why The sph. power is changing to the periphery of the lens

[george dimitrov]

The problem is only with plus power of 1.56 SunSensors.
When I mesure the lens power, at the center is correct, but when I move the lensmeter to periphery, the power begins to changing. The example is as fallows:
Center: Sph. +2.50 Cyl.0.00
Periphery: Sph. +2.25 Cyl. 0.25 ( at any direction 15 mm away from the lens center)

I guess the problem is durring the lens casting? What do you think?
One more, i got several complains from a clients with uncomfortable vision.
The problem is with plus power only! The minus are perfect.

[gunner05]

You won't notice it on lower Rx's as much, but there will be a change in Rx towards the periphery on pretty much all Rx lenses. Corrected curve lenses helped out a lot to begin with then aspheric ones did even more, but none are perfect.

[george dimitrov]

This lenses are keeped as stock lenses. The same producer supply 1.56 White Cr-39 also. No matter the equal endexes, the plus powers of white lenses have no mentioned problem. According to the supplier, both product are done with one and the same casting tools, but the problem apears on 1.56 SunSensors only!!! The other explanation was that the plus powers are produced as Aspherical?

[Carbon Pit]

Consider thickness as well. As the lens gets thicker the curves need to be compensated because the power changes slightly. For a plus lens the thickness change from center to edge could create a power change from center to edge. Or it's aspheric :bbg: