Diopters and VA fraction
Diopters and tenths express two different quantities.
Thediopters (optical power unit or vergence, symbol: D) quantify the importance of an optical defect such as myopia, hyperopia, hyperopia, hyperopiaastigmatism where nearsightedness. They characterize the power (vergence) of an optical system (or the magnitude of an optical defect as often in an ophthalmological context).
The diopter unit is equivalent to the inverse of the metre (m^-1): it is therefore Unlike a distance.
A -2 D short-sighted person can see effortlessly at 1/2 = 0.5 meters (50 cm). A short-sighted person of -4 D sees more blurred from a distance, because he sees sharply and effortlessly at 1/4 = 0.25 meters (25 cm). It is conceivable that the greater the myopia (the closer the effortless clear vision is) the more the visual acuity at a distance will be reduced.
The It allow to quantify theVisual acuity in distance vision (e. g. 10/10, 8/10, etc.) with one eye corrected or not.
This acuity is measured in clinical practice for an almost "maximum" contrast (the letters are black on a white background). Visual acuity is related to the eye's ability to separate, which is based on the Minimum Angle of Resolution (MAR) used to separate two points (see : calculation of the MAR, and MAR).
Diopters of myopia and loss of tenths
The relationship between the magnitude of myopia and the reduction in uncorrected visual acuity from a distance is not linear. A -1D myopia does not necessarily cause a loss of twice as many tenths as a -2D myopia... There is no simple formula for converting diopters to tenths. Of course, the more the number of diopters on the formula of the glasses (or lenses) is high, the smaller the number of tenths will be, but a real proportionality relationship cannot be established.
It is estimated that:
A myopia of-0.25 D reduced acuity to 9/10
A myopia of-0.50 D reduced acuity to 7/10
A myopia of-1 D reduced acuity to 4/10.
A myopia of-1.25 D reduced acuity to 2/10
Role of the pupillary diameter
These values (number of tenths for so many diopters of myopia) vary depending on the pupillary diameter : more myopia is important, and more from rays from a point in the distance converge in front of the retina. As a result, for the same myopia, the pupillary diameter is large, and more the width of the task that these divergent rays form on the retina is important.
It is for this reason (pupillary dilation) that myopes see less well in the evening and at night (the decrease in brightness causes an increase in the diameter of the eye pupilso that it captures more light).
Some weak nearsighted people (e. g. -0.50D) are happy to do without glasses during the day, but feel the need to do so at nightfall, and cannot do without them to drive at night.
The size of the retinal blurred spot can be estimated to be equal to Dp x M/60 where Dp is the diameter of thepupil, and M is the myopia of the eye.
For example, for a myopic eye of 1 D and a pupillary diameter of 4 mm, the task of illumination is equal to 1 x 4 60 or 0.066 mm (66 microns). More this task grows, and more the resolving power of the eye is reduced. To separate two points (it's the resolving power), then their retinal images must be separate from a distance at least equal to half their diameter (here 33 microns).
The angle MAR (Minimum Angle of Resolution or minimum Angle of Resolution or AMR) is the minimum for separate two observed points. L' MAR angle can then be estimatedusing a focal length equal to 17mm: in this example it is 6.5 minutes of arc, which is equivalent to a theoretical visual acuity of 1/6.5=0.15 = 1.5/10.
In clinical practice, -1 D myopia is most often synonymous with visual acuity close to 4 to 5/10: the calculation of MAR does not take into account certain neuro-cognitive factors (even fuzzy views, it is possible to guess the nature of the letters presented during the visual acuity test).
These figures are of course approximate and from simplified geometric models. Even seen a "fuzzy", the letters may be identifiable: reading letters of varying size (from the largest to the smallest) is often used to quantify the Visual acuity, and this type of pattern can "facilitate" a Visual acuity somewhat better than what would be predicted by the theory. Many works are devoted to the study of the relationship between Visual acuity and optical aberrations.