Diopters and VA fraction
Diopters and tenths express two different quantities.
Thediopters (unit of optical power or vergence) quantify the importance of an optical defect such as myopia, hyperopia, theastigmatism where the myopia. A myopic-2 D sees net without effort to 1/2 = 0.5 meters (50 cm). A myopic-4 D sees net without effort to 1/4 = 0.25 meters (25 cm). We conceive that most nearsightedness is important, far more Visual acuity will be reduced.
The It allow to quantify theVisual acuity distance vision (ex: 10/10, 8/10, etc.). This sharpness is measured for a "maximum" contrast (the letters are black on a white background). Visual acuity is related to the resolving power of the eye, which is based on the angle of minimum resolution (Minimum Angle of Resolution: MAR) to separate two points (see:) calculation of the MAR, and MAR).
Diopters of myopia and loss of tenths
However, a myopia of 1 d does not necessarily lose tenths as a myopia of 2D double... There is no simple formula to perform a conversion of diopters in tenths. Of course, the more diopters appearing on the formula of the glasses (or lenses) is high, the more tenths will be low, but cannot establish a relationship of proportionality.
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.
That is why that short-sighted see worse at night and the night (lower brightness causes an increase in the diameter of the pupil). Some low myopic (ex:-0.50D) happen to glasses in day, but cannot stop for driving at night.
It is estimated that the size of this task is equal to Dp x M/60 where Dp is the diameter of the pupil, and M 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 estimated, using a 17 mm focal length: it is in this example 6.5 minutes of arc, which equals an acuity Visual 1/6.5=0.15 theoretical = 1.5/10. In clinical practice, a myopia of-1 D is often synonymous to a Visual acuity close from 4 to 5/10: the MAR calculation ignores some neuro-cognitive factors (even seen blurry, it is possible to guess the nature of the letters presented at 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.