The influence of back vertex distance in ocular refraction
It is well known that the distances between the elements of an optical system have a great bearing on its final refractive properties. This the same as saying that there are as many refractive solutions to correct a refractive error as there are possible coupling distances between the eye and the correcting system. This fact is very familiar to practitioners and is very accurately described by the effective power law. This law dictates the adjustment that is needed for contact lens ordering based on a refraction in the spectacle plane, and the differences between measurements in subjective and objective refraction (for example differences between trial frame subjective refraction and retinoscopy,1 or between phoropters and autorefractors).2 Differences of results from different instruments are also known to increase with increasing refractive error, and studies upon the refraction of aphakes and pseudoaphakes have confirmed this.3,4 In developed countries where refractive surgery is increasingly popular, the importance of reliable and accurate refraction has become a major concern and any minor variation in results must be carefully considered.5 The factors influencing the final refraction may be clearly explained. In the case of distance vision, the most important factors are the pantoscopic angle and, most importantly, the back vertex distance (BVD). The BVD might be described as the distance between the back vertex of the lens closest to the eye and the corneal apex. With near vision there are other influences which must be taken into account, such as the object vergence and the lens form (centre thickness, refractive index and surface power). In this case, the front surface power and BVD have significant influence on the final accommodative demand.6 Other effects related to BVD have been recorded in the literature for years, for example its influence upon prismatic effects and decentration,7 upon residual astigmatism in intraocular lenses8 and upon the use of aspheric concave ophthalmic lenses.9 Although for the purposes of calculation a BVD of 12 to 14mm has been typically chosen when fitting spectacles, the actual BVD in use may be very different in practice. In order to illustrate the significant effect of the BVD on distance vision, consider the following example. Consider a lens positioned 10 mm in front of the patient's cornea which produces the desired optical effect and has a back vertex power (PB) of -15.00D. Parallel rays of light are focused 6.66cm in front of the lens, as illustrated by lens B in Figure 1. However, if the final frame is adjusted in such a way that the patient's lens BVD (A) is 15mm, the back vertex power of the lens should be PA = -16.25D approximately to correct properly the ametropia, almost -1.25D more than the initially prescribed power. Calculations are made using the following well-known expression: The remainder of this article describes a study of a medium to high myopic population where the effects of BVD variation may be significantly important as shown by calculations. It has also served to check the repeatability of subjective refraction with conventional procedures, to compare typical refracting devices (such as trial frames and phoropters) in terms of the final results provided by them and to consider the importance of back vertex distance correction in order to adapt final spectacles or to plan for accurate refractive surgery.