Progressive lens

They are characterised by a gradient of increasing lens power, added to the wearer's correction for the other refractive errors.

Unlike modern PALs, it consisted of a conical back surface and a cylindrical front with opposing axis in order to create a power progression.

[1] While there were several intermediate steps (H. Newbold appears to have designed a similar lens to Aves around 1913), there is evidence[2] to suggest that Duke Elder in 1922 developed the world's first commercially available PAL (Ultrifo) sold by "Gowlland of Montreal".

The breakthrough in user adaptation and comfort, as well as peripheral and dynamic vision however occurred in 1972 with the introduction of Varilux 2, for which Maitenaz created a totally aspheric design and manufacturing process.

[3] Carl Zeiss AG developed freeform technology in 1983 with its own patented progressive series Gradal HS.

Fitting: Progressive lenses require careful placement relative to the wearer's pupil centre for a distance-viewing reference position.

Incorrect specification of the fitting location can cause problems for the wearer including (depending on the design of the lens) narrow fields of view, clear vision in one eye only, on-axis blur, and the need to alter the natural head position in order to see clearly.

Returning to an older prescription or different type of lens design (bifocal, trifocal) only serves to increase the adaptation period to the progressive lenses.

Some wearers find the visual discomfort caused by these distortions outweigh the benefits of wearing PALs; this is known as progressive non-tolerance.