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It is possible to establish the spectacle distance and near prescription (and also prismatic component) of a pair of bifocal spectacles using a lens box according to the principle of lens neutralisation. This estimate may not be as accurate as that established using a focimeter, but lens neutralisation is still a useful skill to have and one that is assessed in the Refraction Certificate Examination.
‘Lens neutralisation’ means using lenses that are equal in magnitude but opposite in direction to neutralise the spectacles, so there is no overall effect. For example, a +2.50 sphere in a spectacle lens is neutralised with a –2.50 spherical trial frame lens. A 2 pd BO prism will be neutralised by a 2 pd BI prism.
First, to establish if the lens is minus or plus, complete the transverse test. Pass the lens horizontally from right to left across a vertical line. If the image of the line moves in the same direction (right to left) as the sweep (‘with’), the lens is minus. If the image of the line moves in the opposite direction to the sweep (‘against’), the lens is plus. You may also notice that minus lenses will minify objects and plus lenses will magnify.
If the transverse test implies the test lens is minus, place a plus lens (say, +3.00 sphere) in direct contact and see if this eliminates the movement of the vertical line image. If the movement is still with the sweep, try a more plus lens; if it is against, try a less plus lens. The reverse is true for plus test lenses. Neutralisation occurs when the image of the vertical line remains still as the lenses are swept across them horizontally.
Note that it is vital that lenses are held in close contact, since if they are held apart their effective power is altered.
Once the lens has been neutralised, the prismatic component (if present) can be neutralised in the same fashion by the application of equal and opposite prisms. Images viewed through a prism are displaced towards their apex. Hence, a 3 pd BU prism will shift the image downwards and be neutralised by a 3 pd BD prism. Neutralisation occurs when the image is not shifted at all by the combined prisms.