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What is CVI?


Visual Acuity

Visual acuity — also known as clarity of vision — is a measure of the finest detail that can be seen, or resolution, just like on a television set. If you watch something with poor resolution the fine detail can't be seen. It is our ability to see the detail in the picture, that is to see things as being separate, that helps us make sense of what we are looking at.

With normal vision, technically referred to as 0.0 (previously referred to as 6/6 or 20/20 vision) we can see in glorious high definition. Look at the pictures below, would you be able to identify the ladybirds in the second and third pictures?

Visual Acuity 6/15 (or 20/50 0.4LogMAR)

Visual Acuity 6/38 (or 20/125 0.8LogMAR)

These images (made with a beta version of SightSim software), give an approximate indication of how someone with the measured levels of vision shown sees when compared with someone with typical vision.

The meaning of the exact measurements will be explained in Section 2.

We are used to having visual acuity measured with a test like the one below, known as logMAR.

If you were having a sight test and could read the third row from the bottom of the standard chart (from 6 meters away), you would be considered to have 0.0 (normal) vision.

Look at the degraded eye charts to the below, what is it about the letters that mean you can't correctly identify them anymore?

Where you can still see a mark, or a blob, or a blur, but can't make out the actual letter, you no longer have clarity of vision — visual acuity is a measure of clarity.

Visual acuity measures tell us the thinnest line that can be seen at the distance tested.

Not everyone will be able to do a conventional eye test, like people who don't understand letters or can't say what they can see. For them, there are other ways to do the tests.

The correct understanding will tell us into how big, far apart, and near things have to be, to be to be seen and understood, and the effect their visual acuity has on how the individual sees the world.

Children can only learn from what they can see, so it is essential to measure this and make sure that everything they have to learn from is visible, and if it cannot be made visible, find different ways to help them learn.

Would the child with reduced visual acuity be able to make sense of the story from the pictures?

Would the child with reduced visual acuity be able to read the writing?

Where have The Gruffalo's eyes gone?


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At CVI Scotland we are devoted to helping people understand cerebral visual impairments, and together working towards developing the understanding of this complex condition.