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. A gradient of increasing lens power is added to the correction for the other refraction error, going from a minimum or nothing at the top of the lens to maximum magnification at the bottom of the lens. A wearer can then adjust the lens power required for clear vision at different viewing distances by tilting his or her head to place the line of sight through different parts of the lens. A bifocal corrective eyeglasses lens A corrective lens is a lens worn on or before the eye, used to treat myopia, hyperopia, astigmatism, and presbyopia. ... A pair of modern glasses A pair of more traditional glasses Glasses, also called eyeglasses or spectacles, are frames, bearing lenses worn in front of the eyes normally for vision correction, eye protection, or for protection from UV rays. ... Presbyopia (Greek word presbyteros (πρεσβύτερος), meaning elder) is the eyes diminished ability to focus that occurs with aging. ... Human eye The Accommodation Reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object (and vice versa). ... Optical power or dioptric power or refractive power is the degree to which a lens or mirror converges or diverges light. ... Refraction error is an error in the focussing of light by the human eye. ... A lens. ... Magnification is the process of enlarging something only in appearance, not physical size. ...

Progressive addition lenses avoid the discontinuities in the visual field created by bifocal and trifocal lenses. The lenses are also more cosmetically attractive. The lenses suffer the disadvantage of creating regions of aberration away from the optic axis, yielding poor visual resolution. As the lenses combine a range of powers in a single surface there are geometric distortions to the visual field, some wearers find the visual discomfort caused by these distortions outweigh the benefits of wearing PALs, however acceptance rates are claimed by manufacturers to be in the 95% to 98% range. Bifocals are eyeglasses that have corrective lenses containing two different lens powers. ... Trifocals are eyeglasses where the lenses have 3 regions to correct for distance, intermediate (arms length), and near vision. ... This article or section is in need of attention from an expert on the subject. ... The term optical axis has the following meanings: 1. ... Resolving power is the ability of a microscope or telescope to measure the angular separation of images that are close together. ...

The first patent for a PAL was British Patent 15,735, granted to Own Aves with a 1907 priority date. Aves patent included the progressive lens design and the manufacturing process. However this was unlike modern PALs. It consisted of a conical back surface and a cylindrical front with opposing axes in order to create a power progression. This design was never commercialized.

While there were several intermediate steps (H. Newbold appears to have designed a similar lens to Aves around 1913), there is evidence to suggest that Duke Elder in 1922 developed the worlds first commercially available PAL (Ultrifo) sold by "Gowlland of Montreal". This was based on an arrangement of aspherical surfaces. The next commercial product appears in Italy during the 1950s. Offcine Galileo di Milano developed a PAL they called the "Varifocal" lens, based on a variation on Aves design, the surface has been called an 'elephant trunk', in that it resembles a section from a conical shape with the apex pointed downward and bent backwards. This creates the same optical effect as Aves lens, however requires only one surface (front) to do it.

The first PAL of modern design (the Varilux lens) was developed by Bernard Maitenaz and introduced by the Societe des Lunetiers (Essel) in 1959. Early progressive lenses were relatively crude designs but modern sophisticated progressive lenses have gained much greater patient acceptance and include special designs to cater to many separate types of wearer application, for example lenses may be customized for use with computers, or to offer enlarged near and intermediate view areas. Over the 1980s through to today manufacturers have been able to minimize unwanted aberrations by 1. Improvements in mathematical modelling of surfaces allowing greater design control, 2. Extensive wearer trialling where lenses are improved by successive approximation to an ideal design based on wearer feedback and 3. Improved manufacturing and lens metrology technology. Today the complex surfaces of a progressive lens can be cut and polished on computer controlled machines referred to as 'freeform surfacing', to distinguish this process from the previous technology that required the progressive surfaces of these lenses be cast from moulds.

Particularities of progressive lenses

• For those new to progressive lenses, an accommodation period is required - basically, the brain needs to learn to work with them. This period varies from a few days for some individuals to a couple of weeks. During this period, side effects can include headache and dizziness. It is advised that, when these symptoms set in, the progressives are replaced by normal glasses for the rest of the day.
• Depth perception and distance estimation can be influenced during the accommodation period.
• Progressive lenses can produce distortion - lines and surfaces do not appear as straight, this is most likely to appear in the lower half of the lens where the near prescription is blended in. It is noteworthy that all lenses (single vision included) with power can cause the same effect in their periphery, causing straight lines to appear bent.