Question on Dioptre

[Engineer]

Can any one enlighten me on how the dioptre limits -3 or -5 relates to the Snellen eyesight chart measurements ie 6:6 or 20:20(for the imperial people amongst you).

Ta!

[FireDragon]

This can be an interesting one... To explain:
As you are probably aware, the '20', (or '6')mark on the snellen chart indicates 20:20 vision - this is not ACTUALLY perfect, (some people can see better than this), but is the perfect 'standard' that we strive to achieve.

Correction, however, may depend on the individual. To quote a case, take 2 people, both with 20/200 vision: (You would normally find correction for this around -3 to -4 prescription-wise). BUT, one person may respond to a -2 for required correction, and the other person require a -5 to achieve the same clarity. (I have personally seen evidence of this, but it may not be the norm - if anyone knows differently, please feel free to correct me).

I have found that the reason is usually down to factors such as previous prescriptions. For example, a person who is 'monocularly challenged' (could have 20:20 in one eye, and 20/200 in the other), may have gotten along for years without correction, the brain will compensate for the weak eye; when they DO decide to correct the weak eye, (for whatever reason), they usually find that a much weaker dioptre strength will be required for clarity. Another person, (20/200 in both eyes, and so NEEDING the correction at a much earlier age) - THEY CAN'T SEE! - who's occular system has become accustomed to repeatedly stronger prescriptions, will normally require much stronger lenses to achieve that same clarity.

Unfortunately, it appears to be the case of: "if you don't use it, you will lose it", with perhaps a better option being some form of vision training, (such as the Bates method).

FD

[Engineer]

FD
Thanks but still not clear My limited understanding is that the 6m line on the chart(fourth line up correct me if I am wrong)when read at a 6m distance gives 6:6(20/20) vision

My question is can you relate the dioptre figure given on your prescription to your visual acuity without refractive correction.

My interpretation of your post is that two people with say 6:24 vision could be prescribed with different refractive correction correct me if I am wrong "excuse the pun"

[inverted flatspin]

Engineer, you can not equate prescription strength with unaided visual acuity. The diopter is a unit of measurement for Lenses, so that the guy who makes your specs is on the same page as the optician. It serves no other purpose. Visual acuity is measured on the eye chart. The JAA are on very weak ground scientifically on this one. There should be no Diopter limit, just correctable to 20/20 or 6/6 which ever you prefer. This concept is proven safe beyond all possible doubt from the experience of the FAA.

[FireDragon]

Engineer

You are correct in your interpretation of my post - you cannot simply give a universal prescription of minus whatever for everyone who has, say, vision of 6/24 - people will react differently.

I can see, (pardon pun!), where your question is coming from, but as Inverted Flatspin says, you cannot equate prescription strength with UNAIDED acuity.

Hope this helps

FD