CORRECTIVE LENSES AND PRIMATIC EFFECTS
[ This article was published in CONVERGENCE, N 36. January / February, 1988, p. 28-29. Given the difficulties to get this ancient issue of Convergence, we think it is possible to put it on this web page]
There is in France 24 million subjects who wear corrective glasses, that is 44 % of the French population. These subjects whose the acuteness is low without correction, find, for the greater part, an excellent acuteness and do not seem handicapped by their visual defect.
Nevertheless putting corrective glasses in front of an eye is not completely harmless.
The optical lens, indeed, due to its geometry and its power, modifies the visual space, the structural marks and even the intrinsic balance of the ametropy which perceives image of objects and of the world in which it evolves, through the correction given by corrective glasses.
In dynamic vision, the optical correction infers disturbances caused by the marginal prismatic effects of glasses.
During head movements, the image of objects perceived through the correction is animated by a different movement as the subject is equipped with convex or concave glasses.
So for nearsighted wearing concave lenses, the movement of the images is made in the same direction as the movement of the head: " he takes his environment with him ". Conversely for the farsighted, the movements are made the other way around: " the environment appears and comes to him ".
The adaptation of the ametrope to these phenomena comes true generally very gradually in the course of the changes of glasses, but one can understand easily that there are moments when it will be more difficult.
The pseudophake, for example, whose the ametropia is brutally transformed during the operation, sees his spatial balance suddenly destroyed, even if the residual ametropia is weak.
All the corrective lenses, due to their prismatic effects, engender spatial distortions: in shape of small cushion for the convex glasses and diabolo for the concave glasses.
In that case still, the progressive adaptation of the subject to these distortions is good, while they remain perturbantes during rough changes of correction.
In cases of anisometropia, the difference of the marginal prismatic effects entails an abnormal disparity of the eye vergences. This error of appreciation of the space is quite particularly felt in the downward glance (stairs, pavements...) because it is in these conditions that the fusion reserves are the weakest.
The sensory modifications led by these various phenomena due to the geometry of the glass are essentially perceived in spatial vision. In private hospital, the possibilities of adaptation of the subject can be controlled by the Hess-Weiss coordimeter.
These inconveniences vary with the type of the chosen glass: they can be sharply eased by advising the customer to choose glasses of small dimensions, having a reduced glass surface, or glasses of new generation with aspherical surface.
These geometries, indeed, allow a net decrease of the thickness as well as the marginal prismatic effects.
If the phenomena quoted first are essentially perceived by the peripheral retina, the prismatic correction acts on the retina as a whole . The prism is, indeed , placed in primary position of the glance and acts on the visual space as a whole. It aims at moving the perceived image in a direction well determined by the strabologist, the orthoptist or the posturologist.
These movements can be obtained by means of organic flexible lenses, like press-on®, or of conventional prisms specially surfaced or, when the power of the glass allows it, by a shift of the correction with regard to the position of the pupils in the glance far off straight front.
So, an indistinct centring of corrective lenses, a collapse of the plaques of deformed frame, or which does not remain suitably put by the patient on his nose, can introduce uncontrolled prismatic effects.
To get a first idea of this phenomenon, within our research department, we measured the average prismatic effect worn by " Sir everybody ".
To do it, we tracked down the position of the pupils of the subject when he looks far away, straight ahead in front of him, by means of Victorin's method.
This process, where one points directly on the glass of the patient the place of the pupils, allows to respect the usual position of the frame when it is worn, what is important for obtaining a measure as close as a possible of the reality.
After this operation, the power of glasses found in the frontofocometer, it is easy to calculate the prismatic effects worn by means of one abacus or the formula:
D = h x Delta
Where h represents the shift expressed in cm,
Delta the power of the glass in diopters,
and D the prismatic effect expressed in prismatic diopters.
It is by this method that 250 adults, subjects suffering from ametropia, spread out from + 5,50 to - 12,00, were examined.
DISTRIBUTION OF THE MEASURED PRISMATIC EFFECTS
|Number of subjects||Perrcentage|
|D = 0||
|0 < D < 0,50||
|0,75 < D < 1,25||
|1,25 < D < 2,00||
|2 < D < 3||
|3 < D < 4||
|D > 4||
DIRECTION OF THE PRISMATIC EFFECTS
51% basis upwards
8% basis downwards
12,5% basis lst quadrant, 0 < a < 90'
14% basis 2nd quadrant, 90 < a < 180'
11,5% basis nasal
3% basis temporal
Let us underline that 49 % of the subjects wore perfectly centred glasses, and this is happy.
Among 9 % of the subjects which have the most important prismatic effects, one often meets nearsighted equipped of frames RayBan with large surface glasses often worn far from eyes.
Finally 47 % of the examined subjects undergo a prism included between 0,75 and 2,00 diopters.
This result seemed to us particularly interesting as far as these powers are exactly those of postural prisms.