... there will be no movement of the image of the target. At this point the unknown lens has been neutralised and its power is equivalent to that of the trial case lenses but with opposite sign. In order to ensure accuracy the neutralising lens must be placed in contact with the back surface of the spectacle lens. This is not always possible with some curved lenses leaving a 3-6mm gap which more often than not affects the accuracy of the result. Therefore it has become very common for trial lenses to be placed against the front surface of spectacle lenses whereby the front focal power is measured. The front vertex power is not the same as the back vertex power (which in most cases is stronger) and therefore this technique is not accurate enough when lenses have powers of more than two dioptres. * Lens Measure:- Geneva Lens measures can be ...

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