Perfect Vision, Visual Acuity, and 20/20 Vision – What’s The Difference?
Do you know what is perfect eye vision? Well, before this, you must understand the difference between eyesight, visual acuity, and vision. Because only then will you understand the vision measurements like 6/6 or 20/20 vision. Therefore, in this blog, we will discuss the following points about vision: –
- What is perfect eye vision?
- Difference between visual acuity, eyesight, or vision
- Factors determining the visual acuity
- Types of vision problems
Eyesight is a generic term used to refer to vision or the viewing ability of the eye.
Vision or Sight is one of the five basic human senses. It refers to the act or power of sensing with the eyes. Thus, vision is the manner in which an individual sees or visually perceives something.
Both these terms describe the ability of the eyes to sense but they do not quantify the ability.
Visual acuity is the term used to quantify the ability of the eyes to see i.e. eyesight or vision.
What is perfect eye vision?
As such the concept of perfect vision may not be scientifically correct. The Visual Acuity is tested on a standardized chart, at a standard distance in a testing room, or recommended dimension and lighting conditions. Hence, we compare the visual acuity in each eye with a standard that has been arrived at with some scientific and mathematical assumptions. Hence, the concept is more of a minimum standard measure of vision rather than what is perfect or normal vision. This standard visual acuity of 6/6 or 20/20 is interchangeably referred to as “normal” or “perfect” vision. However, a person can have better than 6/6 (20/20) vision in one or both eyes.
What is 6/6 vision?
We discussed that It is found that light rays traveling from a distance of 6 meters are almost parallel. When we look at an object that is 6 meters away, our eyes’ internal focusing mechanism, known as accommodation, is generally found to be at rest. Hence, it is ideal to check the visual acuity of the eye without the internal focussing of the eye interfering with the measurement.
The expression 6/6 is made of two numbers. The numerator (the number before the ” / ”) refers to the distance at which the visual acuity is being tested. “ 6/6 “ means that the eye can resolve or see an object (a letter) of a size meant to be seen by standard visual acuity, at a distance of six meters from a distance of six meters.
If the vision is less than standard vision, it could be 6/9 i.e. the eye can resolve letters meant to be seen at 9 meters only when they are placed at 6 meters. If the vision is better than 6/6, it could be 6/5, then it means the eye can resolve objects (letters) meant to be resolved at 5 meters even if they are moved further away at 6 meters. Hence, the denominator (the number after the “ / “) refers to the smallest object that the eye can resolve.
What is 20/20 vision?
Effectively 20/20 is the same as 6/6. It is only a difference in the different systems of measurements followed in different countries. While most countries use the metric system in which distance is measured in meters, other countries like the US follow the imperial system of measurement in which the distance is measured in feet. Six meters is approximately 20 feet (19 ft 8.2 inches to be precise). Hence, in countries following the imperial system, standard visual acuity is expressed as 20/20.
What is the purpose of visual acuity?
Visual acuity is a quantitative measure of the eye’s ability to see. It helps determine the standard vision for human beings or what may be called “normal” vision. Anything less than the “normal” value determined may need clinical investigation and remedial measures.
It also helps to compare the improvement in eyesight over time if a person is undergoing any treatment or vision therapy or the deterioration in eyesight with age, due to some disease, etc.
How is Visual Acuity measured?
Visual Acuity is most commonly measured using a Snellen’s Chart. Snellen’s is the most popular Vision Chart that is easily identifiable. It has a single largest letter on the top. This is followed by a row of two to three slightly smaller letters, followed by rows of smaller but more number of letters. Typically a Snellen’s chart begins with a single letter that an eye with standard visual acuity should be able to read at 60 meters. This is followed by a row of two letters of a size that an eye with standard visual acuity should be able to read at 36 meters. This is followed progressively with rows with numerous letters of sizes that an eye with standard visual acuity should be able to resolve at 24, 18, 12. 9, and 6 meters. Most charts also have lines with smaller letters that should be readable at 5 and 4 meters. This chart is presented for a test at a distance of 6 meters from the eye. This is a standard distance at which visual acuity is checked normally. One eye is checked at a time. More elaborate charts have additional lines with letters that should be readable at 48 meters and 7.5 meters.
What is the principle behind measuring Visual Acuity?
In technical terms, Visual Acuity is defined as the resolving power of the eye. It is the ability to see two separate objects as separate. It may be thought of as the ability to see a gap.
There is an interesting history of how this measure was arrived at. The early astronomers found that it was possible to see two stars as separate if the distance between the stars subtended an angle of at least 1 minute of an arc (you may want to brush up on your geometry to understand this). Hence, the “normal” resolving power of the eye was defined as the ability of the eye to detect a gap with a width of 1 minute of an arc.
However, one minute of an arc is not the limit to human vision. Some eyes can resolve less than that as well and can have better than “normal” vision as well.
At the same time, some people with slightly lesser resolving ability than 1 arc of a minute may still be fine for all practical purposes. Hence, calling this “normal” vision would be misleading. We should refer to this as “standard vision”.
Types of vision problems
There are various types of vision problems that can prevent you from carrying sunglasses out even the simplest of daily tasks properly. The problems may arise due to any structural irregularities in the eyes. These irregularities can affect your image perception. Let’s go through a few of the most common types of vision problems: –
Myopia (Near-sightedness or short-sightedness)
Myopia is also known as Near-sightedness or short-sightedness. It occurs when the light enters the eye, after refracting through the cornea, lens and the media inside the eye forms an image in front of the cornea. In other words, the natural focusing power of the eye is higher in proportion to the length of the eyeball. This may occur if a person has an elongated eyeball. In addition, a person who has myopia may have a more curved cornea. DIn this condition, the person cannot see distant objects properly. Moreover, it can be treated by wearing prescription eyewear, contact lenses of minus power.
Opposite to Myopia, Hyperopia occurs when the light enters the eye, after refracting through the cornea, crystalline lens and the media inside the eye forms an image behind the retina. In other words, the natural focusing power of the eye is lesser in proportion to the length of the eyeball or the length of the eyeball is shorter in proportion to the focusing power of the eye. A hyperic eye may have a short eyeball. Apart from this, a less curved cornea can also lead to this problem.
In this condition, the person faces difficulty seeing the nearby objects. In a lot of cases, the vision may be clear, but it takes additional effort on part of the internal muscles of the eye to increase the thickness of the crystalline lens inside the eye, a phenomenon is known as accommodation, to enable clear vision. The additional accommodation causes eye fatigue. It can be corrected by wearing corrective contact lenses or eyeglasses plus power to improve vision.
Astigmatism refers to vision defects that need cylindrical powered spectacles and contact lenses (toric contact lenses) to correct vision defects. In this condition, light traveling through the cornea, crystalline lens, and media of the eye does not form a point image on the retina as is expected. The image of a point object is elongated or spread. A person with very less astigmatic power may feel the vision is better by squeezing the eyelids and making the eyes smaller. While it is common to find this vision defect in lesser powers associated with a modern active lifestyle, high amounts of astigmatism is generally associated with irregular curvature of the cornea.
Spectacles, rigid gas permeable contact lenses, and soft toric contact lenses are easy and a great way to manage this vision defect.
Presbyopia refers to a decline in visual capacity by age. It occurs when the eye’s lens starts losing its flexibility as age increases. It can also result in ineffective light bending. In addition, this can cause various types of vision problems by making it difficult to focus properly.
Multifocal, bifocal, or progressive lenses are recommended to treat presbyopia. Moreover, you must ensure that you are purchasing your lenses from a reliable optician like R.Kumar Opticians to get the best quality at pocket-friendly prices. For more information, you can get in touch with us.
Is it possible to see better than 6/6 or 20/20 vision?
Yes, it is possible to have a sharper visual acuity than 20/20. People with healthy and young eyes can identify a few letters on the 20/`5 line or smaller letters on the standardized chart. Having a 20/20 vision acuity refers to identifying the numbers or alphabets that are smaller than the numbers or letters on the 20/20 vision line of the Snellen chart.
What are the different types of eye vision?
Here are a few of the most common eyesight measurements that you must know: –
What is 6/9 vision?
6/9 Vision means that you see the objects from a distance of 6 meters, which people generally see from 9 meters. Moreover, it indicates that your vision is slightly below normal vision.
What is 6/6 vision?
6/6 vision means that you can see the objects clearly from 6 meters away. It is considered to be the normal condition.
6/12 and 6/18 vision
Having a 6/12 vision means that you can read the letters from 6 meters away while the normal person can read it from 12 meters away.
Hence, we hope that after going through this blog, you can understand what is perfect eye vision, visual acuity and what is the purpose behind measuring visual acuity. Apart from this, we also discussed different types of vision problems to give you a detailed idea about the topic.
Is minus 6.5 eyesight bad?
A -6.5 refers to the quantum or measure of the refractive error or the correction required by the eye to achieve standard vision or normal eyesight. It is not the measure of eyesight or vision per se. Having a -6.5 refractive error means that the person is suffering from the condition known as myopia.
What is the weakest eye power?
Generally, the refractive error or spectacle power i.e. the correction required in spectacles to correct vision defect is measured in steps of 0.25. This may be considered as the lowest eye power. However, at R. Kumar, we are already trying more precise and accurate mesuemnet and vision correction as low as 0.01 D (D = dioptre. Dioptre is the unit used to measure the refractive error)
How do I know my eye power?
You can book an appointment with an eye specialist for your eye exam to know what your eye power is. Also, if you are experiencing any eye issues symptoms like blurry vision, eye pain, etc., you must get your eyes checked. You can also get in touch with R.Kumar Opticians to get your vision checked and get the reading glasses according to your vision strength or prescription.