Abstract:
A pair of progressive ophthalmic lenses ( 1, 2 ) meets special conditions for improving binocular vision of a wearer, while avoiding discomfort for peripheral vision. A first one of the conditions sets a minimum value for the difference between nasal and temporal half-widths of far vision field and/or proximate vision field for at least one of the lens. A second one of the conditions sets a maximum value for the relative difference in mean refractive power gradient between both lenses.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to a pair of progressive ophthalmic lenses, and to a method for providing such lens pair to a wearer. 
         [0002]    In the context of the present invention, a pair of ophthalmic lenses means two eyeglasses which are intended to be provided to one and same wearer when both eyeglasses are mounted into one spectacle frame. Each lens of the pair is therefore dedicated to correcting both an ametropia and a presbyopia of one of the wearer&#39;s eyes in accordance to an ophthalmic prescription obtained for this eye. Visions of both right and left eyes are thus corrected simultaneously. 
         [0003]    The invention specifically relates to provide improved binocular vision to the wearer, by selecting appropriately the progressive lenses to be paired. 
       BACKGROUND OF THE INVENTION 
       [0004]    Progressive ophthalmic lenses allow sharp vision of objects which are located at varying distances in front of the lens wearer, the wearer being presbyopic. For this purpose, each progressive ophthalmic lens has a mean refractive power which varies along a meridian line of the lens. Here, the meridian line corresponds to the main gaze direction used by the wearer to see at varying distances. The vision distances may be defined by using an ergorama. As a general rule, the vision distance decreases from more than 2 meters for a far vision gaze direction, to about 40 centimeters for a proximate vision gaze direction which is oriented below the far vision gaze direction. 
         [0005]    But out of the meridian line, a progressive ophthalmic lens has mean refractive power values and resultant astigmatism values which are not equal to those on the meridian line at constant height for a same gaze direction lowering angle. As a consequence, the optical powers which are actually produced by each lens do not match exactly the prescription out of the meridian line and out of lens zones dedicated respectively to far vision and proximate vision. Many efforts have been spent for years to increase the far- and/or proximate vision areas in combination with reducing the correction deviations out of these areas and the meridian line. But these improvements relate in optimizing each lens separately. 
         [0006]    In addition, it has been discovered that the wearer&#39;s binocular vision can be improved by selecting appropriately both lenses which pertain to one and same pair, each with a different lens design. For example, document FR 2 704 327 proposes selecting one of the ophthalmic progressive lenses of the pair with a large far vision zone, and the other ophthalmic progressive lens of the pair with a large proximate vision zone. As a consequence, the lens with large far vision zone has a reduced proximate vision zone, and the one with large proximate vision zone has a reduced far vision zone. But binocular vision allows the wearer to select the one of his eyes which has sharper vision for each binocular gaze direction. Thus, each of the largest far- and proximate vision zones between both lenses appears to be actually effective for providing sharp binocular vision, whereas the smallest far- and proximate vision zones are useful for providing stereoscopic vision perception. 
         [0007]    U.S. Pat. No. 7,083,277 discloses another example of lens pair with both lenses being different in far- and proximate vision field widths. 
         [0008]    However, vision discomfort may still arise when both lenses of the pair are thus selected with different designs. In particular, such discomfort relates to peripheral gaze directions with respect to the meridian lines of the lenses. 
         [0009]    Starting from this situation, an object of the present invention consists in improving binocular vision of a wearer of ophthalmic progressive lenses, while avoiding discomfort for peripheral vision. 
       SUMMARY OF THE INVENTION 
       [0010]    For meeting this object or others, the present invention proposes a pair of progressive ophthalmic lenses, in which each lens has a prescribed addition ADD which is common to both lenses of the pair and, in as-worn conditions of the lens by a wearer, a meridian line, a nasal side and a temporal side on either side of the meridian line, respectively, a fitting cross, a prism reference point, a proximate vision gaze direction, a mean refractive power PPO α,β  and a module of resulting astigmatism ASR α,β  for each gaze direction through the lens and corresponding to a lowering angle α and an azimuth angle β, relating to a cyclopean coordinate system. 
         [0011]    For each progressive lens of the pair, a mean refractive power deviation DPPO α,β  is defined for each gaze direction through this lens as follows, separately from the other lens of the pair: 
         [0000]        DPPO   α,β   =PPO   α,β   −PPO   α,βmeridian , 
         [0000]    where PPO α,βmeridian  is the mean refractive power on the meridian line for the same lowering angle value α than the gaze direction considered. The meridian line ML corresponds to the main gaze directions used by the wearer to see at varying distances. Then and again for each lens separately, a total deviation P α,β  is further defined for each gaze direction through this lens as: 
         [0000]        P   α,β   =m *( DPPO   α,β ) a   +n *( ASR   α,β ) b , 
         [0000]    where m, n, a and b are parameters such that m≧0, n≧0, (m, n)≠(0, 0), 0&gt;a≦2 and 0&lt;b≦2. For each lens, such total deviation quantifies the difference between the actual optical power which is produced at a given gaze direction and corresponding optical power selected from the meridian line of the lens for the same height. Thus, the m-, n-, a- and b-parameters are selected initially for allowing the total deviation P α,β  to be calculated for each gaze direction. The values selected for these m-, n-, a- and b-parameters are constant for all features relating to the lens pair considered. In particular, they are constant whatever the gaze direction for which the total deviation is calculated. 
         [0012]    First nasal and temporal half-widths are also defined for each lens separately as the angular distances from a gaze direction through the fitting cross respectively to a first gaze direction on the nasal side of the lens and to a second gaze direction on the temporal side of the lens, both first and second gaze directions with lowering angle α equal to that of the gaze direction through the fitting cross, and each where the total deviation P α,β  satisfies: 
         [0000]        P   α,β   =m *( ADD/ 8) a   +n *( ADD/ 4) b . 
         [0000]    Such first nasal and temporal half-widths quantify the horizontal spread of the far vision area of each lens of the pair, on nasal side and on temporal side respectively. Thus, m*(ADD/8) a +n*(ADD/4) b  is a reference value for the total deviation P α,β , which is used for defining the first nasal and temporal half-widths, when the total deviation P α,β  is calculated as above-indicated using the selected values for the m-, n-, a- and b-parameters, for any gaze direction. 
         [0013]    Second nasal and temporal half-widths are also defined for each lens separately as the angular distances from the proximate vision gaze direction respectively to a third gaze direction on the nasal side of the lens and to a fourth gaze direction on the temporal side of the lens, both third and fourth gaze directions with lowering angle α equal to that of the proximate vision gaze direction, and each where the total deviation P α,β  satisfies: 
         [0000]        P   α,β   =m *( ADD/ 4) a   +n *( ADD/ 2) b . 
         [0014]    Such second nasal and temporal half-widths similarly quantify the horizontal spread of the proximate vision area of each lens of the pair, on nasal side and on temporal side respectively. So m*(ADD/4) a +n*(ADD/2) b  is the reference value for the total deviation P α,β , which is used for defining the second nasal and temporal half-widths. It is different from the reference value used for the first nasal and temporal half-widths. 
         [0015]    A relative difference is further defined for each lens of the pair separately between the first nasal and temporal half-widths of this lens and/or between the second nasal and temporal half-widths of this lens. According to a first feature of the invention, this relative difference is greater than or equal to 0.25 in absolute value. This first feature sets a minimum threshold for the difference between nasal and temporal half-widths of far vision zone or proximate vision zone for at least one lens of the pair. 
         [0016]    Each lens separately also has a maximum mean refractive power gradient produced for a gaze direction corresponding to a cyclopean gaze direction which is comprised:
       inside a zone centered on to the cyclopean gaze direction corresponding to gaze directions passing through the PRP and containing all cyclopean gaze directions (α, β) respecting the following inequality (|α| 2 +|β|) 1/2 ≦40°, and   outside a central optical zone; the central optical zone comprising a main line ML_min (α min , β min ) of cyclopean directions for which, for each angle α min , the resulting astigmatism reaches its minimum, the central optical zone being delimited on either side of the main line by cyclopean gaze directions whose azimuth angle is equal to β min ±5°.       
 
         [0019]    Then, according to a second feature of the invention, a relative difference in the maximum mean refractive power gradient between both lenses of the pair is less than or equal to 0.08 in absolute value. This second feature sets a threshold condition relating to the mean refractive power gradient, for both lenses not to have respective designs in peripheral part which are too different from each other, which can cause discomfort to the wearer. In particular, it avoids assigning a lens with hard design to one of the wearer&#39;s eyes together with another lens with soft design to the other one of the wearer&#39;s eyes. 
         [0020]    Hence, the invention consists in an advantageous combination of two features which provide an improved binocular vision by using a specially selected progressive lens pair, while ensuring comfort in peripheral vision. 
         [0021]    In the context of the invention, the cyclopean coordinate system denotes any coordinate system suitable for identifying a gaze direction which is originating from one point located between both eyes of the wearer, and which is oriented towards an object currently looked at by the wearer. Such direction is called cyclopean gaze direction, and separate gaze directions respectively for both eyes can be derived from the cyclopean gaze direction. But, when characterizing a lens pair according to the invention without a wearer of these lenses being individually identified, the cyclopean coordinate system to be used is defined by a separation distance which is equal to 64 mm (millimeter) between both pupils of a standard wearer, and for a cyclopean eye which is located at mid position between both eyes of this standard wearer. 
         [0022]    Also in the context of the invention, the relative difference between any two values denotes the ratio of the difference between these values over the sum of the same values. Thus, if X and Y denote both values considered, then their relative difference is Ratio=(X−Y)/(X+Y). 
         [0023]    The lenses may further have first and second width values defined in the following manner for each lens separately:
       a first width value defined as the sum of the first nasal and temporal half-widths of said lens; and   a second width value defined as the sum of the second nasal and temporal half-widths of said lens.       
 
         [0026]    Hence, the first width value relates to the horizontal spread of the far vision zone of each lens of the pair and the second width value relates to the horizontal spread of the proximate vision zone of each lens of the pair. 
         [0027]    A total horizontal width ratio is further defined for the pair of lenses as the relative difference between the respective first width values of both lenses of the pair, and/or between the respective second width values of both lenses of the pair. According to a first optional improvement of the invention, this total horizontal width ratio may be lower than or equal to 0.05 in absolute value. This optional condition sets a maximum threshold for the difference in width of far vision zone or proximate vision zone between both lenses of the pair. Preferably, the total horizontal width ratio may be lower than or equal to 0.03 in absolute value. Thus, symmetrical designs for both lenses can be used. 
         [0028]    According to a second optional improvement of the invention, the lens pair may be such that the relative differences between the first nasal and temporal half-widths of both lenses of the pair, and/or the relative differences between the second nasal and temporal half-widths of both lenses of the pair have a same sign. Thus, a sharp vision zone which is narrow on one of the left and right sides for one of the lenses is compensated by the other lens with respect to binocular vision. 
         [0029]    According to a third optional improvement of the invention, the lens pair may be such that the relative differences between the first nasal and temporal half-widths of both lenses of the pair, and/or the relative differences between the second nasal and temporal half-widths of both lenses of the pair are substantially equal to each other in absolute value. Thus, symmetrical designs for both lenses can be used. 
         [0030]    According to a fourth optional improvement of the invention, each lens of the pair may be such that the relative difference between the first nasal and temporal half-widths of this lens and the relative difference between the second nasal and temporal half-widths of this lens have opposite signs. 
         [0031]    The following parameter values may be used for determining either the first and second gaze directions, or the third and fourth gaze directions, or all first to fourth gaze directions, in alternative embodiments of the invention:
       m=0, n=1 and b=1, so that the criterion for determining the width values is based on the resulting astigmatism only;   m=1, n=0 and a=1, so that the criterion for determining the width values is based on the mean refractive power deviation only; or   m=4, n=2, a=2 and b=2, so that the criterion for determining the width values is that of Y. Le Grand as disclosed in the article entitled “Sur le calcul des verres de lunetterie”, Revue d′Optique, Paris, 1966.       
 
         [0035]    The invention may be advantageously applied when an absolute difference in prescribed far vision mean refractive power between both lenses is less than or equal to 1.0 diopter. Out of these cases, optical aberrations as power deviation or resulting astigmatism may not be fully mastered as for each vision zones, so that the design differences between both lenses as provided by the present invention can fully appear. 
         [0036]    Similarly, the invention may be advantageously applied when an absolute difference in prescribed astigmatism value between both lenses is less than or equal to 1.0 diopter. 
         [0037]    The present invention also proposes a method for providing a pair of progressive ophthalmic lenses to an identified wearer, which method comprises the following steps:
       /1/ for the wearer to whom both lenses are dedicated:
           obtaining the prescriptions for both eyes including the prescribed addition ADD which is common to both lenses of the pair;   obtaining a value for a separation distance between both pupils of the wearer, and a position of a cyclopean eye for this wearer; and   defining a cyclopean coordinate system for the wearer, based on the pupil separation distance and the cyclopean eye position;   
           /2/ for each lens of the pair separately from the other lens, defining as above the total deviation for each gaze direction through this lens, obtaining the first and second nasal and temporal half-widths of this lens and defining their respective relative differences, and obtaining the maximum mean refractive power gradient for this lens;   /3/ for the lens pair, defining the relative difference in the maximum mean refractive power gradient between both lenses of the pair;   /4/ optimizing at least one lens of the pair so that the relative difference between the first nasal and temporal half-widths of this lens and/or between the second nasal and temporal half-widths of this lens has an absolute value greater than or equal to 0.25 and the relative difference in the maximum mean refractive power gradient between both lenses is less than or equal to 0.08 in absolute value; and   /5/ manufacturing both lenses of the pair in accordance with an optimization result of step /4/.       
 
         [0046]    The values for the m-, n-, a- and b-parameters are selected initially when implementing the invention method, and are kept constant thereafter all along the execution of the method. In particular, these values are re-used without change when calculating the total deviation P α,β  for any gaze direction. 
         [0047]    When the first and second nasal and temporal half-widths are preferred to be based on the resulting astigmatism only, then the m-parameter equals zero, and the n- and b-parameters both equal unity. 
         [0048]    Conveniently, the optimizing process of step /4/ may be implemented using computer means. 
         [0049]    In some implementations of a method of the invention, step /4/ may be based on data linked to the wearer. These data linked to the wearer may be obtained themselves before step /4/, for example at a same time as step /1/ is carried out. 
         [0050]    For a lens pair or a method according to the invention, the relative difference between the first nasal and temporal half-widths and/or between the second nasal and temporal half-widths may have an absolute value greater than or equal to 0.33. In combination or independently, the relative difference in the maximum mean refractive power gradient between both lenses may be less than or equal to 0.04 in absolute value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0051]    These and other features and advantages of the invention will become apparent from the detailed specification below, which is provided only for illustration purpose without inducing any limitation, and based on the appended drawings now listed: 
           [0052]      FIGS. 1   a  to  1   c  illustrate optical parameters related to a lens pair according to the invention; 
           [0053]      FIGS. 2   a  and  2   b  are mean refractive power maps respectively for a right lens and a left lens of a pair according to an embodiment of the invention; and 
           [0054]      FIGS. 2   c  and  2   d  are maps of resulting astigmatism respectively for the right lens and the left lens of  FIGS. 2   a  and  2   b.    
       
    
    
       [0055]    Same notations used in different ones of these figures have same meanings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0056]      FIG. 1   a  is a perspective view representing both lenses of a pair in as-worn conditions for a wearer, and  FIG. 1   c  is a planar view of the lenses with parameters relating to the cyclopean coordinate system. 
         [0057]    In  FIG. 1   a , reference OR denotes the right eye  10  of the wearer equipped with the lens  1 , and reference OL denotes the left eye  20  of the same wearer equipped with the lens  2 . Hence, both lenses  1  and  2  are worn simultaneously by the wearer to provide him a corrected binocular vision. 
         [0058]    The wearing conditions for both lenses  1  and  2  may be the usual ones which are known in the art. In particular, both lenses  1  and  2  are mounted together within a spectacle frame (not shown) so that the back surface of each lens may be located at a distance of about 25.5 mm (millimetres) from the rotation center of the corresponding eye. R OR  and R OL  denote respectively the rotation centers of the eyes  10  and  20 . The pantoscopic angle for each lens  1  and  2  may be 8° (degree), with each lens top edge leaned ahead with respect to the wearer&#39;s face. The wrap angle of each lens  1  and  2  may have a mean value of about 5°, this angle corresponding to a tilt of each lens about a vertical axis so that the temporal edge of the lens is shifted backwards with respect to its nasal edge. 
         [0059]    Progressive lenses comprise micro-markings that have been made mandatory by a harmonized standard ISO 8980-2 published on 2004 Feb. 1 (ISO 8980-2:2004). Temporary markings may also be applied on the surface of the lens, indicating positions of control points on the lens, such as a control point for far vision, a control point for near vision, a prism reference point PRP and a fitting cross FC for instance. If the temporary markings are absent or have been erased, it is always possible to a skilled person to locate the control points on the lens by using a mounting chart and the permanent micro-markings. Indeed, item 7.1 c) of the standard ISO 8980-2 specifies that the manufacturer&#39;s name must be permanently indicated on the progressive addition lens, and item 8.1 of the same standard ISO 8980-2 indicates additional information which must be specified on the lens packaging or accompanying documents. It refers to Article 6 of ISO 14889:2003 for reciting this additional information or information which must be available on request to the lens manufacturer. This latter comprises lens alignment rules for recovering the locations of all reference points and measurement points which may be non-permanently marked on the lens, including the fitting cross, the prism reference point, the far vision control point, the proximate vision gaze direction (item 6.2 f) of ISO 14889 referring back to item 7.2 a) to e) of ISO 8980-2). Therefore, all these points are set unambiguously for any progressive ophthalmic lens, from the initial design and the manufacturing of this lens, and cannot be selected in a different way when examining this lens later. 
         [0060]    The prism reference point PRP is considered here at the midpoint of the straight segment which connects the circle-like micro-markings. When the lens is mounted in front of the eye, the fitting cross is placed before the pupil or before the eye rotation center of the eye for a primary gaze direction. The primary gaze direction corresponds to the situation where a wearer is looking straight ahead. In the chosen frame, the fitting cross FC corresponds thus to a lowering angle α of 0° and an azimuth angle β of 0°. 
         [0061]    Then, each lens may be mounted within the spectacle frame so that the gaze direction of each eye  10 ,  20  passes through the fitting cross FC of the corresponding lens  1 ,  2  when the wearer is approximately looking horizontally at an object located in front of and far away from him, with the wearer&#39;s head itself approximately vertical. G FC  denotes this gaze direction, which is commonly called primary gaze direction. Additionally for each one of the lenses  1 ,  2 , G PRP  denotes the gaze direction which passes through the prism reference point PRP of this lens. 
         [0062]    N and T denote respectively the nasal side and the temporal side of each lens  1 ,  2 . Both nasal and temporal sides are separated from each other by a meridian line ML within each lens. Within each lens, the meridian line ML is the track of the gaze direction of the corresponding eye when the wearer is looking successively at objects located at varying distances in front of him, as defined by an ergorama. To determine this meridian line ML by ray tracing, one can consider that for each gaze direction, the wearer is looking at an object located at the distance given by the ergorama and contained in the sagittal plane of the wearer. Usually, manufacturers will match this meridian line ML of an ophthalmic lens with approximately the line ML_min containing the gaze directions that corresponds to the minimum of resulting astigmatism or the line located at almost equal distance from two gaze directions through the lens respectively on the nasal side and the temporal side, with same values for the lowering angle and also same values for the resulting astigmatism. According to the monocular coordinate system, each meridian line ML or each line ML_min are usually contained in a vertical plane above the fitting cross FC, and deflected towards the nasal side N below the fitting cross FC. According to the cyclopean coordinate system, the meridian line ML is usually contained in a vertical plane for each lens containing the fitting cross FC. The line ML_min is called main line for each lens. 
         [0063]    Each lens  1 ,  2  is selected based on a prescription obtained for the wearer and indicating a prescribed far vision mean refractive power SPH_M VL , a prescribed astigmatism value CYL VL  and a prescribed addition ADD suitable for correcting the ametropia and presbyopia of each eye. The mean refractive power is obtained by summing the half value of the prescribed astigmatism to the prescribed sphere: SPH_M VL =SPH VL +CYL VL /2. For the present invention, the prescribed addition ADD is equal for both eyes  10  and  20 . Then, the mean refractive power for each eye for proximate vision is obtained by summing the prescribed addition ADD to the far vision mean refractive power SPH_M VL  prescribed for the same eye: SPH_M VP =SPH_M VL +ADD. For each lens, the prescribed far vision mean refractive power SPH_M VL  is produced for a gaze direction intersecting the lens front surface at a far vision control point VL. For example, this far vision control point VL may be located so that the corresponding gaze direction is 8° above the gaze direction G FC  within a common vertical plane. 
         [0064]    Also for each lens  1 ,  2  separately, the calculated proximate vision mean refractive power SPH_M VP  is produced for a gaze direction for which the lowering angle is positive, that is to say below the fitting cross FC. For the invention implementations which are described here, a proximate vision gaze direction G VP  is defined on the meridian line ML where the mean refractive power has reached 100% of the prescribed addition ADD, with respect to the prescribed far vision mean refractive power. Usually, for a regular progressive lens, 85% of the prescribed addition is reached between 22° and 26° from the fitting cross FC, and generally at 24° below the fitting cross FC, and 100% of the prescribed addition is reached between 34° and 38° from the fitting cross FC, and generally at 36° below the fitting cross FC. The intersection of G VP  with the front surface is called the proximate vision point VP and does not necessarily match the proximate vision control point as indicated by the manufacturer of the lens. 
         [0065]    Any gaze direction G through one of the lenses  1 ,  2  is originating from the rotation center R OR  or R OL  of the corresponding eye  10  or  20  respectively. It is identified using a lowering angle value α m  and an azimuth angle value β m . 
         [0066]    Actually, the lowering angle α m  and the azimuth angle β m  as just defined relate to each lens separately, but do not suit for describing binocular vision and assessing binocular vision performances. So the above-defined angles α m  and β m  which relate to monocular vision for each eye are to be replaced with a common angular system which defines the respective gaze directions of both eyes when operating together for binocular vision.  FIG. 1   b  illustrates the definition of such system suitable for binocular vision, as known from WO 2011/061267 for example. 
         [0067]    The rotation centers R OL  and R OR  of both eyes are represented as before, and a cyclopean eye denoted CE is defined on the straight segment which connects R OL  and R OR . The location of the cyclopean eye CE between both rotation centers R OY  and R OR  may be determined in several alternating manners. It may be at the middle position between R OY  and R OR , or shifted along this segment depending on the wearer. In particular, the Man skilled in ophthalmics knows methods for determining the location of the cyclopean eye CE from measurements performed on the wearer. Such a method may be based on the measurement of a dominance between the right eye  10  and the left eye  20 . A dominance rate comprised between −1 and +1 may be determined for the wearer in this way. The cyclopean eye CE is then superposed with the right eye rotation center R OR  when the measured value of the dominance rate equals −1, and it is superposed with the left eye rotation center R OY  when the measured dominance rate equals +1. Then, for a measured value of the dominance rate which is intermediate between −1 and +1, the cyclopean eye CE is moved from R OR  towards R OY  proportionally to the measured value. 
         [0068]    When the wearer is looking at an object point OP contained in his visual environment, a cyclopean gaze direction G CE  connects the cyclopean eye CE to the object point OP. This cyclopean gaze direction G CE  is identified itself using a lowering angle α and an azimuth angle β defined as before but with the cyclopean eye CE used as the angle apex. Then, lowering angle values and azimuth angle values are defined for both eyes, respectively α R  and β R  for the right eye  10  and α L  and β L  for the left eye  20 , corresponding to the respective gaze directions of the eyes when the wearer is looking at the object point OP through the lenses  1  and  2 . Thus, the respective gaze directions of the eyes  10  and  20  for binocular vision are defined by the values of a and  13  relating to the cyclopean gaze direction. These values are used for the maps of  FIGS. 2   a  to  2   d . According to the cyclopean coordinate system, gaze directions which are oriented downwards with respect to the fitting cross FC have positive values for the lowering angle α, and gaze directions which are oriented towards the right side according to the point of view of the wearer with respect to the fitting cross FC or meridian line ML have negative values for the azimuth angle β. Both angles α and β are zero for gaze direction G FC . 
         [0069]    Each lens  1 ,  2  is of progressive type for presbyopia-correction. Then it produces a mean refractive power PPO α,β  and a lens astigmatism AST α,β  which vary continuously depending on the gaze direction G through the lens. So the PPO α,β - and AST α,β -values vary as two functions of the α- and β-angle values for each lens  1 ,  2 . The resulting astigmatism ASR α,β  of one lens for any gaze direction G through this lens is the difference between the actual astigmatism value AST α,β  of the lens for this gaze direction and the prescribed astigmatism for the same lens. It is also commonly called unwanted astigmatism. 
         [0070]    For each lens  1 ,  2  separately from the other lens, a mean refractive power deviation for each gaze direction G(α, β) through this lens is defined as follows: 
         [0000]        DPPO   α,β   =PPO   α,β   −PPO   α,βmeridian , 
         [0000]    where PPO α,βmeridian  is the mean refractive power on the meridian line ML of the lens for the value of the lowering angle α. DPPO is thus calculated between the gaze direction G and the meridian line ML of the lens, at constant value for the lowering angle α. 
         [0071]    A total deviation combines the mean refractive power deviation and the resulting astigmatism for any gaze direction G(α, β) through one of the lenses, as follows: 
         [0000]        P   α,β   =m *( DPPO   α,β ) a   +n *( ASR   α,β ) b , 
         [0000]    where m and n are fixed positive coefficients which can not be zero simultaneously, and a and b are exponents higher than zero and less than or equal to 2. The total deviation is also defined for each lens separately from the other one. 
         [0072]    Then, two threshold values are defined for the total deviation based on the prescribed addition ADD, respectively for far vision and proximate vision. The threshold value for far vision is m*(ADD/8) 2 +n*(ADD/4) b , and that for proximate vision is m*(ADD/4) 2 +n*(ADD/2) b . 
         [0073]    For each one of the lenses  1  and  2  independently, two gaze directions denoted G 1  and G 2  are defined both with the same value for the lowering angle α as the gaze direction G FC , and where the total deviation is equal to the above-threshold for far vision. β 1  and β 2  are the respective azimuth angle values of the gaze directions G 1  and G 2 . The absolute difference in the azimuth angle β between the gaze directions G 1  and G 2 , namely ABS(β 2 −β 1 ), is a measurement of the width of the far vision field of the lens of concern, and has been called first width value in the general part of the description. ABS stands for the absolute or unsigned value. In  FIG. 1   c , this first width value is denoted H_OR_VL for the right lens  1  and H_OL_VL for the left lens  2 . In this notation, H stands for horizontal measurement direction, OR and OL stand respectively for right eye and left eye, and VL stands for far vision. 
         [0074]    Similarly, two other gaze directions denoted G 3  and G 4  are further defined for each of the lenses  1  and  2  independently. They both have approximately the same value for the lowering angle α as the proximate vision gaze direction Gyp. The total deviation equals the threshold for proximate vision as defined above at the gaze directions G 3  and G 4 . β 3  and β 4  are the respective azimuth angle values of the gaze directions G 3  and G 4 . The absolute difference in azimuth angle β between the gaze directions G 3  and G 4 , ABS(β 4 −β 3 ), is a measurement of the width of the proximate vision field of the lens of concern. It