visible lines makes a progressive lens appear similar to the single vision lenses worn before the onset of presbyopia. Disadvantages Peripheral Distortion: Progressive lenses suffer regions of aberrations and geometric distortions in the periphery, leading to poor vision when turning the eyes down and to the sides. Different designs of progressive lenses have more or less of this distortion. 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. Cost: Progressive lenses are more expensive than bifocal and single-vision lenses due to higher manufacturing and fitting costs. Some research has been conducted to reduce the fabrication cost by precision injection molding. When recommending a progressive lens design, an eyecare practitioner will usually ask the customer some questions about their lifestyle, which coupled with prescription restrictions or recommendations and cost can establish the suitability of various models of progressive lens. Different lenses have different glazing restrictions, lens material availabilities, maximum and minimum fitting heights, prescription ranges and as such the variation in quality between higher and lower end varifocal lenses is considerable. Adaptation For those new to progressive lenses, an accommodation period is often required because the brain needs to learn to adapt to them. This period varies from a few hours for some individuals up to around two weeks. During this period, side effects can include headache and dizziness. It is advised that, when these symptoms set in, the progressive lenses be removed for a short period and replaced after symptoms have subsided. 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. However, manufacturers claim acceptance rates of 90%–98%. Depth perception and distance estimation can be influenced during the adaptation period. References Corrective lenses