- –“Field is wider because the surface is nearer to the eye.” Indeed the back surface is a little bit nearer, but also there is less room for progression on the back, so there is not net improvement.
- –“Magnification is more stable because front refractive power is constant.” This effect is quite small. Magnification is mainly due to power. As power increases in a PAL, so does magnification.
- –“Back side PALs produces less distortion”. Distortion depends on the power variation. The faster the variation, the larger the distortion. This is a characteristic of the design, whether it is back of front.
miss-conceptions about back side progressives-Asphericity
Asphericity is more effective on the more curved surface:
- –In positive lenses, ideal Free-Form surface should be the front surface
- –In negative lenses, ideal Free-Form surface should be the back surface
- –But manufacturing currently requires to have the Free-Form surface at the same side à Back side.