The College of Optometrists

hi had a quick quesion on contact lenses. is there a easy way of working out the effective power of RGP lens required. in the fitting part of the exam we will be given spec rx with BVD lets just say spec rx is -12/-1.50 x 180 BVD 12mm normally i would look on the table and choose a power but in the exam we are not allowed to use the table. is there a easy way??????

You can do it by calculator without much fuss.

I expect, for the exam what they are looking for is a working knowledge of BVD's and how they affect the refraction. If you said that 5ds to 8ds you take off a 0.25 and take off 0.5 for near ten ds = you won't be far out.

You will not be expected to calculate the BVP of your trial lens in the exam, as frankly very few candidates will bring a huge range of fitting sets with varying BVPs (if I remember you have to bring your own fitting set of 6 RGP lenses with BVPs of around -3.00). You would only do this if you were fitting empirically.

You would need to understand the relationship between the over refraction of your trial lens, the BVD and the final contact lens power ordered.

As an examiner I would be quite happy if you indicated you would refer to a table to calculate this.