The In’s and Out’s of Telescopes for Low Vision: Achieving a Balance between Acuity and Field of View in Prescribing.

As we know, patients often explain that they cannot see far enough away to perform whatever activity they have in mind. They must move closer in order to see it adequately—which they may or may not be able to do, or feel comfortable doing! Patients with 20/40 vision, however, rarely complain of difficulty seeing. Individuals are licensed to drive without restrictions with 20/40 acuity, and children are unencumbered in the classroom with that level of vision.

It is reasonable to assume then, that if we can provide 20/40 acuity through the telescope, most patients should be reasonably satisfied with the functional benefit it provides. So, if the goal is 20/40, a 2x device should be adequate for an individual with 20/80 acuity, 4x for 20/160, and 6x for 20/320. If we prescribe higher magnification to achieve better acuity it will be at the expense of a narrower field of view, which is the major complaint of telescope users. Also, the higher the magnification, the greater the image motion due to head movement that may actually undermine the benefit of the higher power device.

Clinically we find that users have increasing difficulty finding what they’re looking for when fields of view become narrower than about 10 degrees. So we always need to balance adequate acuity with field of view. Personally, I prescribe Galilean telescopes for powers 2.2x and lower, and Keplerian at 3x and higher.

I have found that patients respond most well to telescopic low vision aids when their acuities are 20/200 (6/60) or better. Keplerian telescopes in 4x and 5x can readily provide 20/40 and the telescope fields of view are greater than 10 degrees. I like to tell my ophthalmology colleagues that their goal is to keep their patient’s vision 20/200 or better. Now with the wonderful success of Anti VegF treatments, many patients have acuity much better than that, making their response to low vision aids even more robust.