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Vertex distance is the distance between the back surface of a corrective lens, i.e. glasses (spectacles) or contact lenses, and the front of the cornea. Increasing or decreasing the vertex distance changes the optical properties of the system, by moving the focal point forward or backward, effectively changing the power of the lens relative to ...
c is decentration (the distance between the pupil centre and the lens's optical centre, in millimetres) f is lens power (in dioptres) The primary use of Prentice's rule is that under certain circumstances, the prescribed prism can be obtained without grinding prism into the lenses, by decentering the lenses as worn by the patient.
If correction (glasses or contact lenses) was worn at the time of measurement; The fixation distance where the prism neutralised the deviation. That way we can gain an understanding as to whether it increases or decreases in size in different positions of gaze and therefore make a diagnosis of whether it is a concomitant or incomitant deviation.
In glasses with powers beyond ±4.00D, the vertex distance can affect the effective power of the glasses. [4] A shorter vertex distance can expand the field of view, but if the vertex distance is too small, the eyelashes will come into contact with the back of the lens, smudging the lens and causing annoyance for the wearer.
When testing at near, the patient is to fixate on light source at 33 cm, which is held at eye level. When testing at distance, the patient is to fixate on a light source at 6m. Patient is instructed to fixate on the light source with both eyes opened. The Maddox rod is then placed over the fixating eye.
BVD Back vertex distance is the distance between the back of the spectacle lens and the front of the cornea (the front surface of the eye). This is significant in higher prescriptions (usually beyond ±4.00D) as slight changes in the vertex distance for in this range can cause a power to be delivered to the eye other than what was prescribed.
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