covex-lenses
Astigmatism: The eye needs correction in two different directions at right angles to each other; correction with cylindrical lenses
Presbyopia: Everyone above 40 sooner or later starts having trouble reading, as the lens of the eye becomes less flexible and focusing on nearby objects becomes difficult. In the single vision category, reading glasses can be the solution
Bi/trifocal
A bifocal lens corrects both far and near vision, while a trifocal also has an intermediate zone. These lenses are recognizable by their two visible bifocal lines. Although the design is extremely practical, it is being increasingly overtaken by progressive lenses (sometimes called no-line bifocals), which guarantee clear vision in all directions and at all distances with smooth transitions. Bi/trifocal designs can be made of various materials and are suitable for almost everyone.
Progressive Addition Lenses (PAL's)
Progressive lenses, also called progressive addition lenses (PAL), progressive power lenses, graduated lenses and varifocal lenses, are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation. They are characterised by a gradient of increasing lens power, added to the wearers correction for the other refractive errors. The gradient starts at a minimum, or no addition power, at the top of the lens and reaches a maximum addition power, magnification, at the bottom of the lens. The length of the progressive power gradient on the lens surface is usually between 15 and 20 mm with a final addition power between 1.00 to 2.50 Dioptres for most wearers.
Teresa Lui, O.D. Palo Alto Optometrist Optical Boutique 650-321-9525 info@drteresalui.com