I Fetchenheuer, E Peli, R L Woods
Schepens Eye Research Institute, Harvard Medical School, Boston, United States
Bioptic telescopes are used intermittently by shifting the eye position with or without head movements. The impact of bioptic telescopes on the monocular visual field is usually specified only as the field seen through the telescope and a ring scotoma. For safety and orientation the full binocular visual field is just as important. We measured monocular and binocular visual fields of different types and brands of monocular bioptic telescopes using standard kinetic perimetry. Magnified fields of view varied minimally between brands. For the same magnification, Keplerian fields were larger than Galilean fields. All telescopes caused an optical ring scotoma in the monocular field with an outer diameter equivalent to the visible field diameter multiplied by the magnification. The physical structure of some telescopes caused a secondary scotoma. In the binocular field there was usually only a small scotoma, where the physiological scotoma of the other eye overlapped the ring scotoma. However, with the Ocutech VHS a significant binocular scotoma remained. The magnified image may be shifted relative to the unmagnified view seen outside the telescope. With sufficiently large shift the magnified and non-magnified view of an object could be seen simultaneously, as previously reported with a micro-Galilean telescope (BITA). We found a similar effect for the Behind-the-Lens telescope, where the magnified field of view was also separated from the scotoma. A user's central scotoma through the telescope is smaller (1/magnification). That scotoma plus the telescopic scotomata may overlap a user's central scotoma(ta) in the other eye to create a binocular scotoma . Visual field interactions should be considered carefully in the design of bioptic telescopes as well as in individual fitting.
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