Some miss-conceptions about Freeform progressives

Written by Optician Club on . Posted in Freeform Rx Lenses, Optician Tools, Progressive Lenses

Some miss-conceptions about back side progressives:
  • –“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 Freeform progressives

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.

 

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