At what prescription do you become legally blind?

You would think knowing if you were blind or not would be a simple thing.

You can’t see nothin’. The world beyond your nose is just one big blur, and trying to do anything without your contacts or glasses always ends in harm or disaster.

But nothing is ever that simple, and not least our vision. Your eyes are one of the most complex systems in the known universe, and there are as many types and degrees of vision as there are shades of colours you can see — approx 100 million.

The result: knowing if you qualify as blind or not is a little more tricky than you first may think.

There are around two million people living in the UK with some level of sight loss, whether that be due to longsightedness, shortsightedness, or astigmatism. Around 360,000 of them are registered as blind or partially sighted.

To be registered as blind or partially sighted involves undergoing various measurements and scans performed by an eye specialist or “consultant ophthalmologist”. Through this process, it is determined that you meet the criteria for certification of blindness or partial sightedness by the Royal National Institute of Blind People (RNIB).

The examinations involve testing your level of visual acuity — the clarity or sharpness of your vision — and the quality of your visual field — the area that can be seen when the eye is directed forward, including peripheral vision — and then combining the two results together.

This is where things get tricky. For example, if you have a good visual acuity, your visual field will have to be severely compromised in order for you to be certified as blind (severely vision impaired) or partially sighted (vision impaired). Similarly, if you have all your visual field, your visual acuity will need to be very poor to meet the RNIB criteria for certification.

Figuring out if you are blind or partially sighted

Although it’s not so straightforward to know yourself whether you qualify as blind or partially sighted, the process for finding out is well-established and pretty straightforward.

As mentioned, your eligibility is dependent on how your eyes perform in two areas, the first being visual acuity.

The familiar Snellen Test, which consists of reading rows of smaller and smaller black letters on a white chart, is the widely used method of measuring for visual acuity. The average standard of visual acuity is 6/6. This level means you can read all or most of the chart from six metres away, and it acts as the standard for which all other measurements are based.

For instance, if you can only read the top line of the chart, your visual acuity would be 6/60 — i.e. you can see at six metres what someone with standard vision can see at sixty metres.

A visual field test is more interactive. Typically, the ophthalmologist will cover one of your eyes, ask you to focus on a target object, and pose questions about what you can see on the far edges and periphery of your vision.

With the results from your visual acuity and visual field tests — which are performed while wearing your glasses or contacts — the eye specialist will determine if you are severely sight impaired or blind, by seeing if your results fall within one of the following categories (as outlined on the RNIB website):

  • Visual acuity of less than 3 / 60 with a full visual field.
  • Visual acuity between 3 / 60 and 6 / 60 with a severe reduction of field of vision, such as tunnel vision.
  • Visual acuity of 6 / 60 or above but with a very reduced field of vision, especially if a lot of sight is missing in the lower part of the field.

The following categories are used to determine if you are sight impaired or partially sighted:

  • Visual acuity of 3 / 60 to 6 / 60 with a full field of vision.
  • Visual acuity of up to 6 / 24 with a moderate reduction of field of vision or with a central part of vision that is cloudy or blurry.
  • Visual acuity of 6 / 18 or even better if a large part of your field of vision, for example, a whole half of your vision, is missing or a lot of your peripheral vision is missing.

But wait, there’s more still to consider. For instance, you may not qualify if you only have poor vision in one of your eyes, your sight loss isn’t permanent, or if you’re currently receiving treatment that could improve your sight.

Whatever the case, it’s worth getting a referral from your GP or going straight to an expert ophthalmologist to figure out exactly what’s what. That way you’ll find out with certainty what your level of vision is. Once you know that, then you can start focusing on your treatment options — a process that could be surprisingly positive if you contact the right clinic that can safely treat very high prescriptions.