Figure 6.1 A focimeter
Recording distance prescription
Turn the focimeter on and set the focusing wheel to zero. Then turn the viewing eyepiece fully anticlockwise and look down the eyepiece, turning it clockwise until the dots and graticule are in focus (this reduces instrument accommodation, which will give a false recording).
Place the spectacles on the focimeter with the arms facing backwards, to ensure that the focimeter measures the back vertex power of the lens. Conventionally, the distance then near prescriptions are established for the right lens and then for the left lens.
If the spectacles are bifocals, check that it is the upper distance segment that is orientated on the focimeter. You may need to move the lens around until the ring of dots is centralised on the graticule. If this is not possible, this is due to a prism in the lens (see p. 75).
Once the spectacles are placed on the focimeter, a ring of dots is only seen if the lens only contains a sphere and when the collimating lens is focused. Therefore, rotate the focusing wheel until a crisp ring of dots is seen then note the power value and sign (+ or –) on the wheel. This will give the distance spherical prescription.
As with most cases, the prescription will have an astigmatic element, so, rather than a ring of dots being observed, a ring of fine lines is observed. Turning the focusing wheel will bring these lines into focus and turning the wheel further will bring a set of perpendicular lines into focus (the previous lines will become blurred or will disappear). It is necessary to adjust the axis of the graticule so that the lines are made linear. Once the axis has been corrected, turn the focusing wheel to bring the lines into sharp focus. Failure to first match the axis will result in an inability to sharply focus the lines. Record the power and the axis – this is the value of the cylindrical component in one of the two principal meridians. Then turn the focusing wheel until the perpendicular lines appear. Again, fine-tune the axis of the graticule until the lines are linear then alter the focus wheel until in sharp focus. Record the power and axis of this perpendicular principal meridian.
It is quite simple to convert the two cyl recordings into a spectacle prescription. If working in plus cyls:
- the sphere is the most negative recording
- the cyl is plus and is the difference between the two recordings
- the axis is the same as the most plus recording.
