Front vs. Back Sided Designs –"The Myth”

"The position of the progressive surface (even if there are two progressive surfaces) is not that important." The real key is the progressive surface (or both) being FreeForm and computed with sophisticated software, not the position on the front or back. The performance of any front side progressive can be mostly reproduced in a back side progressive. The opposite is also true.

“Field is wider because the surface is nearer to the eye.” Even though the back surface is a fraction nearer to the eye, there is less room for progression on the back, so there is no net improvement

“Magnification is more stable because front refractive power is constant.” That’s true, but the effect is very small. Magnification is mainly due to power. As power increases in a progressive, so does magnification.

“Back side PALs produces less distortion." This is false. Distortion depends on the power variation. The faster the variation, the larger the distortion. This is a characteristic of the power design, whether it is back of front.

The real limitation is the existing FreeForm lab machinery. Presently most FreeForm designs can only be ground on the back side by the machinery. Once this limitation is removed, LDCA will offer designs on either side.