To address a separate request, I am also including a version of the formula for compensating the add power for a new near working distance that also takes into account the position of the spectacle lens relative to the principal plane of the eye, which will result in more accurate results in higher Rx powers (from

__System for Ophthalmic Dispensing__):

where

*Vertex* is the vertex distance of the lens in meters (allowing for an extra 1.5 mm to the principal plane of the eye),

*Power* is the specified distance Rx power of the lens,

*Add*_{NEW} is the new addition power for the new working distance,

*Demand*_{NEW} is the dioptric demand associated with the new working distance,

*Add*_{OLD} is the original near addition power, and

*Demand*_{OLD} is the dioptric demand associated with the original near refraction distance.

The dioptric demand is equal to 100 /

*D*, where

*D* is the working distance in centimeters. For a typical near refraction distance of 40 cm (16 in), the dioptric demand is 100 / 40 = 2.50 D.

Best regards,

Darryl

## Bookmarks