10.1 Lensmeter

A lensmeter (“Lensometer”, “Focimeter”, “Vertometer”) measures the:

  1. Back “Vertex” (effective) power of a lens
  2. Axes and major powers of an astigmatic lens
  3. Power of a prism

To use the lensmeter:

  1. Turn the lensmeter on
  2. Adjust your eyepieces. Turn the eyepiece towards the “plus” until the reticule is blurred. Turn it back the other way (towards the “minus”) until the reticule is just in focus
  3. Mount and centre the mires of the unknown spectacle lens (start with the right side of the spectacle, later do the left side) with its back surface against the rest (i.e. away from the examiner, towards the target
  4. Adjust the power (dioptric) dial until the long thin line and short fat line mires are both in focus. If this can be achieved, the lens has sphere only (no cylinder) and the power can be read off directly. If only one set of mires can be focused at any one time, the lens has cylinder. In this case:
    1. Adjust the power (dioptric) and axis dials until the thin line mires are straight and in focus
      1. = Spherical Power
    2. Adjust the power (dioptric) dial until the thick line mires are straight and in focus
      1. Cylindrical power = 2nd reading – 1st reading
      2. Axis of cylinder = Axis on the axis wheel (this hasn’t changed from the 1st reading)
  • NB: in some lensmeters the axis is that of the 2nd reading. It is important to become familiar with your particular model of lensmeter to ensure the correct axis is read
Figure 9.8.1 Phases of a Normal Fundus Fluorescein Angiogram

Figure 10.1.1
Reading the Lensmeter

First adjust the power (dioptric) and axis dials until the thin line mires are straight and in focus. This is the spherical power. If the thick line mires are still out of focus, adjust the power (dioptric) dial until they come into focus to determine the cylinder (see text).

Prism

  • If the spectacle has prism, the image will be decentred and it will be impossible to bring it to the target centre. The amount and direction of prism can be determined by Prentice’s rule:
    • P = D * F
    • P: Prismatic Power (Δ)
    • D: Decentration / Distance from optical centre (cm)
    • F: Lens power (D)

Measuring the Bifocal (Near) add

  • Unlike the distance part of a lens, the power of the bifocal add should be measured from its front surface. The reason why is complicated but due to thick lens theory

Bifocal add = Front vertex power of near – Front vertex power of distance.

          

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