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!

[optical24/7]

A lens has the least thickness along the axis of least power. The example you give would be thinnest at 180. Transposed to plus, your Rx would be Pl + 7.00 X 180. The power is plano at 180, thus the thinnest meridian. Another example would be +7 -7.00 X 045. Transposed it would be Pl + 7.00 X 135. If it was OU the lenses would be thinnest on the OD upper temporal and thinnest at the OS lower temporal.

As far as edge thickness, it will depend on it's final shape and decentration. A plus lens can be ground down to a knife edge to where a plano or minus lens can not. The more plus power, the smaller the diameter. So your +7.00 - 7.00 x 90 lens in an oval shape could have it's thinnest edge at 90.