This article outlines the practical applications of telescopes and how these devices should be integrated into low vision practice for the benefit of our patients.
Telescopes have long since been an integral part of low vision practice and yet over the past 15–20 years prescribing rates have dropped significantly. Academic literature on visual performance when using spectacle lens magnifiers or telescopic magnification is scarce with little published data on prescribing rates and success either with specific pathological conditions or with age groups. However, anecdotally, practitioners still recognise the benefit to patients from the use of both distance and near telescopes and, therefore, these devices should still be incorporated into any low vision service.
A significant factor contributing to the decline in prescribing rates has undoubtedly been that of practitioner confidence. Lack of experience and product knowledge together with practitioner isolation has resulted in only a core number of optometrists and dispensing opticians routinely offering telescopes as part of their service.
This article aims to review the basic optical principles of telescopic devices and provide an overview of currently available designs. The majority of these products do not require complex dispensing, therefore providing the practitioner with the core knowledge to confidently offer the benefits of telescopic magnification to visually impaired patients.
Why should we consider a telescope?
There is an upward trend for patients to use electronic devices or smart phone applications offering distance or near magnification. There is also a growing number of GPS-led navigation applications that offer assistance to visually impaired users when traveling on foot or using public transport. However, telescopes and binoculars are still the primary method of magnifying distance and intermediate objects. For near-magnification tasks, spectacle-mounted telescopes offer a hands-free solution at a longer working distance compared to a spectacle lens magnifier (defined as the addition or incorporation of increased plus power in the spectacle plane). For example, in Figure 1, the patient is using a clip-on binocular telescope to read a newspaper at a comfortable working distance of 35cm. In comparison, a 2x (+8D) spectacle magnifier would have a working distance of 12.5cm due to the focal length of the additional plus lens used. Figure 1: The patient is wearing a 2.1x clip-on Galilean telescope offering a hands-free view at 35cm. The unit adds just 23g of additional weight and has an 18 degree wide field of view. The unit can be easily removed from the spectacles when not required. Image courtesy of Associated Optical Ltd.
History of telescopes
In 1608, two Flemish spectacle-makers, Hans Lippershey and Jacob Metius independently of each other applied for patents for the first refracting telescope. News of these novel devices, described as ‘for seeing things far away as if they were nearby’, soon reached Galileo Galilei who then worked out the principles for himself building his own 8x magnification version in 1609. Galileo was subsequently accredited as the first to use a telescope to observe the stars. Johannes Kepler improved the design further building his own device in 1610.
The history of using telescopes as an aid to low vision is scant. Whether intended originally for those with visual impairment or not, telescopes have been used to magnify distant objects that have been difficult to see, therefore, their use as a low vision aid has developed naturally over time.
Basic principles of telescope designs
All telescopic low vision aids are based upon two principal designs of distance telescopes: the Galilean, and the Keplerian telescope. The original telescopes were afocal, being designed to study the stars from which incident light reaching the telescope would have zero vergence.
In reality, when a telescope is used as a low vision aid, the majority of objects that a visually impaired person needs to be able to view will not be at infinity. Commonly reported tasks include, for example, viewing train indicator boards, watching the television, or reading music placed on a music stand. At distances closer than infinity, incident vergence entering the telescopic system is no longer zero and must now be corrected to focus the image upon the retina. Secondly, eyes tend not to be emmetropic and, therefore, for the most part the basic afocal telescope will require modification. Any modification will ultimately have an effect on the telescope design and resultant magnification.
In basic terms, a telescope is constructed by an objective lens (Fo) at the far end of the tube (also known as the entrance pupil) and the eyepiece lens, which is closest to the user’s eye. To achieve magnification (M), the eyepiece lens (Fe) must always be the stronger in power: M = -Fe/Fo
In a Galilean system, the eyepiece is a concave lens and the objective lens is convex. The resultant magnification of the telescope is positive, indicating that the image is erect. When viewing an object at infinity, the length of the telescope (t) is calculated as the sum of the focal lengths of both the objective and eyepiece lenses of both the objective (fo’) and eyepiece (fe’) lenses.
Galilean telescope worked example:
Fe = -40D
Fo = +20D
M = - (-40)/20
M = 2x
Telescope length (t):
t = fe’ + fo’
t = (-25) + (+50)
t = 25mm
In a Keplerian system, the eyepiece (Fe) is a convex lens and thus positive in power. The image viewed is now inverted and laterally reversed; this also indicated by the negative sign on the magnification calculation as below.
Keplerian telescope worked example:
Fe = +40D
Fo = +20D
M = - 40/20
M = -2x
t = fe’ + fo’
t = (+25) + (+50)
t = 75mm
The Kepler design will be longer in length compared to the Galilean telescope of equivalent magnification. From the above equation it also follows that the higher the power of the lenses incorporated within the unit, the shorter the focal lengths and thus the shorter the tube length. Consequently, compound lens systems (consisting of two or more lenses) are often used to increase overall power of the eyepiece and objective ‘lenses’ and thus create a more compact unit. However, the combination of more than one lens in a compound lens system has the disadvantage of increasing the number of lens surfaces. Coatings that are then added to compensate for the consequence of increased reflections will ultimately push up the cost of the device.
Prism erecting systems
In Kepler’s design the image of the stars was inverted and laterally reversed but it was not until Ignazio Porro in 1854, over 200 years since Galileo and Kepler’s designs, that a prism erecting system was developed to place the image the ‘right way up.’ In the 1890s the Carl Zeiss Foundation, with the help of Ernst Abbe and Mortiz Hensoldt refined Porro’s prism erecting system further developing new designs with so-called roof prisms and incorporating them into their own optical devices.
Incorporating a prism within the telescope adds weight and depending upon the type of prism or prism system used will impact upon the physical design of the telescope and image quality. The incorporation of a double Porro prism system will result in a bulkier unit compared to a telescope containing a roof prism system. This is because a Porro prism system alters the line of sight with the added effect of reducing the overall length of the tube (see Figure 2: Two Porro prisms are placed at right angles to each other within the barrel of the telescope to produce an image that is erect and laterally corrected. The light path is 'folded' resulting in a shorter but bulkier unit). Incorporation of a roof prism system, of which there are several designs, creates a straight optical light path and thus a slimmer, longer unit (see Figure 3: A roof prism system is placed within the barrel of the Keplerian telescope, which produces a straight optical light path). The Porro prism system benefits from total internal reflection across its surfaces, whereas some roof prims require coated surfaces to improve reflectivity with the result of increasing overall cost.
Entrance and exit pupils
Light enters the telescope through the objective lens (entrance pupil). The telescope concentrates the light gathered by the objective into a beam, which leaves the telescope through the exit pupil – a point that does not correspond with the eyepiece lens. In the Galilean system the exit pupil appears within, or internal to the telescope system and in the Keplerian telescope the exit pupil is external and virtual to the system, that is to say, it appears between the eye and the eyepiece (see Figure 4: A) The smaller exit pupil of the Keplerian telescope is closer to the user's own pupil and is external to the barrel of the device; B) The Galilean exit pupil is larger and further away from the eye and is contained within the barrel of the device. The additional auxiliary lens may be glazed with the patient's distance correction and added to the eyepiece).
Telescopes and binoculars are described in terms of their magnification and objective lens diameter. For example, an 8x20 unit has a magnification of 8x and an objective lens diameter of 20mm. The field of view of a device marginally increases with the diameter of the objective and larger lenses will collect more light resulting in a brighter, better quality image. Figure 5 shows three 8x telescopes used as low vision devices each with increasing diameter of objective lenses. In producing the same level of magnification in each device the units that contain larger diameter lenses often use lower powered lens powers, which will result in larger and longer telescope units.
From a practical point of view, the patient will achieve the best field of view and the brightest image through a telescope when the exit pupil is the same size as the patient’s own ‘entrance’ pupil. If the telescope exit pupil is larger than the patient’s pupil then the excess light is wasted. Patients are also encouraged to hold the telescope as close to the eye as possible; those who have small pupils may need assistance to locate the telescope’s exit pupil, especially when using a Keplerian device as the exit pupil is so much smaller than that found in a Galilean design. A rubberised eyepiece cup, present on the end of a Keplerian telescope, assists patients in locating the exit pupil as it allows the patient to rest the telescope within the orbit and closer to the eye. For those who continue to wear a spectacle correction while using the telescope, the eyepiece cup may then be folded back, allowing a protective, rubberised end to come into contact with the spectacle lenses and thus bring the exit pupil closer to the patient’s eye (see Figure 6: A) The rubberised eyecup of the telescope may be folded back allowing the telescope to be placed on the anterior lens surface; B) By increasing the telescope length the patient can focus this 4x monocular as close as 23cm).
Modifications: using a telescope at a finite distance
Vergence is amplified, or magnified, when it passes through a telescope system. The calculation of vergence amplification is complex.2 However, an approximate working formula is described below:3 Emergent vergence = (magnification)2 x incident vergence. Therefore, if an 8x afocal distance monocular were used to view a train indicator board at a distance of 3m, then the incident vergence from the target entering the telescope would be -0.30D: Emergent vergence = (8)2 x -0.30D = -19.20D
Even at distance of 3m, a patient using an 8x telescope would require a large degree of accommodation to focus the target. To compensate for this amplification effect, a number of modifications can be made to the telescope:
- Increase the separation between the eyepiece and objective lenses: incident vergence from a near object may be corrected as it travels through the telescope by increasing the separation between both the eyepiece and objective lenses (see Figure 6B). Variable focus telescopes and binoculars require the use of both hands to focus the image, whereas the non-adjustable telescope that the patient is using in Figure 7 only requires the use of one hand
- Adding an end cap: vergence may be corrected before it enters the telescope by adding a simple plus lens ‘end cap’ to the objective; this method is used typically for spectacle mounted telescopes such as those seen in Figure 8 where a distance unit is mounted onto the patient’s spectacles and an additional end cap may be added converting the telescope for use at near.
Worked example 1
If a 2.2x distance Galilean spectacle-mounted telescope is used to view an object at 10cm, an end cap of +10D will neutralise the incident vergence entering the telescope. The power of the end-cap will have an effect on the overall magnification of the system. The total magnification will be the product of the magnification provided by both the distance unit and the reading cap:
Magnification of telescope = 2.2x
Magnification of the end cap = (M=F/4) = 10/4 = 2.5x
Total magnification at near = 2.2 x 2.5 = 5.5x
Worked example 2
A patient requires a 5x spectacle-mounted telescope for use at near:
If the Galilean spectacle mounted distance unit has a magnification of 2.5x, then the patient would require 5x/2.5x = 2x magnification in the end cap to achieve this. The power of the end cap will, therefore, need to be +8.00D (M=F/4). The working distance that the patient will need to hold the object will be the focal length of the end cap; in this case 12.5cm.
The main purpose of using a telescope for near is to provide the patient with an extended working distance compared to using a spectacle lens magnifier offering the patient the same level of magnification. In the above example, if a patient needs 5x magnification for near, then the equivalent spectacle lens magnifier power would be a +20D lens (M=F/4) with a focal length working distance of 5cm.
The 5x telescope example above offers over twice the working distance of the spectacle lens magnifier. However at the higher powers of magnification of 8–10x or more, the difference in working distance between the telescope and spectacle lens magnifier becomes negligible.
Correcting for refractive error
When a patient needs their full spectacle correction in place, a telescope may then be screwed into a simple, self-adhesive carrier on the front surface of the spectacle lens as seen in Figure 8 (A distance 2.5x Galilean base unit is screwed into a self-adhesive mount on the front surface of the patient's spectacles. To view an object at a finite near distance the additional of a clip on end-cap corrects for the incident vergence). These adhesive mounts significantly reduce chair time and cost compared to previous carriers that required cementing onto the spectacle lens by a prescription house. Alternatively, some telescope designs may utilise a simple clip so that the telescope may be removed when not required, such as the type shown in Figure 1.
However, with some designs the patient’s spectacle correction may be glazed into a small auxiliary lens unit that may then be clipped directly over the telescope’s eyepiece lens (see Figure 4). This is particularly useful for those patients who have a significant cylindrical error. Adjustable telescopes and binoculars such as the popular ‘TV glasses’ (see Figure 9: Binocular 'TV' glasses offering 2.1x magnification. The unit is only 49g in weight and the adjustable cogs compensate from -3.00 to +3.00D of spherical refractive error. The negative eyepiece lenses identify this unit as a Galilean design. Image courtesy of Associated Optical) are able to compensate for spherical refractive error depending upon the design of the unit. When an ametropic patient uses a device without his or her spectacles there will be a change in the level of magnification achieved:
Galilean telescope worked example:
M = –Fe/Fo = 2x
Tube length = fe’ + fo’ = (–25)+ 50 = 25mm
If a 5D myope were to use the above telescope they would essentially ‘borrow’ 5D from the –40D eyepiece, hypothetically changing it to a –35D eyepiece lens:
M = 1.75x
Tube length: 21.4 mm
If the above Galilean telescope is used again by a 5D hyperope then theoretically the uncorrected hyperopia is ‘added’ to the eyepiece:
M = 2.25x
Tube length: 27.7 mm
Therefore a myope will achieve less magnification from using a Galilean telescope and will shorten the tube length, whereas the hyperope will achieve more magnification and will extend the tube length when using the same telescope. The reverse is true in both cases when using a Keplerian design. However, in practical terms the above change in magnification is relatively small. Also, higher degrees of refractive error cannot be corrected in this way due to the restrictions in altering the tube lengths.
BiOptics have been around for some time and in recent years have made a resurgence within the UK marketplace with several designs available. Figure 10 shows a patient using a BiOptic consisting of a lightweight monocular Keplerian unit mounted across the spectacles rather than projecting outwards. The single eyepiece is placed superior to the visual axis and is angled slightly upwards by approximately 100. The frame is glazed with the patient’s habitual correction for general viewing and when magnification is required, the patient drops his head to look through the telescope. This type of telescope boasts a range of working distances from infinity to 25cm and can be used for intermediate tasks such as reading music and playing bridge. The telescope is comfortable to wear for long periods of time and several users wear their unit while outdoors to improve navigation and mobility. (Figure 10: This 26g adjustable BiOptic unit is mounted superior to the line of sight at 10 degrees. The patient drops his head to view the object when required. The unit compensates for +/-12D of spherical refractive error and up to 3D of cylindrical error).
In some US states and a few non-EU member states BiOptic units have been approved for a ‘restricted’ driving license. At the time of going to print, BiOptics are currently not recognised in the UK for drivers and the reader is directed to the current online statement from the DVLA for any changes to the current legislation following the UK’s decision to exit the EU.4
However, it was not until the 1950s that the provision of telescopes as an aid to those with a visual impairment became a professional service, with Charles Keeler being one of the more notable members of the optical profession to develop this area of clinical practice.1 The original stargazing telescopes built by Galileo and Kepler were long in design due to the use of relatively low powered lenses, amongst other factors. By using higher-powered lenses with significantly shorter focal lengths, telescopes used for low vision are now shorter in length and thus more compact and portable. Optical performance is further improved with the use of high quality lenses and coatings that together produce clearer, brighter images.
Telescopes and binoculars are used by all age groups as mobility aids and as magnification devices offering magnification for the full range of object distances from infinity to near. This makes them the most versatile of low vision aids and an indispensible tool to improve independence and allow patients to perform daily living tasks.
About the author
Jane Macnaughton FCOptom, Prof Cert LV is currently an associate trainer for Associated Optical Ltd, and a postgraduate student at Anglia Ruskin University. She has lectured widely in the practice of low vision from undergraduate to post graduate level and is the author of Eye Essentials: Low Vision Assessment (2005).
- Keeler C (1956) Visual aids for the pathological eye; excluding contact lenses. Transactions Ophthalmological Society of the United Kingdom, 76, pp.605–614
- Fried AN (1977) Telescopes, Light Vergence and Accommodation. American Journal of Optometry and Physiological Optics, 54(6), pp.365–373
- Bailey IL (1978) New Method for Determining the Magnifying Power of Telescopes. American Journal of Optometry and Physiological Optics, 55(3), pp.203–207
- Driver and Vehicle Licensing Agency, 2011. BiOptics: current GB driving standards. Available at: https://www.gov.uk/government/publications/bioptics-current-gb-driving-standards.