I was asked to write on the above subject, but what is it? You would think something to do with buying cheap glasses online if the request landed here on a glasses blog………..obviously something to do with the back; distance between something or other and something or other; must surely be the distance separating the vertebrae in the backbone. So after hours of research and five hundred laboriously written words, I had to witness my work of art being tossed unceremoniously into HIS wastepaper basket.
It seems that the above (hereinafter called BVD for short) is the distance between the eye and the lens of a pair of glasses. Very important apparently particularly when the spectacle lenses are of a medium or high power.
If you are a short sighted person and you pull your glasses a little way away from your eyes (i.e. increasing the BVD) you will effectively reduce the power of the lens; if on the other hand you are long sighted and you do the same thing, the increase in BVD will effectively increase the power of the lens. In either case your distance vision will be impaired to a greater or lesser extent depending on how strong your lenses are, but things very close up might be a little clearer as the magnification will be greater.
This has a practical application for the middle aged or older persons who might have chosen to have two pairs of glasses instead of bifocals or varifocals. If one pulls one’s distance glasses on to the end of one’s nose they could effectively become the strength of one’s reading glasses depending of course on the length of the nose – sorry, one’s nose. Crossword puzzlers will have noticed that one’s is an anagram nose, if one ignores the apostrophe. I mention that only because there is a limit to the number of words one can write on the subject of BVD.
In the days before the use of intraocular lenses following cataract operations, it was necessary for the patient to wear spectacles with very high powered lenses, of the order 10D (ten dioptres) usually also incorporating a correction for astigmatism, generally about 3D. For these patients it was absolutely essential for the Optometrist or Ophthalmologist to provide a BVD.
Even small prescriptions can vary when presenting with different BVD’s. Contact wearers are sometimes a little confused by the difference between their spectacle prescription, which might be say: -4.25sph –0.25 cyl axis 180 and their contact lens specification which could be 8.4 14 –4.00. The essential elements in these two prescriptions are the –4.25 in the spectacles and the –4.00 in the contact lenses. A difference of –0.25 due to the BVD.
I trust that this subject is now quite clear to all readers, if not please let me know and I will look for a new career as a crossword puzzle compiler.