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One of my friends went to an eye specialist for checkup and the doctor told him that his eye power changed from -6 from -4.5. He is really very upset and doesn’t want to discuss anything on this. I can understand that -6 means the vision is in very poor state but don’t have any idea how much worse it is.

  1. Could someone explain me how what eye power -6 means in respect to closeness of blindness?
  2. What can be the minimum negative eye power and maximum positive eye power of an eye?
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  • I'm voting to close because I think the question as posed is unclear and too broad to be answered. For starters I think you need to define what you consider to be blindness. The numbers you mention appear to be the myopia correction values used by optometrists when writing lens prescriptions, but does severe myopia constitute blindness?
    – Carey Gregory ♦
    Commented Oct 10, 2015 at 2:03
  • I think you don't understand. -6 doesn't mean your eyes are weak, it means they are focusing too close. It's a measure of the corrective lenses needed to give normal vision. My wife has been extremely nearsighted her whole life, -9 or worse in the decades I've known her. If anything her vision has improved slightly over that time--it's an annoyance, not a sign of impending blindness.
    – Loren Pechtel
    Commented Oct 11, 2015 at 4:06
  • "He is really very upset and he even doesn’t want to discuss anything on this." Yes, and this is why people are entitled to privacy of their medical information. Prying is rude. Asking around (including the internet) instead of the person directly doesn't make it less so. Voting to close.
    – Lucky
    Commented Oct 19, 2015 at 12:44

2 Answers 2

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Your units of measure are likely "diopters." If someone needs glasses for reading (because the person is far sighted), then they would be given an prescription with a +[digit], and note that each eye could be different (and usually is, at least slightly). An example might be:

left eye: +1.5
right eye: +0.75

If he has a negative number of diopters in his prescription, it just means he is instead near sighted, or myopic: he sees things up close probably pretty well (in the eye with a negative diopter). If both eyes are around -6 diopters (and assuming he doesn't have severe astigmatism on top of this strong near sightedness), he probably needs relatively strong lenses to see distances clearly, and can probably only read an average size font (such as 12-14 points) if it less than a foot (roughly) from his face.

See also: http://www.britannica.com/technology/diopter-optics and "Amplitude of accommodation" on wikipedia

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I am going to assume with - 6 you mean a measurement of - 6 Dioptre . If you are talking about the strength of the glasses he needs, the minus indicates that this eye is nearsighted, a positive value would indicate farsightedness. If that is a measurement of his eyesight, he is farsighted. A measurement of 0 indicates that the eye can adapt to both short and long distances without problems (though corrections may still be necessary, for example for astigmatism). Because of the definition of a Dioptre, there is no maximum/minimum number.

A dioptre (uk), or diopter (us), is a unit of measurement of the optical powerof a lens or curved mirror, which is equal to the reciprocal of the focal length measured inmetres (that is, 1/metres). It is thus a unit ofreciprocal length. 

A measurement of -6 is certainly not nothing and will require corrective lenses. It is, however, far from being blind and can be corrected by wearing glasses or contact lenses. For nearsightedness, -6 is where high-degree myopia begins.

If you are talking about whether he could be considered legally blind , that is a definition that only applies to how much you can see with correction. Since myopia of -6 can be corrected well with glasses, there should be no risk of being defined as legally blind.

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