The use of ophthalmic lenses for the correction of ametropia is well known. For example, multifocal lenses, such as progressive addition lenses ("PALs"), are used for the treatment of presbyopia. Typically, a PAL provides distance, intermediate, and near vision zones in a gradual, continuous progression of increasing dioptric power. PALs are appealing to the wearer because the lenses are free of the visible ledges between the zones of differing optical power that are found in other types of multifocal lenses, such as bifocals and trifocals.
As the wearer's eyes move from the distance, through the intermediate, and into the near vision zones of a PAL, the wearer's eyes converge bringing the pupils closer together. Ideally, the design of a PAL would be such that the power progression from the distance zone, through the intermediate and to the near zone matches the wearer's requirements as the eye scans the lens. However, in the design of conventional PAL's, a trade-off is made between the power progression distribution and the level of unwanted astigmatism of the lens.
Unwanted astigmatism is astigmatism introduced or caused by one or more of the lens surfaces resulting in image blurring, distorting, and shifting for the lens wearer. In order to reduce unwanted astigmatism, the power progression is distributed over a greater length in some designs. Due to this lengthened distribution, the power distribution may not meet the wearer's requirements and the wearer may have to alter their natural viewing posture, or head and eye position, in order to use the intermediate and near vision zones of the lens. In those lens designs in which the power progression distribution is over a shorter length, a more natural viewing posture may be provided, but the level of unwanted astigmatism is increased reducing the useable area of the lens.
Thus, a need exists for a PAL that provides a power progression distribution tailored to the requirements of the wearer's natural eye path, but that does not increase the lens' unwanted astigmatism. Additionally, a need exists for a method to determine the preferred viewing posture of the wearer so that the wearer is not required to make significant viewing posture adaptations to use the lens.