Progressive-power spectacle lens design method
A progressive-power spectacle lens design method, the progressive-power spectacle lens including a distance portion, a near portion, and a progressive portion provided between the distance portion and the near portion, the method comprising: decreasing a distance along a principal meridian from a fitting point to a progression start point and increasing a length of a progressive corridor defined by the progression start point and a progression end point when addition power is large, whereas increasing the distance along the principal meridian from the fitting point to the progression start point and decreasing the length of the progressive corridor when the addition power is small, wherein the distance from the fitting point to the progression end point is fixed irrespective of the addition power.
1. A progressive-power spectacle lens design method, the progressive-power spectacle lens including a distance portion, a near portion, and a progressive portion provided between the distance portion and the near portion, a power of the lens changing along a principal meridian between a progression start point provided at a distance-portion-side end of the progressive portion and a progression end point provided at a near-portion-side end of the progressive portion, a position of the progression start point when an addition power is large and a position of the progression start point when the addition power is small being different, the method comprising:
decreasing a distance along the principal meridian from a fitting point to the progression start point and increasing a length of a progressive corridor defined by the progression start point and the progression end point when the addition power is large; and
increasing the distance along the principal meridian from the fitting point to the progression start point and decreasing the length of the progressive corridor when the addition power is small, wherein
the distance from the fitting point to the progression end point is fixed irrespective of the addition power, and
with a position of the fitting point being zero, a side where the distance portion being positive with respect to the fitting point, and a side where the near portion is present being negative with respect to the fitting point, the following relationships are satisfied:
S mm= a ×Add−b (1)
3 ≦a≦ 5 (2)
8 mm≦ b≦ 12 mm (3)
where S represents the position of the progression start point with respect to the fitting point along the principal meridian, Add represents the addition power, and “a” and “b” represent parameters.
2. The progressive-power spectacle lens design method according to claim 1 , wherein a limiting parameter “c” that limits the position of the progression start point S satisfies the following relationships:
−3 mm≦ c ≦0 mm (4)
S mm≦ c (5).
3. The progressive-power spectacle lens design method according to claim 2 , wherein when the addition power is equal to or greater than 2.00 dioptre, the position of the progression start point S with respect to the fitting point is set at a fixed value of −2 mm.
4. The a progressive-power spectacle lens design method according to claim 1 , wherein a limiting parameter “f” that limits the position of the progression start point S satisfies the following relationships:
−6 mm≦ f (6)
f≦S mm (7).