Low Vision Distance Devices

PLV0

reference distance magnification (RDM)

• original distance/ new distance

• no lens input

• patient moves closer to object of interest

relative size magnification

• new size/ reference size

• object size enlarged

• no lens input

angular magnification

• angular subtense of the image as viewed with the device compared with ratio and form of angular subtense of object without the device

• lens input

lens vertex (spectacle) magnification

• magnification provided by magnifying lens

• object not moved

• magnifier held at focal length

lens vertex magnification formula

• LVM = 1-dU/1-dV

• d = vertex distance

• U = object vergence incident at the lens

• V = image vergence leaving the lens

magnification from multiple sources

• Mag total = M1 x M2 x M3

required magnification

• M = reference size/ goal size

• same for distance and near

• gives how much mag

Kestenbaum’s Rule (ADD or Mag)

• gives how much mag using distance VA

• add = denominator/ numerator

• M = F/4

Lebensohn’s Reciprocal

• gives how much mag

• Mag = Best distance VA/ target NVA

magnification labels - effective mag or rated mag

• M = D/4

• MC in US

magnification labels - conventional mag

• M = D/ 4+1

• better optics, accounts for varying distance

• european