MXPA98008608A - Multifo oftalmico lens - Google Patents

Abstract

In a multifocal ophthalmic lens comprising an aspherical surface having at each point thereof a middle sphere and a cylinder, a far vision region, a near vision region, and an intermediate vision region, the length of progress, i.e. , the length over which the power of the lens varies by an amount established in different regions of the lens by an amount established in different regions of the lens, is short. To avoid distortion at the periphery of the lens, which would otherwise cause this, the isosphere and isocylinder lines are distributed over the surface of the lens, in order to ensure that variations in the sphere are not too sudden throughout of a circle of a radius of 20 millimeters centered on the geometric center of the lens, and that the variations in the cylinder on the lens surface inside this circle are also very small. The lens has an enlarged near view region, and progress is less noticeable to the user

Description

MU LT1FOCAL OPHTHALMIC LENS BACKGROUND OF THE INVENTION The present invention relates to a multifocal ophthalmic lens, having a spherical surface having a middle sphere and a cylinder at each point thereof, far, near and intermediate vision regions, and a main meridian of progress passing through of these three regions. These lenses are well known; Among multifocal lenses one can distinguish lenses known as progressive lenses adapted to view at all distances, and lenses that are more specifically dedicated to near vision and intermediate vision. Progressive multifocal ophthalmic lenses comprise a far vision region, a near vision region, an intermediate vision region, and a main meridian of progress that passes through all three regions. French patent application 2,699,294, which is incorporated herein by reference, discloses, in its preamble, the different elements of a progressive multifocal ophthalmic lens (principal meridian of progress, far vision region, near mink region, etc.). , as well as the work carried out by the applicant to improve the comfort of the user of said lenses.

The applicant has also proposed, in order to better meet the visual needs of people with eyestrain and to improve the comfort of progressive multifocal lenses, to adapt the shape of the main meridian of progress, as a function of the power addition value A (application French Patent FR-A-2,683,642). For these lenses, the power addition value A is defined as the variation in the mean sphere between a reference point in the far vision region and a reference point in the near vision region. Such progressive lenses are usually prescribed as a function of the user's ametropia and the power needed for near vision. The lenses also exist that are dedicated more specifically to near vision; said lenses do not have a far vision region with a defined reference point as conventional progressive lenses do. These lenses are prescribed depending on the near vision power that the user needs, regardless of the far vision power. Said lens is described in an article in "Opticien Lunetier" dated April 1988, and sold commercially by the applicant under the Essilor Delta brand; This lens is also as simple to use and easy to put on as a progressive lens, and is attractive to people with eyestrain not accustomed to progressive lenses. This lens is also described in the French patent application FR-A- 2, 588, 973. It has a central portion that is equivalent to the lenses of an approach that would normally be used to correct eyestrain, in order to ensure satisfactory near vision. In addition, it has a slight reduction in power in the upper portion, ensuring that the user has a sharp vision beyond the usual near field of vision. Finally, the lens has a point at a power value equal to the rated power for near vision, a higher power region in the lower portion of the lens, and a lower power region in the upper part of the lens. The existing multifocal lenses, either progressive or dedicated to near vision, can be further improved in terms of their foveal vision operation, in order to improve user comfort. Users of multifocal lenses do in fact sometimes feel uncomfortable with dynamic vision. Users of multifocal lenses do in fact sometimes feel uncomfortable with dynamic vision, which can cause symptoms such as nausea and headache. Such lenses can also be improved by conserving a near vision region that is sufficiently high to ensure the comfort of the optimal user, along with wide visual fields in near and intermediate vision.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a multifocal lens that overcomes the disadvantages of the lenses of the prior art and ensures vision Improved peripheral, at the same time maintaining good functioning foveal vision, so that it facilitates the adaptation of users to their lenses. However, the invention ensures rapid progress of the middle sphere, so that the presence of a large near vision area is ensured. A balanced distribution of the isosphere and isocylinder lines is also achieved. The invention provides a multifocal ophthalmic lens comprising a spherical surface having at each point thereof a middle sphere and a cylinder, said lens having an addition of power and comprising a far vision region V, a region of near vision VP, an intermediate vision region VI, a principal progress meridian MM 'passing through said three regions, and an addition of power, wherein a main length of progress as defined herein is shorter than 16 mm , a maximum value | ds / d? | ma? of a module of the tangential derivative of the middle sphere in a circle of diameter of 40 mm centered on a geometric center of said lens is less than a quarter of the maximum value Pmer of an inclination of the middle sphere on said meridian: a maximum cylinder value Cmax within said circle is less than a nominal power addition of said lens: (- 'ma x * ~ A n o.

Advantageously, the main meridian of progress is made by mid-points of horizontal segments joining the respective lines formed by points where the cylinder is 0.50 diopters. The near vision region, delimited in an upper portion of said lens by the lines formed by the points where the cylinder is equal to the power addition ratio may have a width that is greater than 14.5 mm at a point of reference for near vision. The far vision region defined in an upper portion of said lens by the lines formed by the points where the cylinder is equal to the power addition half, preferably contains at least one angular sector formed by two directed middle lines. upwards that originate in a geometric center of said lens and that have an included angle of at least 1 50 °. Preferably, the cylinder on the surface of said lens is less than the power addition, preferably less than 90% power addition. Advantageously, the difference in the maximum cylinder values in the two portions of said lens of the imitated by the principal meridian of progress is less than 0. 1 diopter, and is preferably less than 0.05 diopter. In one embodiment, the lens is a m ultifocal lens dedicated to near vision and intermediate vision, said lens having an addition. of power defined as a difference between the maximum and minimum values of the average sphere in said meridian of progress, within a circle with radius of 20 mm centered on the geometric center of said lens. In this case, the main length of progress is defined as a relation between the addition of power and a maximum value of inclination of the average sphere Pmer on said meridian. In another embodiment, the lens is a progressive multifocal lens having a reference point for a near vision region, a reference point for a far vision region and a power addition defined as a difference between the values of the average sphere in these two points. In this case, the main length of progress is defined as a difference in height between a mounting center and a point on that meridian where the average sphere is equal to the sum of said reference point for far vision, plus 85% of said power addition. Other features and advantages of the present invention will become clearer from the following description of an embodiment of the invention provided by way of non-limiting example with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DIAMETERS Figure 1 is a diagrammatic front view of a multifocal progressive lens. Figure 2 shows graphically the variation in power over the meridian of the lens according to the invention. Figure 3 is a front view of the lens in Figure 2, showing the main meridian of progress and the lines indicating the level of the middle sphere. Figure 4 is a front view of the lens in Figure 2, showing the main meridian of progress and the lines indicating the level of the cylinder. Figure 5 shows the derivative of the sphere as a function of angle in a circle with radius of 20 mm centered on the geometric center of the lens. Figures 6 to 8 are views similar to those in Figures 2 to 4, for a power addition of 2 diopters.

DETAILED DESCRIPTION OF THE PREFERRED MODALI DAD Later, an orthonormal coordinate system will be used where the x axis corresponds to the horizontal axis and the y axis corresponds to the vertical axis; center 0 of the reference frame is the geometric center of the lens.

Figure 1 is a diagrammatic front view of a known progressive ophthalmic lens, showing the different elements thereof. Figures 2 to 5 show the optical characteristics of the lens according to the invention, this lens having a diameter of approximately 60 mm. In Figures 2 to 5, a lens having a power addition of 2 or 3 diopters is described. With reference to Figure 1, the different elements of a multifocal ophthalmic lens will now be described. Said lens generally has a spherical face shown in Figure 1 and a second face which may be spherical or toroidal. For each point on the spherical surface, an average sphere D is defined from the formula: 2 R R2 where: Ri and R2 are the maximum and minimum radii of curvature expressed in meters, and n is the refractive index of the lens material. The indro C is defined by the formula: C = (n - 1) | 1 - 1 | The lines of the isosphere are lines formed by the projection in a plane tangential to the progressive surface at the geometric center O of the points on the surface of the lens. has the same value of the middle sphere. Similarly, the lines of the isocylinder are lines formed by the projection in this same plane of points that the same cylinder has. Conventionally, lens 1 comprises in its upper portion a far vision region VL, in its lower portion a near vision region VP and, between these two regions, an intermediate region VI. For a progressive lens, a reference point P is defined in the near vision region where near vision is measured and a reference point L where far vision is measured. For a lens dedicated to near vision, a reference point P is defined in the near vision region to measure near vision; however, no corresponding reference point is defined for the far vision region. In Figure 1, the principal meridian of progress 2 of the lens is shown, which passes through the far vision region, the intermediate vision region and the near vision region. This meridian is defined as the place of the midpoints of horizontal segments delimited by the isocylinder line of 0.50 diopters. In the example of figure 1, the meridian is essentially composed of three segments, the first extending substantially vertically from the top of the lens, passing through the point L, to a point D, referred to as the center of adjustment, and located between the far vision control point L and the geometric center O. The second segment extends from the point D obliquely towards the nasal fado of the lens, and the third segment begins from the end C of the second segment and passes through the near vision control point P. Other meridian forms are possible.

In the case of progressive multifocal ophthalmic lenses, a power addition is defined in a manner known per se, this being the difference in the average sphere between a reference point P in the near vision reigon and a reference point L in the region of far vision. For multifocal lenses dedicated to near vision and intermediate vision, the minimum and maximum sphere values are measured in the meridian thus defined within the limits of a circle with a radius of 20 mm centered on the geometric center of the lens. The power addition is now the difference between three minimum and maximum values of the sphere; this definition is substantially equivalent, for progressive lenses, to the conventional definition of power addition and the difference in sphere between the reference points for near and far vision. With these definitions, it is generally considered that the limit of the far vision region in the upper portion of the lens is formed by isocylinder lines of a value equal to half the power addition. Similarly, the boundary of the near mink region in the lower portion of the lens is established by lines of the indile isocil of a value equal to half the power addition.

The inner circle shown in Figure 1 represents the region detected by the eye when performing everyday tasks. The size and position of this portion, known as the foveal vision region, is has determined by several series of measurements carried out in the applicant's laboratories; reference can be made to I EEE, portable eye movement recorder by T. Bonnin and N. Bar, Proceedings of the 14th annual international conference of the Society of Engineering in Medicine and Biology of I EEE of 1992, part 4, pages 1668 to 1669, to AAO 1993, for "Optimization of ophthalmic spherical lenses: recording of eye movement for everyday tasks ", N. Bar, T. Bonnin and C. Pedreno, Optometry and vision science 1993, No. 12s, volume 70, page 154, or once again to ECEM 93, "The use of visual space", a poster by N Bar. region covers a disk with a diameter of 30 mm centered on the assembly center. To ensure maximum visual comfort for the user, the disc with a diameter of 40 mm centered on the geometric center of the lens, covering the foveal vision region, is considered and it has been established to limit the tangential variations in the sphere on this circle. The control of the variations in the sphere on this circle makes it possible to dominate the deformations in the optical characteristics of the multifocal surface; in this way it improves the peripheral vision of the user. It is also desired to overcome defects such as the cylinder within the 40 mm circle, as much as possible, acute vision within the foveal region. This circle is shown in Figure 1 and Figures 3, 7 and 10. In order to improve the fragility of the progress of the lenses, and to facilitate the user to adapt to the lens, the present invention considers a new definition of the characteristics of the lens surface, explained with reference to the following figures. The figures cover the case of progressive multifocal lenses; The invention applies mutatis mutandis to multifocal lenses dedicated to near vision. Figure 2 is a graph showing the power over the meridian of the lens according to the invention, the power addition of this lens being one diopter. The coordinates of the y-axis of the graph of Figure 1 are the coordinates of the y-axis of the lens; the coordinates of the x axis give the difference in power, in diopters, from the reference point in the far vision region.

The point that has the value y = 8 mm on the y-axis on the meridian corresponds to the reference point L for far vision, which, in the case of Figure 2, is the point of the smallest sphere; at this point, the average sphere is 5.2 diopters and the cylinder is 0.01 diopter; the point that has a value of the axis and 14 mm in the meridian is the reference point P for near vision; in this position, the average sphere is 6.20 dios and the cylinder is 0.01 dio. For a progressive multifocal lens, a main length of progress Lp p is defined as a difference in height between the value of the y-axis of a mounting center and the value of the y-axis of a point of said meridian where the middle sphere is equal to the sum of the average sphere at said reference point for far vision, plus 85% of said power addition. In the example of Figure 2, the Medium sphere is 85% higher than the power addition at the far vision reference point at a value point y = - 8.4 mm; where a mounting center is located on a y-axis value of y = 4 mm, the main length of progress is 12.4 mm. For a dedicated progressive multifocal lens for near and intermediate vision, the main length of progress is the ratio between the power addition as defined above and the inclination of the middle sphere on the meridian; this can be written as: Lpp - (max _ m¡n) 'r mer Where Smax and Sm¡p are respectively the maximum and minimum values of the sphere in the meridian, and Pmer is the maximum value of the inclination of the sphere average on the meridian; the inclination of sphere corresponds to the maximum modulus of inclination of sphere with respect to x and / or y. This ratio Lpp is equivalent to a length, and represents the length over which the average sphere increases by a value corresponding to the power addition. In both cases, the main length of progress represents a position on the meridian that corresponds to a variation in the average sphere substantially equal to the addition of power. Figure 2 shows that, at the beginning, the sphere remains substantially constant in the far vision region above point L. It also shows that the sphere remains substantially constant in the near vision region, around point P. Finally, it shows that the Main length of progress is low, and it is noticeably less than 16 mm. This ensures close vision satisfactory in a region that extends over the near vision control point, making obvious the need for the user to move his head. Close and comfortable close vision is assured. The maximum inclination of sphere in the meridian is 0.085 diopters per mm. Figure 3 is a front view of the lens in Figure 2, showing the principal meridian of progress and the lines of equal mean sphere. Those elements shown in Figure 2 will also be found in Figure 3 with the addition of isosphere lines. The isosphere lines in figure 3 are the lines 1 1, 12, 13 and 14 that represent respectively the average sphere which is greater by 0.25, 0.5, 0.75 or 1 diopter to the middle sphere at the far vision control point L Finally, a circle with a diameter of 40 mm centered on the geometric center of the lens is shown. Figure 4 is a front view of the lens in Figure 2, showing the principal meridian of progress and the equal cylinder lines. Those elements shown in figure 2 are also present in figure 7. Since the cylinder is low on the main meridian of progress, there are two isocylinder lines for each cylinder value. The isocylinder lines in figure 5 are lines 16 and 16 ', 17 and 17' and 18 and 18 ', which represent, respectively, a cylinder of 0.25, 0.50 and 0.75 diopters. As indicated above, in the upper portion of the lens the limit of the far vision region is constituted substantially by lines 17 and 17 'of the isocylinder of 0.5. the lens of the invention of this way has a wide distant vision region that extends over almost the entire upper half of the lens. In the lower portion of the lens, the limit of the near vision region is also substantially constituted by lines 17 and 17 'of the isocylinder of 0.5. The maximum value of ^ cylinder on the surface of the lens, inside the circle with radius of 20 mm is 0.88 diopters; this is reached at a point that has the coordinates x = 16, y = -8; this maximum value is less than the power addition. This maximum value is reached on the nasal side; on the temporary side, the maximum cylinder value is 0.83 diopter and is reached at a point x = -8, y = -6, 10 mm from the geometric center of the lens. The difference between the maximum values of the cylinder on the nasal and temporal sides of the lens is less than 0.05 diopters. The invention proposes that this variation be less than 0.1 diopters and preferably less than 0.05 diopters. Figure 5 shows the sphere derivative as an angle function, in a circle with radius of 20 mm centered on the geometric center of the lens; the y-axis is graduated in diopters / 0; the x-axis graduates in degrees, and gives the angle? that a middle line that passes through the center of the circle with radius of 20 mm does with the horizontal; from figure 5, it can be seen that the maximum module of the derivative is 0.018 diopters / 0 and is reached for a value of? about 310 °, that is, in the lower portion of the lens.

The lens in figures 2 to 5 thus ensures that the progress is careful, the adaptation being facilitated by the user. Quantitatively, this is reflected by the following relationships: A / Pmer < 16 mm (1) | ds / d? | Max / Pmer < 0.25 (2) and '-'max "^ nom \ ¿) In these relations, | ds / d? | Max is the maximum modulus value of the tangential derivative of medium sphere in the circle with radius of 20 mm centered in the center geometric lens, Pmer is the maximum value of the average sphere inclination on the meridian, in diopters per mm, Cmax is the maximum cylinder value within the circle with radius of 20 mm mentioned above, Anom is the nominal value of addition of Lens power in diopters The value of 0.25 is thus in mm / ° The ratio (1), as explained above, limits the main length of lens progress The ratio 2 reflects the fact that the variations in the sphere they are not very sudden on the circle with radius of 20 mm.

Instead of a derivative with respect to an angle, one can use a derivative with respect to an x-axis in curve in the circle. The expression Tangential derivative in the middle sphere in the circle means the derivative of the middle sphere with respect to the angle? that a middle line that passes through the center of the circle with a radius of 20 mm does with the horizontal; Calculation of this derivative is a simple mathematical operation. In this relation, the l / Pmer factor is a normalization factor making it possible to compare lenses that have different power additions. The third relationship limits the variations in indro cil on the lens surface. By combining these two relationships it is ensured that the lines of the isosphere and isocylinder are distributed well over the surface of the lens, thus ensuring very careful progress of the lens. Combining the relationships (1), ((2) and (3) is not satisfactory for any of the multifocal ophthalmic lenses of the prior art, as tested by the applicant.The invention provides, for the first time, said distribution of isocylinder lines and Figures 6 to 8 are views similar to those of Figures 2 to 4, but for a lens having a power addition of two diopters and Figures 9 to 11 are similar to Figures 2 to 4 but for a lens having a power addition of 3 diopters In figures 7 and 8, the isosphere and isocylinder lines are shown respectively in 0.50 steps, in figures 9 and 11, the isosphere and isocylinder lines are respectively shown in steps of 0.25 diopters.

For each lens, relations (1) to (3) are satisfied. In the case of the lens of Figures 2 to 5, we have: A / Pmßr = 12.4 mm | ds / d? | max / Pmßr = 0.22 and Cmax = 0.88 < Anom = 1 .00 diopters For the power addition lenses of 2 and 3 diopters, the relations (relations 1 and 2) are substantially identical; The maximum cylinder values are as follows: Cmax = 1 .75 < Apom = 2.00 diopters on one side, and Cmax = 2.65 < Anom = 3.00 diopters on the other. The invention provides other advantageous features which, in combination with the relations (1), (2) and (3), make it possible to improve the operation of the lenses of the invention. As mentioned before with reference to Figure 2, the main length of progress is advantageously less than 16 mm; is 12.4 mm for the power addition lens of one diopter, and has substantially the same value for power addition lenses of 2 and 3 diopters. In lenses of the prior art, said small length of progress would generally lead to distortion at the periphery for the user, in accordance with the invention, thanks to the conditions expressed by the relations (1), (2) and (3) ), this short length of progress does not lead to any inconvenience for the user. The invention also provides that the near vision region has, at the reference point for near vision, a width of at least 14.5 mm; This width is measured in the y-axis coordinate of point P, between the isocylinder lines A / 2 where A is the power addition, defined above. As can be seen better in Figure 3, in the case of power addition of a diopter, the width of the near vision region is 17 mm. For lenses that have power additions of two and three diopters, the width of the near vision region is substantially equal to this value. In one embodiment of the invention, the far vision regions comprise at least one sector formed by two median lines originating from the geometric center of the lens, its included angle directed towards the upper portion of the lens being approximately 150 °. As can be observed in figure 4, in the case of a power addition of one diopter, the angle ß between the corresponding middle lines 19 and 19 'is 153 °. For a power addition of two diopters, just as for a power addition of three diopters, the value is substantially the same. It will be recalled that the near vision region, in the upper portion of the lens, is also limited by the isocylinder lines A / 2, where A is the power addition as defined above. The middle lines involved are drawn in figures 8 and 11. In order to improve binocular vision, the invention also provides the difference in maximum cylinder, between the two portions of the lens delimited by the principal meridian of progress to be less than 0.10 diopter. In the case of FIG. 4, for a power addition of one diopter, the maximum cylinder on the nasal side (the right side in figure 4) is 0.88 diopter; on the temporary side it is 0.83 diopter. For the lens of Figure 8 (addition of power of two diopters), the corresponding values are 1.74 and 1.69 diopters), the corresponding values are 2.65 and 2.60 diopters. Advantageously, the cylinder in the lens is less than the power addition, preferably less than 90% addition. As an example, in the case of a power addition of one diopter, the maximum cylinder is 0.88 diopters; it is 1.74 diopters for a power addition of two diopters and 2.65 for a power addition of three diopters. Now details of the different characteristics that make it possible to provide the different lenses according to the invention will be given. As it is known per se, the surface of the lenses is continuous and can be continuously derived three times. As is known to those skilled in the art, the surface of progressive lenses is obtained by digital optimization when using a computer, establishing boundary conditions for a certain number of lens parameters. A combination of the two relationships defined above can be used as boundary conditions, together, if appropriate, with one or more of the criteria defined above. These criteria apply to a conventional progressive multifocal lens with a reference point in the far vision region and a reference point in the near vision region, as well as for a multifocal lens dedicated to near vision.

One can begin favorably by defining, for each lens of the family, a principal meridian of progress. For this, the teachings of the French patent application FR-A-2,683,642 mentioned above, which is incorporated herein by reference, is used. Any other definition of the principal meridian of progress can be used to apply the teaching of the invention. Obviously, this invention is not limited to what has been described: among other things, the spherical surface may be the surface facing the wearer of the lenses. In addition, although it is not mentioned in the description of lenses that may be different for both eyes, it certainly applies obviously.

Claims (11)

REIVIN DICACIONES

1 .- A multifocal ophthalmic lens comprising a spherical surface having at each point a medium sphere and a cylinder, said lens having an addition of power and comprising a far vision region VL, a near vision region VP, an intermediate vision region VI, a principal progress meridian MM 'passing through said three regions, and a power addition, wherein a main length of progress as defined herein is shorter than 16 mm, a maximum value | ds / d? | max of a module of the tangential derivative of the middle sphere in a circle of diameter of 40 mm centered on a geometric center of said lens is less than a 15 fourth of the maximum value Pmßr of an inclination of the average sphere on said meridian: | ds / d? | Max / Pmßr < 0.25 a maximum cylinder value Cmax within said circle is less than a nominal power addition of said lens: 'O Cm to x < Year .

2. The lens according to claim 1, wherein said main meridian of progress is made by midpoints of horizontal segments joining respective lines formed by points where the cylinder is 0.50 diopters.

3. - The lens according to claim 1 or 2, wherein said near vision region, delimited in an upper portion of said lens by lines formed by points where the cylinder is equal to the power addition half has a width that is greater than 14.5 mm at a reference point for near vision.

4. The lens according to claim 1, 2 or 3, wherein said far vision region defined in an upper portion of said lens by lines formed by points where the cylinder is equal to the power addition half, they contain at least one angular sector formed by two midlines directed upwards which originate in a geometric center of said lens and which have an included angle of at least 150 °.

5. The lens according to one of claims 1 to 4, wherein the cylinder on the surface of said lens is less than the power addition, preferably less than 90% power addition.

6. The lens according to one of claims 1 to 5, wherein a difference in maximum cylinder values in the two portions of said lens delimited by the principal meridian of progress is less than 0.1 diopters, preferably less than 0.05 diopters.

7. The lens according to one of claims 1 to 6, wherein the lens is a multifocal lens dedicated to near vision and intermediate vision, said lens having a power addition defined as a difference between maximum and minimum values of average sphere in said meridian of progress, within a circle with radius of 20 mm centered in the geometric center of said lens.

8. - The lens according to claim 7, wherein the main length of progress is defined as a ratio between the addition of power and a maximum value of average sphere inclination Pmer on said meridian.

9. The lens according to one of claims 1 to 6, wherein said lens is a progressive muitifocal lens having a reference point for a near vision region, a reference point for a far vision region, and an addition of power defined as a difference between the values of average sphere in these two points.

10. The lens according to claim 9, wherein the main length of progress is defined as a difference in height between the value of the y-axis of a mounting center and the value of the y-axis of a point on said meridian in where the average sphere equals the sum of the average sphere at said reference point for far vision, plus 85% of said power addition. SUMMARY In a multifocal ophthalmic lens comprising a spherical surface having at each point thereof a mean sphere and a cylinder, a region of far vision, a region of close mink, and a region of intermediate vision, the length of progress, that is, the length over which the power of the lens varies by an amount established in different regions of the lens, is short . To avoid distortion at the periphery of the lens, which would otherwise cause this, the isosphere and isocylinder lines are distributed over the surface of the lens, in order to ensure that variations in the sphere are not too sudden throughout of a circle of a radius of 20 millimeters centered on the geometric center of the lens, and that the variations in the cylinder on the lens surface inside this circle are also very sma

ll. The lens has an enlarged near view region, and the progress is less noticeable to the user.