Astigmatic Lens Thickness

[Burfordmatt]

Hi,

Just having a blank moment, but is the maximum lens thickness of a minus cyl always perpendicular to the axis, even in a positive lens?

For instance in the Rx +7.00/-7.00x90 is the lens thickest along 180 even though the effective power in this meridian is 0.00?

Thanks in advance!

[MikeAurelius]

Your question is somewhat misleading...

For a plus power lens, thickness is going to depend on decentration and axis, but also the frame measurements. If your axis is at 90, and you have two frames with the same amount of decentration, the frame with the smaller B measurement is always going to be thinner than the frame with the larger B measurement. Center thickness, that is. And, as you rotate the axis towards 0/180, your thickness is going to increase, the thickest lens would be where the axis is aligned with the ED angle. This is one reason why strong plus lenses should be dispensed in round or oval frames.

For a minus power lens, center thickness will always be the same, but edge thickness will vary based on decentration and frame size (and power of course).

[shanbaum]

Hi,

Just having a blank moment, but is the maximum lens thickness of a minus cyl always perpendicular to the axis, even in a positive lens?

For instance in the Rx +7.00/-7.00x90 is the lens thickest along 180 even though the effective power in this meridian is 0.00?

Thanks in advance!

No.

[Darryl Meister]

Just having a blank moment, but is the maximum lens thickness of a minus cyl always perpendicular to the axis, even in a positive lens?

As the others have pointed out, it will depend upon the decentration, frame (or uncut) shape, et cetera. However, in the simple case of a round uncut with no decentration, the maximum edge thickness of either a plus or a minus lens would be located at the cylinder power meridian, 90 degrees away from the axis meridian.

The center thickness will obviously be fixed, either way. For a minus lens, the center thickness will typically be equal to the minimum lens thickness, say 2.0 mm. For a plus lens, the center thickness will be constrained by the power, diameter, and minimum edge thickness.

In your example of a +7.00 -7.00 x 090, for instance, the thinnest point along the edge of a round uncut would occur at axis 90. The thickest point would occur at axis 180. Yes, this meridian is also Plano, so the edge thickness will actually be roughly equal to the center thickness, which is in turn constrained by the +7.00 meridian.

Best regards,
Darryl