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Diopters and VA fraction

Diopters and tenths express two different quantities.

The diopters

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 tenths

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.

myopia blurred retinal pupillary diameterRepresentation of the extent of retinal blur caused by a distant point light source, as a function of the diameter of the retina pupilin a nearsighted eye. When the pupillight expands (e. g. at night), the diameter of the light beam collected by the light pupilincreases. The geometry of the figure explains the increase in retinal blur, due to the spatial extension of the bouquet of rays beyond the focus located in front of the retina (myopia). The extent of the blur (exaggerated) is represented by the double arrow. In clinical practice, the existence of high degree aberrations also contributes to the increase in retinal blur.

Representation of the extent of the retinal blur caused by a point light source away, depending on the diameter of the pupil in a myopic eye. When the pupil expands (ex: at night), the diameter of the light collected by the pupil increases. The geometry of the figure explicit increase the blur retinal, share the spatial extension of the bunch of rays beyond the home located in front of the retina (myopia). The extent of the blur (exaggerated) is represented by the double arrow. In clinic, the existence of aberrations of high degree also participates in the increase of the retinal blurring.

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.

19 responses to "Dioptries and tenths"

  1. Stephanie says:

    Thank you doctor for this article!

    My 6 year old son with very high farsightedness since birth and has a Visual acuity of 4/10. He sees airplanes in the sky, is not disrupts classroom for the moment (large section) but he does not see the faces in the distance and is clearly not reading the first lines of letters on the Snellen chart... and cela created stress.

    Blind student, explained 4/10 back to myopia-1 d has so much more sense to me! and finally a little reassuring... We hope that he can win in acuity in the coming years.

    Thank you for your lights.
    Kind regards

  2. Mendonca Taylor says:

    I would like to know what is the equivalent in diopter of a myopia of 600 degrees.

    Thanks if possible to reply to this email address: Thom0899@gmail.com

  3. Dr. Damien Gatinel says:

    Myopia is not expressed in degrees, unfortunately I can not answer your question which may be misformulated.

  4. Maite says:

    I'm very myopic and astigmatism.
    My ophthalmologist indicated that my eyesight did not change and prescribed the following corrections: OD-7.5 (105-0.75) and OG-7
    However, the previous requirement was OD-7.25 (180-0.50) and OG-6.75 (70 °-0.25)
    The previous time I had to go back to my ophthalmologist and redo the glasses because I could not stand the correction of the right eye originally prescribed OD-7 (175-1.25).
    So I have a doubt and would like to know if the new correction is important. I am afraid I will not support it or worsen my myopia if it is "overcorrected". Indeed, I have the same correction for many years.
    Thanks in advance for your help
    Well cordially

  5. Dr. Damien Gatinel says:

    The increase in the correction is quite low, but if you are not embarrassed with the current lenses there is no reason to change your correction and make this new pair of corrective lenses.

  6. Dick Dimitri says:


    I'm nearsighted (-3.25 left eye and-3.75 right eye). I would have liked to know what visual acuity that would be roughly?
    Even approximately because as indicated above, there is no simple formula for converting diopters to visual acuity.

    Thank you very much for your eventual answer.

  7. Dr. Damien Gatinel says:

    This correction is certainly at the origin of a acuity of less than 1/10th. It is normal not to find reliable equivalence tables, because one of the parameters likely to vary the acuity is the diameter pupillary (myopics badly or uncorrected squints to reduce slightly the diameter of the pupil input, i.e. the diameter of the light beam allowed by the eye) to reduce the retinal blur (the diameter of the defocusing spot is narrower when the permissible beam is narrower: cf. Thales theorem!).

  8. Mina says:


    I am myopic, OD-2.5 and OG-15/-16 not corrigable, can you give me the conversion to 10th and is it spawnn for the passage of the permit or not?

    Thank you in advance for your help.

  9. Dr. Damien Gatinel says:

    It is not really appropriate to convert the diopters to tenth in your case. What is more important is to verify that you have the number of tenths required for driving (with or without correction, and it will be with course in your case). If your myopic eye-2.50 reads 8/10e (or more) with correction, you will be able to pass the driver's license without problem a priori.

  10. Chateau says:

    Oschenros Thomas says:
    9 JULY 2017 AT 23 H 22 MIN
    I would like to know what is the equivalent in diopter of a myopia of 600 degrees.
    Hello, I am faced with this same problem to buy scuba goggles on the Internet
    The choices are expressed in degrees
     » 200-700 degrees Nearsightedness Replacement lens for professional scuba diving mask goggles Water Sports equipment YS-»
    Thank you for your reply

  11. Dr. Damien Gatinel says:

    Myopia is not expressed in degree, there must be an error on the site concerned!

  12. Olivier says:

    In the image of the people above, I came across a job offer that requires a myopia less than 600 °:

    "Myopia of not more than 600 degrees and astigmatism of not more than 200 degrees per eye"

    So this expression in degrees means something, right? Maybe she's converting to diopter? (600 degrees = 6 diopters?)

  13. Dr. Damien Gatinel says:

    This is probably a myopia less than 6.00 diopters (6 diopters) and not a number or "degrees" score that would not correspond to anything in this case . For astigmatism, this is a limit set at 2.00 (two) diopters.

  14. Sébastien says:

    Hello, I would like to know my visual acuity with this correction
    Right eye (165 °-1.25) + 0.50
    Left eye (0 °-1.75

  15. Dr. Damien Gatinel says:

    It is difficult to predict your visual acuity from your correction: Many factors intervene to modulate it, in particular the diameter of the pupil ; It is certain that your visual acuity is better in situations where the brightness is high (outside in good weather), because the reduction of the diameter of the pupil Induces a reduction in retinal blur. Under such conditions, the visual acuity of your right eye must be at least 8/10th.

  16. Lili says:

    Hello, I unfortunately developed a myopia of-0.75 for the 2 eyes. As a result, I wondered what it was like in Visual acuity. Thank you in advance for your reply.

  17. Dr. Damien Gatinel says:

    It is not really possible to give you a precise answer: the acuity can be around 8/10 with both eyes in case good lighting and good contrast of the observed target, but be much lesser under low light conditions, due to the increase in the pupillary diameter that mechanically induces an increase in retinal blur.

  18. Vero says:

    I want to buy a diving mask at my sight. I carry-2.25 (-0.75 to 90 °) and-3.25 (-0.75 to 90 °)
    Should it have astigmatism corrected or can it be neglected or make a spherical equivalent?

  19. Dr. Damien Gatinel says:

    I think that the correction of astimgatism is not really required, and that a spherical equivalent is simply corrected in the mask.

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