Telescopic systems are the only optical devices that magnify the image of distant, intermediate, and near objects. There are two types of telescopic systems used for low vision: Galilean and Keplerian. The Galilean telescope has a plus objective and a minus ocular lens, and produces a virtual, erect image. The Keplerian telescope has a plus objective and a plus ocular lens that produces a real, inverted image. Internal prisms to produce an upright image are incorporated in a Keplerian telescope, which can have either straight-ahead or lateral optics.
Telescopes are either hand-held or spectacle-mounted. They can be for monocular or binocular use. If they are spectacle mounted, telescopes can be placed in various positions in a carrier lens or are available with the optical system as part of the frame. Most telescopes are focusable. The autofocus telescope changes focus automatically as the user changes the direction of gaze from near to intermediate to distant objects.
More so than with other types of optical devices, a variety of factors is important in selecting and prescribing telescopes, and providing effective instruction in their use. Considerations include: diagnosis, patient objectives, optical design, power, refractive correction, weight, light-gathering, field of view, exit pupil, and cosmesis. Task requirements suggest whether a distance, intermediate, near, or reading telescope is prescribed, whether a hand-held or spectacle mounted telescope is indicated, and whether a fixed or variable focus is preferable.