SURVEY OF OPHTHALMOLOGY VOLUME
PERSPECTIVES
36
l
NUMBER 2 * SEPTEMBER-OCTOBER
1991
IN REFRACTION
MELVIN L. RUBIN, EDITOR
Designing Half-eye Binocular Spectacle Magnifiers GERALD
E. FONDA,
M.D.
Saint Barnabas Low Vision Center, West Orange, New Jersey
Abstract. The process of designing half-eye frames with a smaller than usual eye size and narrower distance between lenses (DBL) in order to produce additional base-in prism effect is described. This method enhances binocular function while the spectacle magnifier is used. The design also provides a lighter, thinner, and cosmetically more acceptable optical aid. Binocular half-eye spectacle magnifiers are useful optical aids, especially for patients with binocular vision, because they are produced in strong powers with strong prism effects incorporated in the lenses. The magnifiers range in power from +6.00 D C 8’ base-in to + 14.00 D S lSA base-in for vision ranging from 20/60 (6/l@ to 20/320 (6/96). Binocular half-eye spectacle magnifiers are recommended for prolonged reading because the strong base-in prism effect relieves the convergence effort, also providing a large field of vision with both hands free. (Surv Ophthalmol 36:149-154, 1991)
Key words.
half-eye binocular spectacle magnifiers
Binocular spectacle magnifiers are useful optical aids for low vision patients with binocular vision because they are the strongest powers allowing binocular vision. The strong prisms and frames with a narrow interpupillary distance (PD) relieve the convergence effort needed to read at close distances. There is no need to measure the PD, because the narrow PD on the frame produces the greatest base-in effect which supplements the base-in prism. Binocular half-eye spectacle magnifiers have been prescribed for 64% of a series of 120 patients who received binocular magnifiers.’ Without any training in the use of the glasses, 88% of patients were judged to be successful in using them. Based on experience with more than 500 binocular half-eye spectacle magnifiers, it is clinically evident that no cylinder under 2.50 D needs to be incorporated in the lens, as it does not improve the magnified image significantly. Anisometropia as great as 3.00 D has no unfavorable effect, nor does it
l
low vision aids
l
optical aids
make any difference if the vision is much better in one eye. A stereotest, such as the Titmus test, conducted at the focal distance of the binocular magnifier, is useful in establishing the presence of binocular vision. A range of powers from +6.00 D z 8’ base-in to + 14.00 D Z 18A base-in is available for visual acuity from 20/50 (6/15) to 20/320 (6/90). A person with 20/320 will be able to read 12 point (typewriter type). While success has been achieved with powers up to + 18.00 D Z 24A base-in, powers over + 14.00 D Z lSA base-in are not recommended, because the probability of achieving binocular function with powers greater than + 14.00 D is remote; the use of monocular magnifiers is recommended for stronger powers.
Design Calculations Adequate decentration or base-in prism incorporated into reading lenses is imperative to attain bin149
150
Surv Ophthalmol
36 (2) September-October
ocular vision. It does not seem possible to create too much base-in prism, unless the patient manifests high esophoria. There is a direct relationship between convergence and accommodation. Every meter angle (MA) of convergence is associated with one diopter of accommodation or reading addition. Reading at a distance of 25 cm requires 4 MAs of
1991
FONDA convergence and 4.00 D of accommodation, or a + 4.00 D reading addition for clear vision; reading at a distance of 10 cm requires 10 MAs of convergence and 10.00 D of accommodation, or a + 10.00 D reading addition. Figure 1 shows convergence required to read at 1 meter, 25 cm and 10 cm. Meter angle is defined as the angle formed at the distance of one meter by the median line and fixation axis. The magnitude of the MA varies with the interpupillary distance (PD) and may be converted to prism diopters by multiplying the number of MAS by one-half of the PD in centimeters. For example, MAs for PD of 60 mm = 3’; MAs for PD of 65 mm = 3.25’; MAs for PD of 70 mm = 3.5’. At a reading distance of 10 cm, an individual with a PD of 65 mm would require 32.50’ of convergence for each eye, and a PD of 70 mm, 35’ of convergence for each eye.
Conversion of Decentration Prism Diopters
to
Decentration is the distance between the optical center of a lens and the point where the fixation axis passes. Prism power is equal to decentration expressed in centimeters times lens power (Prentice’s Law). P = prism d = decentration 1 = lens power P = d(cm) x 1
10
nmer angles (309
Fig. 1.
Convergence
and 10 cm.
required to read at 1 meter, 25 cm
Fig. 2.
Method of calculating tance (PD).
near inter-pupillary
dis-
HALF-EYE BINOCULAR SPECTACLE MAGNIFIERS TABLE
151
1
Power, Focal Distance and Convergence of Half-eye Binocular Spectacle Magnifiers
Convergence Needed Wearing Focal Distance
Power + + + + + +
6.00 8.00 10.00 12,00 14.00 16.00
S Z ^, S Z S
8* lOA 12* 14” 18A 22*
BI BI BI BI BI BI
16.6 12.5 10 8.3 7 6.2
cm cm cm cm cm cm
(5.5”) (5”) (4”) (3.3”) (2.9”) (2.5”)
60 mm
53 mm
PD Frame
PD Frame
19; ix* 43* 49* 52*
29” 36* 39* 41*
Fig. 2 shows method of calculating near interpupillary distance (PD) at spectacle plane. = distance between centers of rotation GG (equivalent to distant PD) = distance between visual axis at spectaV,V, cle plane (equivalent to near PD) D = distance of object from spectacle plane D + 25 = distance of object from centers of rotation A = fixation point for near oc = optical center of lens d = decentration (distance between optical center and visual axis for near) C,C, -=V,V,
D+25 D
60 -=V,V,
125 loo
6000 V,V, = -= 125
48
This is an accurate method, but is awkward for rapid calculations. Lebensohn’s rule for determining the difference between distant and near PD is a simple and accurate method.3 Lebensohn’s rule states that the difference between distant and near PD is determined by dividing the distant PD in millimeters by the reading distance in inches, plus one. For example: distant PD, 60 mm; reading distance, 4 inches, plus one: 60/5 = 12. Therefore the near PD is 48 mm. Fig. 3 shows that an emmetrope wearing a + 12.00 D spectacle magnifier at spectacle plane, 25 mm from the center of rotation,* is converging at a distance of 108 mm (the focal distance of a + 9.26 D lens). 1. This will require convergence at 9.26 MA (27.78*) for each eye (55.56* for both), assuming the patient’s PD is 60 mm and the PD on the frame is 60 mm. *The center of rotation is 13 mm behind the surface of the cornea. The vertex of the lenses is 12 mm.
Fig. 3. Convergence D spectacle magnifier
needed to read through at the spectacle plane.
a + 12.00
2. Induced base-out effect while reading produced by a + 12.00 D spectacle magnifier with a near PD of 46 mm is 8.4* for each eye (16.8* for both). 3. Total convergence needed is 55.56’ + 16.8” = 72.36”. 4. Total convergence needed while wearing O.U. + 12.00 D Z 14* base-in is 72.36* - 28* = 44.36*. 5. Total convergence needed while wearing a 53 mm PD frame is 44.36” - 8.4” = 35.96*, demonstrating the advantage of a frame with a narrow PD. A frame with a 53 PD worn by a patient with a 60 PD produces an additional 8.4* base-in (+ 12.00 D x .7). Since the meter angle of convergence is measured from the center of rotation and the dioptric power (focal length of the magnifier) is measured from the spectacle plane, less convergence for the corresponding dioptric power of the spectacle magnifier is needed than would be required if the dioptric power were produced by accommodation or high myopia. The amount of accommodation normally required for reading at a 10 cm distance is 10.00 D, and the amount of convergence is 10 MA because one diopter of accommodation is equal to one MA of convergence.* *While the diopter and prism powers are calculated to the nearest hundreth, this does not imply that the diopter or prism effect corresponds to this precision. The clinical prism power varies from the calculated power because the person’s fixation axis is not perpendicular to the lens surface.
152
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36 (2) September-October
1991
The purpose of all spectacle magnifiers is to bring the object of regard closer to the eye, thereby producing a larger retinal image. If the object of regard cannot be brought closer by a myopic eye, accommodation, or a combination of both, a strong plus lens is needed. It is not the strong plus lens per se that produces the magnification, but rather the close distance to the eye. Table 1 summarizes the power of half-eye spectacle magnifiers, focal distance and convergence needed, using a 60 PD frame and a 53 PD frame. The use of a 53 PD frame reduces convergence by about lOA,and allows the use of thinner and lighter lenses. This is the reason for using 53 mm PD frames with 36 eye size for + 10.00 D and stronger spectacle magnifiers. Patients wearing this range of spectacle magnifiers experience comfortable binocular vision. The narrower the PD on the frame, the greater is the amount of prism base-in produced on a convex lens. Prismatic power is produced by decentering the lens in from the patient’s fixation axis; the greater the distance of the point where the fixation axis passes through the lens from the optical center of the lens, the greater is the prism produced. For example, if the patient’s PD is 60 mm and the PD on the frame is 53 mm with + 10.00 D lens, the prism base-in created is 7’ (.7 x 10). Calculations for convergence needed for reading through a + 12.00 D spectacle tignifier at spectacle plane for a patient with 70 mm PD, in order to show the maximum increase in convergence caused by a wide PD.
The difference between distant and near PD is (70 mm + 4.3”) = 16 mm; therefore, the near PD is 54 mm. Consequently, decentration is 8 mm for each lens. Convergence needed to read at 108 mm from the center of rotation = 9.26 MA x 3.5 = 32.41” for each eye, 64.82A for both eyes. The induced base-out effect while reading produced by a + 12.00 D spectacle magnifier for a near PD of 54 mm is .8 x 12 = 9.6” for each eye, 19.2A for both. The total convergence needed is 64.82A x 19.2A
TABLE
FONDA = 84.02A. The total convergence needed for a patient with a 70 mm PD who is wearing a frame with a 60 mm PD containing a correction of O.U. + 12.00 D S 14A base-in is 84.02’ - 28” = 56.d2A. This compares to a prismatic effect of 44.36A in the case of a patient with a PD of 60 mm. This indicates that an additional 11 .66A of convergence is needed for a patient with a 70 mm PD. Total convergence with a 70 mm PD wearing a frame with a 53 mm PD is 56.02A - 8.4” = 47.62A, compared to 35.96A for a patient with a 60 mm PD. This also demonstrates that it is advisable to use a frame with a 53 mm PD for strong binocular spectacle magnifiers.
Lens Material The lens material used for the lenses is crown glass (1.523) and CR-39 (1.498). The optical quality is abodt the same for CR-39 and crown glass. While lenses made of plastic are lighter, they have the disadvantages of being less scratch-resistant and thicker. The cost of plastic is comparable to glass. Lenses consisting of + 12.00 D Z 14” base-in and the + 16.00 D Z 20A base-in have been made of high-index glass (1.806), which reduces the thickness by 20%, which is cosmetically impressive. However, severe chromatic and spherical aberrations of high-index glass create color fringes and blur print. In addition, high-index glass is more costly. Because of the disadvantages of high-indexof-refraction glass, this material in lenses is not recommended. Crown glass (1.523) and CR-39 (1.498) are preferred materials because they are relatively free of optical aberrations and are modest in cost.
Examples Table 2 summarizes the range of powers of available binocular spectacle magnifiers with corresponding visual indications. The visual indication depends on the size and contrast of the type which the patient needs to read,
2
Binocular SpectacleMagnifiers: Range of Powersand indications Range of Powers (O.U.) + 6.00 + 8.00 + 10.00 +12.00 +14.00
Z Z z 2 s
8’ base-in 1OAbase-in 12’ base-in 14” base-in 1aA base-in
Visual Indication 20160 2OllOO 20/125 201160 201240
(6/18) (6/30) (6/38) (6/48) (6172)
-
20/100 201125 20/160 201240 201320
(6/30) (6138) (6/48) (6172) (6196)
153
HALF-EYEBINOCUIARSPECTACLEMAGNIFIERS TABLE
3
Indications, Advantages, and Disadvantages Half-eye Binocular Spectacle Magnifiers 1. Patient 2. Patient
of
Indications with binocular vision. who needs no significant
distant correction. 3. Patient who needs strongest binocular magnifier. 4. Patient who needs to read for prolonged periods.
Frame with 60 Fig. 4. ?bp:+8.00 D 2 10” base-in. mm PD; 22 mm DBL, 38 mm eye diameter. Bottom: + 12.00 D 2 14A base-in. Frame with 53 mm PD; 17 mm DBL, 36 mm eye diameter.
as well as the available illumination. Kesenbaum’s method (reciprocal of vision)’ is based upon the Snellen notation for distant vision. The power of the denominator divided by the numerator of the Snellen fraction gives the dioptric power to read Jaeger 5. For example, & = 5 diopters for reading Jaeger 5. My indication of +6.00 D for 20/100 and + 14.00 D for 20/320 compares closely to the Kestenbaum method. Fig. 4 shows the 60 mm frame with 22 mm DBL and 38 mm eye diameter; and 53 mm frame with 17 mm DBL and 36 mm eye diameter. The 60 mm frame is used for powers +6.00 D S 8* base-in and +8.00 D Z lOA base-in. The 53 mm PD is used for powers + 10.00 D Z 12’ base in to +14.00 D Z I@ base-in. Reducing the PD on the frame produces additional base-in effect by decentration, that is, the distances between the optical center of the lens and fixation axis is increased. For example, reducing the PD on the frame from 60 mm PD to 53 mm PD produces (.7 x 10) 7*base-in on a + 10.00 D lens. Reducing the diameter on the lenses makes them thinner and lighter. Most binocular half-eye magnifiers are made with plastic lenses. Usually the frames are 66 mm PD, 24 DBL, 42 mm to 44 mm eye diameter. Glass lenses 36 mm in diameter are about 20% thinner than plastic lenses 44 mm in diameter. Reducing the PD (1.3 cm on a frame from 66 mm to 53 mm produces x 12) 15.60 base-in on a -t 12.00 D lens; this also makes the lenses thinner and lighter, thereby less conspicuous. Reading binocularly at a close distance demands great convergence effort. The closer the reading distance, the greater is the difference between the distant and the near PD. Therefore, greater decen-
Advantages 1. Produce the maximum practical base-in prism, thereby relieving the greatest convergence effort. practical binocular magni2. Produce the strongest fication: + 14.00 D C 18A base-in (3.5X). lighter and less conspicuous 3. Lenses are thinner, than full-size ones. 4. Cosmetically acceptable for adults. Patients can easily look over the top of the lenses. ;: Instantly available for testing and demonstration at the time of examination. 7. Available for loan or purchase. 8. Inexpensive. Disadvantages 1. Close reading distance. for children. 2. Conspicuous
tration or prism base-in is required. Many people, such as a myope of - 10.00 D and children who can accommodate 10.00 D, can read for 30 minutes and longer exerting 60” of convergence.
Discussion Indications, advantages, and disadvantages of half-eye spectacle magnifiers are summarized in Table 3. Binocular spectacle magnifiers are one of the most useful optical aids for low vision patients with binocular vision because they are produced in pow(3.5X) and with ers up to + 14.00 D S 18’base-in narrow PD frames of 53 mm which relieve the greatest amount of convergence. There is no need to measure the patient’s PD because it is advantageous to use as narrow a PD on a frame as possible since this only produces a base-in prism by decentration. The range of powers available is + 6.00 D S aA + 10.00 +8.00 D C 10’ base-in, base-in, 12* base-in, + 12.00 D ^, 14* base-in and DZ + 14.00 D Z 18* base-in for visual acuities from 20/60 (6/l@ to 20/320 (6/96). The +6.00 D S 8’ base-in and + 8.00 D C 1OA base-in are available in 60 mm PD frames. Stronger powers are available in 53 mm PD frames: Weaker half-eye spectacle magnifiers are sometimes indicated for people with vision better than
154
Surv Ophthalmol
36 (2) September-October
20/60 (6/l@, or for doing fine work, such as needlepoint, with normal vision. Rather than prescribe prisms base-in, the laboratory should be instructed, in case the power is + 4.00 D, to decenter the lenses in 10 mm each, which will produce 4* base-in, a total of 8” for both eyes. This will reduce convergence from 24* to 16*. Decentering a lens should be much less expensive than grinding prisms. The closer the reading distance, the greater is the difference between the distant and the near PD; therefore, greater decentration or prism base-in is required. Many people can read for hours using available accommodation, or read at the far point of their myopic eye, or use a combination of myopia and accommodation exerting 60* of convergence. To date, no data has been presented concerning the production of prismatic effect involved in the design of binocular half-eye spectacle magnifiers. In this article, I emphasize that a small eye diameter makes possible the positioning of the plus lens’s optical center extremely nasally in the frame resulting in maximal base-in prismatic effect. It is also
1991
FONDA emphasized that a narrow bridge results in a nasal shift in the positioning of the lens of the frame. Both of these features decrease the distance between the optical center of the lens and the point on the lens through which the fixation axis passes. The resulting augmented base-in effect makes possible maximum binocular function while using spectacle magnifiers of powers up to + 14.00 D C 18* base-in.
References 1. Fonda G: Binocular
reading additions for low vision. Arch 1970 2. Kestenbaum A, Sturman RM: Reading glasses for patients with very poor vision. Arch Ophthalmd 56:451-470, 1956 3. Lebensohn JE: Practical problems pertaining to presbyopia. Am J Ophthalmol32:22-30, 1949
O~hthalmol83:294299,
Frames are available from May, USA, Box 760, Marion Road, Wareham, MA, 02571. Neither the author nor his family has a proprietary interest in the development or marketing of these frames. Reprint address: Gerald E. Fonda, M.D., 101 Old Short Hills Road, West Orange, New Jersey, 07052.
PERSPECTIVES
36
l
NUMBER 2 * SEPTEMBER-OCTOBER
1991
IN REFRACTION
MELVIN L. RUBIN, EDITOR
Designing Half-eye Binocular Spectacle Magnifiers GERALD
E. FONDA,
M.D.
Saint Barnabas Low Vision Center, West Orange, New Jersey
Abstract. The process of designing half-eye frames with a smaller than usual eye size and narrower distance between lenses (DBL) in order to produce additional base-in prism effect is described. This method enhances binocular function while the spectacle magnifier is used. The design also provides a lighter, thinner, and cosmetically more acceptable optical aid. Binocular half-eye spectacle magnifiers are useful optical aids, especially for patients with binocular vision, because they are produced in strong powers with strong prism effects incorporated in the lenses. The magnifiers range in power from +6.00 D C 8’ base-in to + 14.00 D S lSA base-in for vision ranging from 20/60 (6/l@ to 20/320 (6/96). Binocular half-eye spectacle magnifiers are recommended for prolonged reading because the strong base-in prism effect relieves the convergence effort, also providing a large field of vision with both hands free. (Surv Ophthalmol 36:149-154, 1991)
Key words.
half-eye binocular spectacle magnifiers
Binocular spectacle magnifiers are useful optical aids for low vision patients with binocular vision because they are the strongest powers allowing binocular vision. The strong prisms and frames with a narrow interpupillary distance (PD) relieve the convergence effort needed to read at close distances. There is no need to measure the PD, because the narrow PD on the frame produces the greatest base-in effect which supplements the base-in prism. Binocular half-eye spectacle magnifiers have been prescribed for 64% of a series of 120 patients who received binocular magnifiers.’ Without any training in the use of the glasses, 88% of patients were judged to be successful in using them. Based on experience with more than 500 binocular half-eye spectacle magnifiers, it is clinically evident that no cylinder under 2.50 D needs to be incorporated in the lens, as it does not improve the magnified image significantly. Anisometropia as great as 3.00 D has no unfavorable effect, nor does it
l
low vision aids
l
optical aids
make any difference if the vision is much better in one eye. A stereotest, such as the Titmus test, conducted at the focal distance of the binocular magnifier, is useful in establishing the presence of binocular vision. A range of powers from +6.00 D z 8’ base-in to + 14.00 D Z 18A base-in is available for visual acuity from 20/50 (6/15) to 20/320 (6/90). A person with 20/320 will be able to read 12 point (typewriter type). While success has been achieved with powers up to + 18.00 D Z 24A base-in, powers over + 14.00 D Z lSA base-in are not recommended, because the probability of achieving binocular function with powers greater than + 14.00 D is remote; the use of monocular magnifiers is recommended for stronger powers.
Design Calculations Adequate decentration or base-in prism incorporated into reading lenses is imperative to attain bin149
150
Surv Ophthalmol
36 (2) September-October
ocular vision. It does not seem possible to create too much base-in prism, unless the patient manifests high esophoria. There is a direct relationship between convergence and accommodation. Every meter angle (MA) of convergence is associated with one diopter of accommodation or reading addition. Reading at a distance of 25 cm requires 4 MAs of
1991
FONDA convergence and 4.00 D of accommodation, or a + 4.00 D reading addition for clear vision; reading at a distance of 10 cm requires 10 MAs of convergence and 10.00 D of accommodation, or a + 10.00 D reading addition. Figure 1 shows convergence required to read at 1 meter, 25 cm and 10 cm. Meter angle is defined as the angle formed at the distance of one meter by the median line and fixation axis. The magnitude of the MA varies with the interpupillary distance (PD) and may be converted to prism diopters by multiplying the number of MAS by one-half of the PD in centimeters. For example, MAs for PD of 60 mm = 3’; MAs for PD of 65 mm = 3.25’; MAs for PD of 70 mm = 3.5’. At a reading distance of 10 cm, an individual with a PD of 65 mm would require 32.50’ of convergence for each eye, and a PD of 70 mm, 35’ of convergence for each eye.
Conversion of Decentration Prism Diopters
to
Decentration is the distance between the optical center of a lens and the point where the fixation axis passes. Prism power is equal to decentration expressed in centimeters times lens power (Prentice’s Law). P = prism d = decentration 1 = lens power P = d(cm) x 1
10
nmer angles (309
Fig. 1.
Convergence
and 10 cm.
required to read at 1 meter, 25 cm
Fig. 2.
Method of calculating tance (PD).
near inter-pupillary
dis-
HALF-EYE BINOCULAR SPECTACLE MAGNIFIERS TABLE
151
1
Power, Focal Distance and Convergence of Half-eye Binocular Spectacle Magnifiers
Convergence Needed Wearing Focal Distance
Power + + + + + +
6.00 8.00 10.00 12,00 14.00 16.00
S Z ^, S Z S
8* lOA 12* 14” 18A 22*
BI BI BI BI BI BI
16.6 12.5 10 8.3 7 6.2
cm cm cm cm cm cm
(5.5”) (5”) (4”) (3.3”) (2.9”) (2.5”)
60 mm
53 mm
PD Frame
PD Frame
19; ix* 43* 49* 52*
29” 36* 39* 41*
Fig. 2 shows method of calculating near interpupillary distance (PD) at spectacle plane. = distance between centers of rotation GG (equivalent to distant PD) = distance between visual axis at spectaV,V, cle plane (equivalent to near PD) D = distance of object from spectacle plane D + 25 = distance of object from centers of rotation A = fixation point for near oc = optical center of lens d = decentration (distance between optical center and visual axis for near) C,C, -=V,V,
D+25 D
60 -=V,V,
125 loo
6000 V,V, = -= 125
48
This is an accurate method, but is awkward for rapid calculations. Lebensohn’s rule for determining the difference between distant and near PD is a simple and accurate method.3 Lebensohn’s rule states that the difference between distant and near PD is determined by dividing the distant PD in millimeters by the reading distance in inches, plus one. For example: distant PD, 60 mm; reading distance, 4 inches, plus one: 60/5 = 12. Therefore the near PD is 48 mm. Fig. 3 shows that an emmetrope wearing a + 12.00 D spectacle magnifier at spectacle plane, 25 mm from the center of rotation,* is converging at a distance of 108 mm (the focal distance of a + 9.26 D lens). 1. This will require convergence at 9.26 MA (27.78*) for each eye (55.56* for both), assuming the patient’s PD is 60 mm and the PD on the frame is 60 mm. *The center of rotation is 13 mm behind the surface of the cornea. The vertex of the lenses is 12 mm.
Fig. 3. Convergence D spectacle magnifier
needed to read through at the spectacle plane.
a + 12.00
2. Induced base-out effect while reading produced by a + 12.00 D spectacle magnifier with a near PD of 46 mm is 8.4* for each eye (16.8* for both). 3. Total convergence needed is 55.56’ + 16.8” = 72.36”. 4. Total convergence needed while wearing O.U. + 12.00 D Z 14* base-in is 72.36* - 28* = 44.36*. 5. Total convergence needed while wearing a 53 mm PD frame is 44.36” - 8.4” = 35.96*, demonstrating the advantage of a frame with a narrow PD. A frame with a 53 PD worn by a patient with a 60 PD produces an additional 8.4* base-in (+ 12.00 D x .7). Since the meter angle of convergence is measured from the center of rotation and the dioptric power (focal length of the magnifier) is measured from the spectacle plane, less convergence for the corresponding dioptric power of the spectacle magnifier is needed than would be required if the dioptric power were produced by accommodation or high myopia. The amount of accommodation normally required for reading at a 10 cm distance is 10.00 D, and the amount of convergence is 10 MA because one diopter of accommodation is equal to one MA of convergence.* *While the diopter and prism powers are calculated to the nearest hundreth, this does not imply that the diopter or prism effect corresponds to this precision. The clinical prism power varies from the calculated power because the person’s fixation axis is not perpendicular to the lens surface.
152
Surv Ophthalmol
36 (2) September-October
1991
The purpose of all spectacle magnifiers is to bring the object of regard closer to the eye, thereby producing a larger retinal image. If the object of regard cannot be brought closer by a myopic eye, accommodation, or a combination of both, a strong plus lens is needed. It is not the strong plus lens per se that produces the magnification, but rather the close distance to the eye. Table 1 summarizes the power of half-eye spectacle magnifiers, focal distance and convergence needed, using a 60 PD frame and a 53 PD frame. The use of a 53 PD frame reduces convergence by about lOA,and allows the use of thinner and lighter lenses. This is the reason for using 53 mm PD frames with 36 eye size for + 10.00 D and stronger spectacle magnifiers. Patients wearing this range of spectacle magnifiers experience comfortable binocular vision. The narrower the PD on the frame, the greater is the amount of prism base-in produced on a convex lens. Prismatic power is produced by decentering the lens in from the patient’s fixation axis; the greater the distance of the point where the fixation axis passes through the lens from the optical center of the lens, the greater is the prism produced. For example, if the patient’s PD is 60 mm and the PD on the frame is 53 mm with + 10.00 D lens, the prism base-in created is 7’ (.7 x 10). Calculations for convergence needed for reading through a + 12.00 D spectacle tignifier at spectacle plane for a patient with 70 mm PD, in order to show the maximum increase in convergence caused by a wide PD.
The difference between distant and near PD is (70 mm + 4.3”) = 16 mm; therefore, the near PD is 54 mm. Consequently, decentration is 8 mm for each lens. Convergence needed to read at 108 mm from the center of rotation = 9.26 MA x 3.5 = 32.41” for each eye, 64.82A for both eyes. The induced base-out effect while reading produced by a + 12.00 D spectacle magnifier for a near PD of 54 mm is .8 x 12 = 9.6” for each eye, 19.2A for both. The total convergence needed is 64.82A x 19.2A
TABLE
FONDA = 84.02A. The total convergence needed for a patient with a 70 mm PD who is wearing a frame with a 60 mm PD containing a correction of O.U. + 12.00 D S 14A base-in is 84.02’ - 28” = 56.d2A. This compares to a prismatic effect of 44.36A in the case of a patient with a PD of 60 mm. This indicates that an additional 11 .66A of convergence is needed for a patient with a 70 mm PD. Total convergence with a 70 mm PD wearing a frame with a 53 mm PD is 56.02A - 8.4” = 47.62A, compared to 35.96A for a patient with a 60 mm PD. This also demonstrates that it is advisable to use a frame with a 53 mm PD for strong binocular spectacle magnifiers.
Lens Material The lens material used for the lenses is crown glass (1.523) and CR-39 (1.498). The optical quality is abodt the same for CR-39 and crown glass. While lenses made of plastic are lighter, they have the disadvantages of being less scratch-resistant and thicker. The cost of plastic is comparable to glass. Lenses consisting of + 12.00 D Z 14” base-in and the + 16.00 D Z 20A base-in have been made of high-index glass (1.806), which reduces the thickness by 20%, which is cosmetically impressive. However, severe chromatic and spherical aberrations of high-index glass create color fringes and blur print. In addition, high-index glass is more costly. Because of the disadvantages of high-indexof-refraction glass, this material in lenses is not recommended. Crown glass (1.523) and CR-39 (1.498) are preferred materials because they are relatively free of optical aberrations and are modest in cost.
Examples Table 2 summarizes the range of powers of available binocular spectacle magnifiers with corresponding visual indications. The visual indication depends on the size and contrast of the type which the patient needs to read,
2
Binocular SpectacleMagnifiers: Range of Powersand indications Range of Powers (O.U.) + 6.00 + 8.00 + 10.00 +12.00 +14.00
Z Z z 2 s
8’ base-in 1OAbase-in 12’ base-in 14” base-in 1aA base-in
Visual Indication 20160 2OllOO 20/125 201160 201240
(6/18) (6/30) (6/38) (6/48) (6172)
-
20/100 201125 20/160 201240 201320
(6/30) (6138) (6/48) (6172) (6196)
153
HALF-EYEBINOCUIARSPECTACLEMAGNIFIERS TABLE
3
Indications, Advantages, and Disadvantages Half-eye Binocular Spectacle Magnifiers 1. Patient 2. Patient
of
Indications with binocular vision. who needs no significant
distant correction. 3. Patient who needs strongest binocular magnifier. 4. Patient who needs to read for prolonged periods.
Frame with 60 Fig. 4. ?bp:+8.00 D 2 10” base-in. mm PD; 22 mm DBL, 38 mm eye diameter. Bottom: + 12.00 D 2 14A base-in. Frame with 53 mm PD; 17 mm DBL, 36 mm eye diameter.
as well as the available illumination. Kesenbaum’s method (reciprocal of vision)’ is based upon the Snellen notation for distant vision. The power of the denominator divided by the numerator of the Snellen fraction gives the dioptric power to read Jaeger 5. For example, & = 5 diopters for reading Jaeger 5. My indication of +6.00 D for 20/100 and + 14.00 D for 20/320 compares closely to the Kestenbaum method. Fig. 4 shows the 60 mm frame with 22 mm DBL and 38 mm eye diameter; and 53 mm frame with 17 mm DBL and 36 mm eye diameter. The 60 mm frame is used for powers +6.00 D S 8* base-in and +8.00 D Z lOA base-in. The 53 mm PD is used for powers + 10.00 D Z 12’ base in to +14.00 D Z I@ base-in. Reducing the PD on the frame produces additional base-in effect by decentration, that is, the distances between the optical center of the lens and fixation axis is increased. For example, reducing the PD on the frame from 60 mm PD to 53 mm PD produces (.7 x 10) 7*base-in on a + 10.00 D lens. Reducing the diameter on the lenses makes them thinner and lighter. Most binocular half-eye magnifiers are made with plastic lenses. Usually the frames are 66 mm PD, 24 DBL, 42 mm to 44 mm eye diameter. Glass lenses 36 mm in diameter are about 20% thinner than plastic lenses 44 mm in diameter. Reducing the PD (1.3 cm on a frame from 66 mm to 53 mm produces x 12) 15.60 base-in on a -t 12.00 D lens; this also makes the lenses thinner and lighter, thereby less conspicuous. Reading binocularly at a close distance demands great convergence effort. The closer the reading distance, the greater is the difference between the distant and the near PD. Therefore, greater decen-
Advantages 1. Produce the maximum practical base-in prism, thereby relieving the greatest convergence effort. practical binocular magni2. Produce the strongest fication: + 14.00 D C 18A base-in (3.5X). lighter and less conspicuous 3. Lenses are thinner, than full-size ones. 4. Cosmetically acceptable for adults. Patients can easily look over the top of the lenses. ;: Instantly available for testing and demonstration at the time of examination. 7. Available for loan or purchase. 8. Inexpensive. Disadvantages 1. Close reading distance. for children. 2. Conspicuous
tration or prism base-in is required. Many people, such as a myope of - 10.00 D and children who can accommodate 10.00 D, can read for 30 minutes and longer exerting 60” of convergence.
Discussion Indications, advantages, and disadvantages of half-eye spectacle magnifiers are summarized in Table 3. Binocular spectacle magnifiers are one of the most useful optical aids for low vision patients with binocular vision because they are produced in pow(3.5X) and with ers up to + 14.00 D S 18’base-in narrow PD frames of 53 mm which relieve the greatest amount of convergence. There is no need to measure the patient’s PD because it is advantageous to use as narrow a PD on a frame as possible since this only produces a base-in prism by decentration. The range of powers available is + 6.00 D S aA + 10.00 +8.00 D C 10’ base-in, base-in, 12* base-in, + 12.00 D ^, 14* base-in and DZ + 14.00 D Z 18* base-in for visual acuities from 20/60 (6/l@ to 20/320 (6/96). The +6.00 D S 8’ base-in and + 8.00 D C 1OA base-in are available in 60 mm PD frames. Stronger powers are available in 53 mm PD frames: Weaker half-eye spectacle magnifiers are sometimes indicated for people with vision better than
154
Surv Ophthalmol
36 (2) September-October
20/60 (6/l@, or for doing fine work, such as needlepoint, with normal vision. Rather than prescribe prisms base-in, the laboratory should be instructed, in case the power is + 4.00 D, to decenter the lenses in 10 mm each, which will produce 4* base-in, a total of 8” for both eyes. This will reduce convergence from 24* to 16*. Decentering a lens should be much less expensive than grinding prisms. The closer the reading distance, the greater is the difference between the distant and the near PD; therefore, greater decentration or prism base-in is required. Many people can read for hours using available accommodation, or read at the far point of their myopic eye, or use a combination of myopia and accommodation exerting 60* of convergence. To date, no data has been presented concerning the production of prismatic effect involved in the design of binocular half-eye spectacle magnifiers. In this article, I emphasize that a small eye diameter makes possible the positioning of the plus lens’s optical center extremely nasally in the frame resulting in maximal base-in prismatic effect. It is also
1991
FONDA emphasized that a narrow bridge results in a nasal shift in the positioning of the lens of the frame. Both of these features decrease the distance between the optical center of the lens and the point on the lens through which the fixation axis passes. The resulting augmented base-in effect makes possible maximum binocular function while using spectacle magnifiers of powers up to + 14.00 D C 18* base-in.
References 1. Fonda G: Binocular
reading additions for low vision. Arch 1970 2. Kestenbaum A, Sturman RM: Reading glasses for patients with very poor vision. Arch Ophthalmd 56:451-470, 1956 3. Lebensohn JE: Practical problems pertaining to presbyopia. Am J Ophthalmol32:22-30, 1949
O~hthalmol83:294299,
Frames are available from May, USA, Box 760, Marion Road, Wareham, MA, 02571. Neither the author nor his family has a proprietary interest in the development or marketing of these frames. Reprint address: Gerald E. Fonda, M.D., 101 Old Short Hills Road, West Orange, New Jersey, 07052.
