How can we have more 10/10th of Visual acuity?
This question comes up frequently in the mouth of the patient or consultants. Visual acuity in question corresponds to the resolving power supposed maximum, namely the ability of the eye to distinguish as distinct two points loosely spaced and located at a given distance. That's what ophthalmologist measures when it shows his patient of the letters of decreasing size. To distinguish an 'E', the eye must discern (separate) the dark and light horizontal bars forming the E (in addition to the vertical bar). Plus the letter, the more its constituent elements form a low angle with the eye. Above a certain size, the eye is no longer able to distinguish (separate the constituent elements) the letter projected onto the table. What for?
There are two factors limiting the Visual acuity:
-One is physical, it is the diffraction : the image formed on the retina by a basic point (ex: Star) is not a point but a "disk", whose size depends on the diameter of the pupil.
-The other is anatomical, but the size of the photoreceptors of the fovea (the fine "retinal grain" depends on the size of the cones: more these are small, more spatial density is high, which allows sampling of the image projected on the retina).
(to deepen, consult the training dedicated to Visual acuity page)
It is remarkable to observe that these two factors converge to the same value, close to 2 microns. Indeed, the minimum size of the image of a point source formed on the retina cannot be smaller than 2 microns when the best optical conditions exist (perfectly corrected eye, then limited only by the inevitable diffraction). And precisely, the cones of the fovea, which are smaller, have a diameter close to 2 microns and a distance between the centers of cones almost contiguous but separated by a cone (to separate two points sources) close to 2.5 microns (this distance is less than twice the diameter of the smaller cones because of the paving of hexagonal nature).
A calculation involving the distance between the retina and the system optical of the eye (especially the distance between nodal point and fovea) as well as the spacing between the photoreceptors minium (hexagonal paving of photoreceptors can provide a density of cones near 200 000 cones per mm2, 120 000 cones by degree or 60 cycles per degree) shows that the minimum angle that can solve the eye to separate two distinct points is 30 arc seconds (one half minute of arc), which corresponds to Visual acuity of... 20/10! (By the way, remember that Visual acuity in tenth is a fraction which is equal to the inverse of the minimum separable. This minimum is an angle whose top is the eye, called minimum angle of resolution or MAR - for Minimum Angle of Resolution. This angle is expressed in minutes of arc. A minute of arc is equal to a sixtieth of a degree. For an angle of a minute of arc, acuity is equal to 1/1 or 10/10).
Then why don't we not always achieved this Visual acuity of 20/10? To benefit from this sharpness, that the eye be perfectly corrected in optical terms: glasses correct that the so-called flaws of "low level" and some other defects (aberrations of high degree) can do "lose" a few lines of Visual acuity. There are also the retina (fovea) is in perfect condition (lack of macular abnormalities), and that the cones (photoreceptors by mm2 number) density is high enough: However, significant variations were measured between the eyes of different individuals (100 000 cones by mm2 at more than 200,000 cones by mm2 at the center of the fovea).
In practice, the acuities of 12 to 15/10 or 20/10 can be commonly measured in young patients (17-25 years) and without eye abnormalities (other than a possible refractive defect - ametropia). In older patients, Visual acuity is close from 10 to 12/10.
A Visual acuity of 10/10 is only a threshold below which we can consider that Visual acuity is "abnormally" low.
It should be remembered that Visual acuity is only one of the elements of appreciation of the Visual function. There are many others such as sensitivity to contrast, detection acuity, chromatism, Visual field perception, motion detection, etc.