A eyeglass lens in which the power continuously changes is known as a progressive lens. Unlike bifocal or trifocal lenses, progressive lenses ensure that the presbyopic eyeglass wearer finds the right dioptric power for every distance, guaranteeing smooth, high quality vision.
The power increase is achieved by constantly decreasing radii of curvature in the vertical and horizontal directions.
In the most frequently used zones of vision, virtually aberration-free vision is possible, as here the radii of curvature are almost identical in the vertical and horizontal directions.
A PAL designed for all-round use has three zones of vision: the distance, progressive and near zones. The transitions between these zones are smooth and invisible.
The illustration shows these zones by means of a diagram. The peripheral areas marked in gray limit the zones of the progressive lens used for direct vision. In the peripheral zones the deviations from the required prescription are so pronounced that the wearer can no longer use them for direct vision.
The top area of a progressive featuring the dioptric power required for distance vision is known as the distance zone. In the distance zone the lens displays the power needed to correct the wearer’s ametropia, or no dioptric power if he is emmetropic.
The transition between the distance and near zones in which clear vision is possible is known as the progressive zone. Here, the spherical power increases constantly in a downward direction until the addition is reached. In the progressive zone the lens displays the power required to correct any ametropia present and the additional power needed for vision at intermediate distances.
The width of the progressive zone is depends on the design of the progressive lens and on the power of the addition. One of the factors determined by the design chosen for a PAL is the "distribution" of the areas of blurring on the lens and the length of the progression zone. The following rule applies: the shorter the progressive zone and the higher the addition, the narrower the progressive zone becomes.
The near zone contains the power required for near vision and reading. The near power consists of the distance power and the addition.
Progressives specially designed for vision at intermediate and near distances (e. g. ZEISS Officelens) do not display a fully correcting distance power in the upper area of the lens, but the dioptric power required for intermediate distances. For special visual requirements, this special type of progressive lens is superior to a PAL designed for all-round use.
A special progressive of this type provides the wearer with markedly larger zones for middle distance and near vision.
To meet the demands now being made on our eyes not only at the workplace – and at computer workstations in particular – but also in leisure activities, special-purpose progressives are now gaining increasing popularity.
In an asymmetric progressive the near zone is nasally displaced with respect to the distance zone (nasal inset), with the result that no rotation of the lenses is required during glazing. This design ensures improved binocular utilisation of the zones of vision. In lateral vision the two eyes look through areas with similar image quality.
To ensure maximum wearer acceptance and optimum visual quality, ZEISS goes one step further by incorporating the principle of horizontal symmetry in its progressive lenses.
The curvature of the progressive front surface increases constantly below the major reference point for distance vision. Without the use of a thinning prism, this would mean that the edge thickness in the upper area of the progressive lens would be greater than that in the lower area.
To reduce the weight of the lenses, a base-up prism is ground into the back surface of the lens. A base-down prism is then effective in the finished progressive lens. The magnitude and direction of this thinning prism is equal on the right and left lenses with the same addition and is not therefore effective for the eyeglass wearer.