I never really put much thought into it, but when I was young I thought you either had perfect vision or no vision at all. I didn't realize there was a huge grey area.
Did you know that most people with vision loss have some useable sight? Imagine vision on a continuum in which you have perfect vision at one extreme and absolutely no vision or "no light perception (NLP)" at the other end. Most people lie somewhere between these two extremes. In fact, 90% of clients at CNIB have some sight. This can range from being able to read large print to being able to detect only whether a light is on or off.
There are two measurements that are used to evaluate a person's vision: visual acuity and visual field. Let's take a closer look at these two measurements:
1. Visual Acuity. This measurement refers to your central vision or reading vision. A person with perfect vision has 20/20 vision. In the visual acuity measurement, as the number in the denominator increases (e.g., 20/200, 20/400, 20/800), vision decreases. What does this mean? Someone with 20/200 vision needs to be 20 feet away from an object to identify it. A person with perfect vision would be able to identify this same object from 200 feet away.
2. Visual Field. This measurement of the degrees of vision refers to peripheral vision. A person with perfect vision has 180 degrees of vision in the horizontal plane.
It is important to note that these two measurements (visual acuity and visual field) are determined with the best possible correction (i.e. prescription lenses) in the better seeing eye.
What a person is able to see will depend on whether the sight loss is a central vision loss, a peripheral vision loss or both.
Central vision is your detailed vision and colour vision. People with central vision loss will have trouble reading fine print or identifying details like faces, stairs, curbs or uneven pavement. Macular degeneration is an example of a central vision loss. A central vision loss can be simulated by smearing Vaseline on your eye-glasses.
Peripheral vision is your travelling vision. People with a peripheral vision loss may have problems navigating their environment (e.g. bumping into door frames, people, or other obstacles). Glaucoma and retinitis pigmentosa are examples of peripheral vision loss.
A visual field loss can be replicated by holding two toilet paper rolls up to your eyes.
Blindness is often misunderstood by the general public, but it makes more sense if you take into account the type of vision loss that the person is experiencing. For example, people with central vision loss may not be able to see a pin on the floor if they stare straight at it. But they may be able to see the same pin on the ground in their peripheral vision if the pin reflects sunlight. Similarly, people with peripheral vision loss may require a white mobility cane or guide dog to get around but the same people may be able to read the newspaper if they have good central vision.
If you would like to get an idea of what it would be like to look through the eyes of someone with a central or peripheral vision loss, you can download the app called, "iSimulator", for free on iTunes.
Do you know what legally blind means? Legally blind is a term that is often misunderstood. It is simply a point on the vision continuum described earlier. It is denoted by a visual acuity of 20/200 or a visual field of 20 degrees. A person who is legally blind has vision of 20/200 or worse or less than 20 degrees of vision. If you can see only the E at the top of the Snellen eye chart, you have 20/200 vision and you are considered legally blind.
I hope this has brought some clarity to blindness. After all, it's not all black and white!