What if the vertex is different?

In our continuing series looking at the use of calculations in dispensing, Janet Carlton this month considers the importance of vertex distance on refraction

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Ideally, the lens should sit in the chosen spectacle frame in exactly the same position as in the trial frame during refraction. If this is not the case, the way the lens behaves (and consequently the patient's visual comfort) may not be as the optometrist intended. The main concern is that the lens may be sitting closer or farther from the eye, effectively altering the power of the lens. Failure to take into account any difference between the dispensed and the trial frame back vertex distances (BVDs) will result in an incorrect final lens power.