Measuring Visual Acuity: Dioptres, Visus and Percentages Explained
After an eye test, the optician hands you a certificate with numbers on it, and the conversation turns to dioptres, Visus and percentages, often in the same sentence. Most learners nod politely and take the paper home, without really knowing what the numbers mean. Measuring visual acuity is simple once the three scales are separated, because each one measures a different thing, even though they are all about the same pair of eyes.
This guide explains the three numbers you will meet when your eyes are measured for a German driving licence: Visus, the decimal scale of visual acuity, dioptres, the strength of the lenses you need, and percentages, the everyday translation of Visus. You will learn how the test is done, what the minimum of 0.7 really means, and what to do when your numbers are close to the limit.
Measuring Visual Acuity: The Three Numbers You Will Meet
Visual acuity is how sharply you can see. It is measured with a chart, and the result is given as a number. The trouble is that there are three ways to give that number, and opticians, eye doctors and licence documents use all three. The first is Visus, a decimal number such as 0.7 or 1.0. The second is dioptres, a number with a plus or minus sign, such as minus 2.50, which is your prescription. The third is a percentage, such as 70 percent, which is the same as a Visus of 0.7 written as a share of normal vision.
The confusion comes from mixing them up. A person with a prescription of minus 3.00 dioptres can have a Visus of 1.0 with glasses and a Visus of 0.2 without them. The dioptre says how strong the correction is, and the Visus says how well you see with or without it. The driving licence only cares about the Visus you reach with your usual correction, because that is what decides whether you are safe on the road.
Think of it like a car. The dioptre is the size of the engine you need, the Visus is the speed you actually reach, and the percentage is the same speed written as a share of the speed limit. You need all three numbers to understand the whole picture, but for the licence, the speed you actually reach is the one that counts.
Visus: The Decimal Scale
Visus is the standard scale used in Germany for visual acuity. It is a decimal number, and 1.0 means normal vision. That does not mean perfect vision: it means you can see at a given distance what a person with standard vision can see at the same distance. A Visus of 0.7 therefore means you can see at seven metres what a person with normal vision can see at ten metres. A Visus of 2.0 would mean you see twice as sharply as normal, which is possible for young people with very good eyes.
The Visus is measured one eye at a time and with both eyes together. For a car licence, the value that matters most is the binocular Visus, with both eyes open, and it must be at least 0.7. Each eye is also measured on its own, because the licensing rules look at both values. If one eye is much weaker than the other, the tester will note it, and the driving licence may still be issued, as long as the binocular value is met and the visual field is wide enough.
Visus is sometimes written as a Snellen fraction, especially in English-speaking countries. On that scale, normal vision is 20/20, which is the same as a Visus of 1.0, and a Visus of 0.7 is about 20/28. In Germany, the decimal scale is the everyday one, and that is the scale your certificate uses. The value is always written with one decimal place, such as 0.5, 0.7, 0.8 or 1.0, and the tester measures it by asking you to read the smallest row you can manage on the chart.
Dioptres: The Prescription Number
A dioptre is the unit used to describe the power of a lens. The number tells you how much the lens bends light. A lens of minus 1.00 dioptres is weak, a lens of minus 3.00 dioptres is moderate, and a lens of minus 6.00 dioptres is strong. A plus sign means the opposite: a lens of plus 2.00 dioptres is used for long-sightedness, where the eye focuses light behind the retina instead of on it. Astigmatism is given as a second number, the cylinder, with an axis that says which direction the irregularity lies in.
The dioptre is not a measure of your eyesight. Two people with the same prescription can have different Visus values, and the same person can have a different Visus on different days. The dioptre says what correction you need, the Visus says how well you see with it. That is why the optician measures both: the prescription tells them what glasses to make, and the Visus tells them whether those glasses give you the sharpness the licence requires.
Your prescription is written on the glasses slip you get from the optician. It usually lists the sphere, which is the main dioptre value, the cylinder, which is the astigmatism value, and the axis, which is a number from zero to 180. If your prescription is high, your optician may suggest thinner lens material or a smaller frame, because a strong dioptre in a large frame can be heavy and can distort what you see at the edges. Those details matter for comfort, but they do not change the Visus you reach.
Percentages: The Everyday Translation
Percentages are the third way to write visual acuity, and they are the simplest to understand. A Visus of 1.0 is 100 percent, a Visus of 0.7 is 70 percent, and a Visus of 0.5 is 50 percent. The percentage tells you, in everyday language, how much of normal vision you have. It is not a different measurement; it is the Visus multiplied by one hundred.
Percentages are often used in conversation and in popular articles, because saying seventy percent sounds clearer than saying zero point seven. Official documents, however, usually use the Visus decimal, because it is the scale the charts are built on. If someone tells you that your vision is 70 percent, they mean the same thing as a Visus of 0.7, and if they tell you that your vision is 50 percent, that is a Visus of 0.5.
There is one trap to avoid with percentages. A percentage of 70 does not mean that you see 70 percent of the world, or that 30 percent of your vision is missing. It only means that you can see at seven metres what a person with normal vision sees at ten. Your field of vision, your colour vision and your night vision are not included in the percentage, and a person with 100 percent acuity can still have a problem elsewhere. Percentages describe one number, not the whole of your eyesight.
How the Test Is Done: Charts, Rings and Trial Lenses
The measurement itself is simple. You sit or stand at a fixed distance from a chart, usually three to five metres, and you read rows of symbols. The chart may use letters, numbers, or Landolt rings, which are circles with a gap on one side. The gap can point up, down, left or right, or diagonally, and you say which direction it points. Landolt rings are used because they cannot be memorised: the tester can present a new ring in each position, and they test the eye optical sharpness rather than the ability to read words.
Each row of the chart corresponds to a Visus value. The tester starts with a large row and moves down to smaller rows until you can no longer read reliably. The smallest row you can read for most of the symbols is your Visus for that eye. The tester then repeats the process for the other eye and for both eyes together, and sometimes with a trial frame if a new prescription is being tested. Our article on Landolt rings in the eye test explains how to read the openings correctly and what the eight directions mean.
Lighting matters more than most people realise. The chart must be lit in a standard way, and the room should not have glare from a window or a bright light behind you. A good optician checks the light before the test starts. If the chart is too bright or too dim, the result can be one line different, which is enough to matter when you are near the limit. That is why the test is standardised, and why taking it at home with a printed chart is only a rough guide.
The 0.7 Minimum and What It Means in Practice
For a standard car licence in Germany, the minimum binocular Visus is 0.7, and the minimum for each eye is lower, because the two eyes work together. In practice, that means most people with glasses comfortably exceed the standard, many people without glasses meet it easily, and some people with a stronger prescription need to wear their correction to reach it. The standard is deliberately set at a level that is safe for traffic but realistic for the population, including older drivers who may have slightly reduced sharpness even with glasses.
The 0.7 minimum is a pass mark, not an average. The average healthy young adult has a Visus of 1.0 or higher, and many children have a Visus of 1.2 or more. As we get older, the average falls, but a value of 0.7 remains common well into later life, especially with glasses. If your value is 0.7, you have met the standard by the smallest possible margin, and your eyesight is not perfect, but it is good enough for driving a car.
For professional drivers, the minimum is higher, and the examination is more detailed. Lorry and bus drivers need a higher binocular value and a minimum in each eye, which is why their examination is done by an eye doctor rather than an optician. Our guide to visual acuity 0.7 explains the car standard in more detail, including the rules for people whose two eyes differ a lot.
What to Do If Your Numbers Are Borderline
If your measured Visus is close to the limit, the first question is whether your prescription is up to date. A pair of glasses that is two years old may no longer be right, and a new refraction can move your value by one or two lines. Have your eyes measured again, and if the new glasses give you a comfortable margin over 0.7, take the test again with them and the problem is solved.
Some people are borderline on some days and comfortable on others. Tiredness, dry eyes and poor light can each cost a line, so it is worth taking the test when you are rested and alert, with your eyes moist and the room well lit. If you are unsure whether your result was typical, ask the tester to repeat the key rows, or come back another day and take the test again. A repeat test is cheap, and it gives you a more reliable picture than a single tired morning.
If your value is below 0.7 even with the best correction, see an ophthalmologist. There may be a reason, such as a change in the lens of the eye, a problem with the retina, or an uncorrected astigmatism that needs a toric lens. The eye doctor can find the cause, and in many cases a new treatment or a better correction brings you over the line. Do not give up on the licence after one measurement: find out what the numbers mean and fix the cause.
Conclusion
Dioptres, Visus and percentages are three ways of writing the same story. The dioptre says how strong the correction is, the Visus says how sharply you see with or without it, and the percentage is the same Visus written as a share of normal vision. For a German car licence, the number that matters is the binocular Visus, which must reach at least 0.7. Measure it properly, with up-to-date glasses and rested eyes, and the number on your certificate will speak for itself.
Frequently Asked Questions
What is the difference between Visus and dioptres?
Visus is a decimal number that says how sharply you see, for example 0.7 or 1.0. Dioptres are the strength of the lenses you need, for example minus 2.50. A strong prescription does not mean poor vision: with the right glasses, a person with minus 3.00 dioptres can have a Visus of 1.0.
What does 70 percent vision mean?
Seventy percent vision is the same as a Visus of 0.7. It means you can see at seven metres what a person with normal vision can see at ten metres. That is the minimum for a standard car licence in Germany when both eyes are tested together.
Is a Visus of 1.0 perfect vision?
A Visus of 1.0 is normal vision, not perfect vision. Some young people have a Visus of 1.2 or higher. Normal vision means you see as sharply as the standard expects, and that is enough for safe driving.
Why do I need a chart and trial lenses?
The chart measures how small a detail you can see at a fixed distance, and the trial lenses test which correction gives you the sharpest result. Together, they give the optician both numbers: your Visus and your dioptre value.
What if my Visus is just under 0.7?
First, check whether your prescription is up to date. A new pair of glasses often brings you over the line. Also take the test when you are rested, because tiredness and dry eyes can cost a line. If you are still under 0.7 with the best correction, see an ophthalmologist.
Do lorry and bus drivers need a higher Visus?
Yes, professional drivers have a stricter standard and a more detailed examination, usually by an ophthalmologist. The minimum is higher than the 0.7 used for car licences, and each eye is given a minimum value as well.