Updated June 2021
Reduced visual acuity can be caused by the eye or brain conditions, or both. With CVI we are looking at visual acuity in relation to the brain.
Visual acuity if often thought of in terms of the smallest thing you can see, for example a marble, a peanut, a grain of rice. These are not measurements, they are observations.
To be useful, visual acuity needs to be measured.
The measurement of visual acuity is the measurement of the width of the line that allows you to separate things, to let you see them clearly.
Visual acuity is a measure of the thinnest black / white line gap and line thickness that can be seen apart from each other.
As things look smaller the further away they are, measures of visual acuity have to take this into account.
This is done in two different ways.
This is a complex calculation preferred by some optometrists, ophthalmologists and vision scientists, because the result is more precise, and the scale is decimal. Visual acuity when measured this way is recorded as LogMAR.
Normal (6/6 or 20/20) vision equates to 0.0LogMAR (see conversion table below).
There are two types of visual acuity
1. Threshold visual acuity
Meaning a measure of the smallest black / white detail that can be seen. This is tested with each eye in turn.
2. Functional Visual Acuity
Meaning the measurement of the smallest detail (not blob or mark) that can be seen comfortably at the highest speed possible with both eyes open in normal lighting conditions, even at the end of the day when the person is getting tired.
As clarity of vision is lost with reducing acuity in these LogMAR charts (below), look at what has happened to the clear lines that defined the letters.
These images are made using a programme called SightSim which gives an approximate indication of how someone with the measured levels of vision shown sees when compared with someone with typical vision
The lines of separation are rarely nice neat straight white lines, with thick black borders as with the letter E.
On our Visual Acuity page in the previous section, we explained clarity of vision using some ladybirds as an example.
Clarity of vision, that is to see the ladybirds clearly, can only be achieved if you can see the separate parts. With the bottom picture there is not enough information to confidentially recognise the 'red blobs' as ladybirds.
This is the one of the ladybirds from the picture.
It is our ability to see these things separately that helps us give meaning to the image.
Our visual acuity is the measurement of the minimum thickness of the line that separates things, and the thickness of the things themselves, allowing clarity of vision.
With this measurement you will be able to understand:
To assess visual acuity, we are looking for a line thickness, and identical gap thickness. As with our letter E (above) if the space between the lines is narrower than is visible, then what is on either side merges to become one, meaning the clarity is lost, and the letter cannot be identified.
Above is a conversion chart. 0.00 (highlighted) is considered 'normal' visual acuity, also called 6/6 or 20/20 vision. Numbers less than this mean that vision is better than 'normal'. Numbers bigger than this means that vision is less than normal.
The first two columns, Imperial and Metric, are the same measurements, just using feet or metres as the measure. The first number 20 (Feet) or 6 (Meters) is the distance you are standing from the assessment letters. If the test is conducted in a smaller room without a 6m distance available, then the letters on the testing chart are made proportionately smaller to allow for the closer distance, to ensure a consistent accurate result. The distance from the visual target at which the test is done is key to measuring visual acuity.
6/19 vision would mean that the smallest detail the person with that level of vision sees at 6m, can be seen by a person with normal (6/6) vision at 19m. This can be a useful way of understanding how someone with reduced acuity sees things - finding out how different something looks when viewed from just over three times the distance.
Another way is to look progressively further to the side of the required larger target until you can just make it out. This is quite like what the target looks like for people for whom this is the smallest target they can make out when looking straight at it from the same distance. If you master this method, by knowing how far to the side you need to look to just make out the target, you can then always look this far to the side of anything you are showing the person with reduced visual acuity, to check it is just visible (visual acuity), and how much less to the side you need to look to check if it is easily visible (functional visual acuity). This method works because our clarity of vision is less to the side than in the centre and it gets lower the further away from the centre we are concentrating on what we are looking at.
The line width needed to see nearby relates to the minimum thickness a line has to be, at the person's 'arms-length', approximately which of course gets greater the older the child.
Connor's visual acuity is 1.5 LogMAR. From our table we can see that the metric equivalent is 6/190. That means that Connor's mother would need to stand 190m away from whatever Connor was looking at from 6m (or more than thirty times further away), to achieve a comparable acuity simulation for herself.
Arms-Length - Between 15 and 30cm depending on size.
Typically, it is the distance we:
Connor's visual acuity of 1.5 LogMAR can be converted to a line width of 2.78mm (rounded up to 2.8mm on the table, above). At arms length, anything with a width below 2.78mm (just over 1/4cm) is not visible to Connor. This includes the spaces. In the diagram below both the lines and spaces are of about thickness Connor can see (depending on the size of your screen).
If the lines were kept the same width, but the spaces reduced to 0.5mm (not to scale), as below:
Then this (below) is what Connor would see due to his 6/190 visual acuity.
The spaces are too small to be visible to Connor. Connor could no longer see the lines. Connor saw a big blob of black!
For Connor - everything that is presented to him at arms-length needs to be at least 2.78mm to be visible. This includes the detail - if it is a picture, all the details in the picture need to be at least 2.78mm wide.
For all Connor's favourite picture books his parents have enlarged the details (like eyes and other facial features), so that Connor can see these details to be wider than 3mm and separated by more than 3mm, to make sure he can see these details.
Understanding visual acuity in terms of a minimum line width gives people a critical guide for how big everything has to be for the person to just see it. Not just the book, but the text within the book. Not just the pictures in the book, but the detail in the pictures in the book. Not just the toy but the detail on the toy. Everything.
To see how different levels of visual acuity affect what visual information is available to us, the following picture has been edited to demonstrate the different levels of reduced visual acuity.
Further examples of images that have been edited to show different reduced levels of visual acuity can be found in Simulated Images section.
The reason the images become harder to see clearly and make sense of is because the detail - the visual information that enables us to see things separately - is too small and lost.
One important element to see from these pictures is that variety of the primary colours helps one to see the elements as separate as well.
Assessing visual acuity does not aim to provide the person with the minimum size things have to be to be strained to be seen. In the same way a spectacle prescription should allow comfortable vision, when assessing functional vision the aim is to make things easy and comfortable to be seen all the time.
Once you have the measurement of visual acuity, look at the things the person uses and consider how much of the visual information they can see to make meaningful sense:
Typical young children learning to read, are given books with large simple well separated words, short sentences and pictures.
As the child's reading abilities and understanding of words improve they move on to books with more grown up language. Sometimes those books have much smaller and more compact text. So the child's vision needs to deal with the smaller text size and the crowding of many more words.
This needs the development of their eyesight, and their ability to handle more crowded print to keep up with this change. In our experience of our children this does not happen. This can mean that a child who was a confident learner reader suddenly starts to struggle with reading.
This could be because they can't see the words clearly, because of a combination of there being too many, too small and too closely spaced words. Try the same words in an e-reader like our reading tool LOOK, and experiment with different font sizes and different numbers of words on the screen at a time. This should show if the reason for the difficulty is to do with reading ability, or visual difficulties or both.
There are many resources in our section Visual Acuity - Free Resources.
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