Dr Alan W Johnston AM
MAppSc PhD LOSc FAAO (DipLV) FACO
Optometrist and Low Vision Consultant
East Melbourne Optometry and Low Vision Centre
Since the Kooyong Low Vision Clinic was established in the 1970s, our low vision devices have improved. Optical magnifiers have better aberration correction and wider fields of view. Electronic imaging options on tablets and smartphones offer voice input and output. Increasingly, using a smartphone camera is an alternative to using a hand-held telescope. However, spectacle-mounted telescopes are still valued by busy patients who need enhanced distance and intermediate vision improvement.
Threaded carrier for Galilean telescopes where spectacle correction is essential for correction of high ametropia or astigmatism
Small Galilean telescopes are lightweight but limited to about 2.5x magnification. Most are fixed-focus types bonded to the front of the spectacle lens or mounted on a flip-up carrier. Some are preset for a 35 or 42 cm distance, such as surgical microscopes. Other Galilean telescopes have lens caps for near-viewing distances. For tasks such as playing bridge, watching television, hobbies or sewing machine use, fixed focus types work well. We can mount bifocal lenses into empty carrier caps for more flexible use (Figures above and below).
We determine the telescope mounting position using a small hole in opaque adhesive tape stuck to the front of the lens, mark the back of the lens then centre the telescope on the front of the lens, over the mark. Alternatively, acrylic predrilled carrier lenses can be edged into the frame, centred as described above. Bioptic telescopes mounted in the upper half of a prescription lens are practical for special occasions such as live theatre, religious ceremonies or sports viewing.
Keplerian telescope mounted
Fixed-focus lens types disappoint if changes in viewing distance are needed. For the regular user of telescopes, Keplerian (prism) designs are preferred (Figure above). They tend to be heavier but range up to 6x magnification, usually with objective (front) lens focusing. Keplerians have a more complex eye lens system containing a field lens, an optical element that shortens eye relief but widens the field of view. For equivalent magnification, they offer up to twice the field of view of Galileans.
Surprisingly, correction of ametropia can be achieved by focusing the telescope: higher powers of Keplerians are more forgiving of uncorrected astigmatism, for example, so that focusing caps on the eye lens are rarely needed. Spiral focus rather than a fine screw thread on the objective barrel provides almost instant focus adjustment, over half a turn from distance to about 40 cm. Rapid adjustment is invaluable for the active user in an office or school environment.
Magnification is illusory
More is not always better. More magnification means a narrower field of view and a shallower depth of field that requires exact focusing even for distant objects. The ‘optical lever’ or image movement increases with magnification. A one-degree movement of a 6x telescope gives a six-degree movement of the field in the opposite direction. Spectacle mounting of telescopes provides greater control of this nuisance. A clinical trial to meet a patient’s vision and convenience needs is essential and provides an assured outcome.