Whether you need reading glasses or another type of glasses, choosing the correct strength is crucial. Otherwise, you risk the glasses not helping – and, in the worst case, they could do more harm than good. But how do you find out which strength is right for your eyes? In this guide, we explain how the eyeglass strength scale works and how to find your strength, so you can easily spot the glasses that are perfect for you.
What is an eyeglass strength scale?
The eyeglass strength scale is used by everyone and is a standard scale that both opticians and eyewear manufacturers use to indicate vision strength. This means you only need to relate to one scale – making it much easier for you to find the right reading glasses or glasses, no matter where you buy them. The scale tells you whether you are nearsighted or farsighted, and to what extent you need correction to achieve clear vision. After an eye exam, you will receive an eyeglass prescription with a specific strength value, which you can use as a starting point when choosing glasses.
What does an eyeglass strength scale look like?
An eyeglass strength scale – also called a refraction scale – indicates how much correction is needed in the eyeglass lenses to improve vision. The scale consists of positive and negative numbers, starting from 0 and moving upwards, and is measured in diopters (D). See what the scale typically looks like:
• Plus powers (+): Used for farsightedness (hyperopia) – when you see clearly at a distance but have difficulty focusing on nearby objects. E.g.: +0.50, +1.00, +1.50, +2.00, +2.50, +3.00, etc.
• Minus powers (-): Used for nearsightedness (myopia) – when you see clearly up close but have difficulty seeing clearly at a distance. E.g.: -0.50, -1.00, -1.50, -2.00, -2.50, -3.00, etc.
What is the difference between plus (+) and minus (-)?
The plus and minus values indicate the type of vision problem you have:
• Plus (+): You are farsighted (hyperopic), meaning you have difficulty seeing clearly at a short distance.
• Minus (-): You are nearsighted (myopic), which makes it difficult to see clearly at a longer distance.
This means, for example, that a strength of +2.50 indicates a need for help focusing on close objects, while a strength of -1.00 means your distance vision is blurred.
What do the numbers on the eyeglass strength scale mean?
The numbers on the eyeglass strength scale show the extent of your vision problem – and thus how much correction you need:
• 0: You have normal vision and do not need glasses.
• 0.25 to 1.50 (plus or minus): Mild vision impairment – here, glasses can be a clear advantage and are often recommended.
• Over 2.00 (plus or minus): Moderate to severe vision impairment – here, glasses will be necessary to ensure clear vision.
– the higher the number – whether it is plus or minus – the greater the need for vision correction.
How do you find your strength?
You can determine your eyeglass strength in several ways – here are 3 methods:
1. If you are unsure whether you need glasses, you can start with a quick self-test at home: Cover one eye with your hand, then the other. Do you see well? And with both eyes? Or is there a difference? If everything seems clear, you don't need glasses – unless you love glasses and want a pair of glasses without prescription.
2. If you are age-related farsighted (plus power) – you can use our age-based strength guide to get an idea of whether you need reading glasses – and if so, what strength you might need. But remember, this guide is for guidance only and applies only to age-related farsightedness (presbyopia):
35-40 years: +0.5
40-45 years: +1.0
45-50 years: +1.5
50-55 years: +2.0
55-60 years: +2.5
60-65 years: +3.0
– if you are in doubt between two strengths, we recommend choosing the lower of the two strengths. It can feel uncomfortable to increase strength too quickly. We also recommend that you have your eyes checked by an optician once a year.
3. The most accurate way to find your strength is to have an eye exam with an optician or ophthalmologist. Here you will get the precise eyeglass strength you need – and both your eyes will be measured individually, so the strength fits both eyes precisely. Some opticians offer free eye exams, while others charge a small fee.
When does your strength change?
Vision often changes with age. Already in their 40s, most people will experience changes in their eyes, where the lens's ability to change curvature begins – and which has been given the rather unattractive names 'old sight', 'age-related vision' – or simply presbyopia, as it is called in professional terms. This is a natural development and is completely harmless. You will typically notice that small letters have started to blur together? And that you can barely see what is written on your phone or on the menu! And perhaps you have completely given up on reading ingredient lists on items in the supermarket? In most cases, this can be solved with a pair of reading glasses, which will allow you to decipher small print again.
How often should you have your strength checked?
As you approach the age where reading glasses become relevant, we recommend having your eyes checked by an optician or ophthalmologist once a year. But keep a close eye on whether your vision changes – and if it does, book an appointment with an ophthalmologist or optician. The same applies if you suddenly experience other vision changes such as:
• You squint your eyes when watching TV or driving, which is often due to nearsightedness (-).
• You have to hold your phone or book far away from your face to see clearly, which is often due to farsightedness (+). This is where a pair of reading glasses can often help.
In short: have your eyes checked annually by an optician, and generally monitor your vision – and contact an optician if it changes.
What symptoms can you experience with incorrect eyeglass strength?
Do you wear glasses but still don't feel quite right? Perhaps it's not your eyes, but the eyeglass strength itself that is incorrect. If you wear glasses with the wrong strength – or don't wear glasses at all, even though you need them – you can experience a range of physical symptoms. Look out for these typical symptoms:
Headaches: One of the most common signs. If your eyes overcompensate, it can cause tension headaches – especially late in the day.
Blurred vision: Even with glasses on, vision can appear blurry, especially when shifting between near and far distances.
Dizziness or nausea: Incorrect strength – especially with astigmatism – can create a feeling of imbalance, similar to motion sickness.
Eye strain: If you feel your eyes are 'working hard' when reading or working on a screen, it could be a sign of incorrect correction.
Double vision: Incorrectly adjusted lenses can cause focusing problems – especially if your eyes don't work well together.
Problems with depth perception: Incorrect strength can interfere with your ability to judge distance – particularly dangerous in traffic.
Discomfort when changing focus: If it takes time to focus when looking from, for example, a book to the TV, it could be a sign of incorrect strength.
– if you experience these symptoms, it's time to have your vision strength checked by an optician or ophthalmologist. An eye exam can quickly determine if your eyeglass strength is wrong.
Can you harm your eyes with incorrect strength?
No – incorrect eyeglass strength does not alter your eye structure or permanently worsen your vision. Symptoms usually disappear when you switch to the correct strength. However, prolonged use of incorrect glasses can lead to eye strain, fatigue, and difficulty concentrating, which can affect your daily life.
Why do you often have different strengths for each eye?
It is completely normal to have different strengths for each eye. This is typically due to natural variations in the eye's shape and structure. One eye may be more nearsighted, farsighted, or have a different degree of astigmatism than the other. The difference can also be due to injury, eye diseases, or age-related changes. Although it may seem strange, the brain often adapts to the difference, so you still experience clear vision. However, if the difference is very large, it may require special eyeglass lenses or contact lenses to achieve optimal vision.
How can astigmatism affect your eyeglass strength?
Astigmatism occurs when the cornea or lens in the eye is not perfectly round but more oval in shape. This means that light is refracted unevenly when it hits the retina, which can lead to blurry or distorted vision at both short and long distances. To correct this, a specific power called 'Cylinder' (cyl) is used in your eyeglass lenses to adjust for the uneven light refraction. At the same time, there will be an 'Axis' value that indicates the direction of the astigmatism. Without this correction, you may experience problems with focusing, headaches, and eye strain, especially when reading or working on a screen. Therefore, astigmatism plays a central role in determining the precise eyeglass strength you need to see clearly.
What is the difference between single-vision lenses and multifocal lenses?
The difference between single-vision lenses and multifocal lenses lies in how they correct your vision at different distances:
• Single-vision lenses are designed with one fixed strength across the entire lens. They are typically used to correct either nearsightedness (myopia), farsightedness (hyperopia), or for reading glasses. If you only need vision correction at one specific distance – for example, for reading or for clear distance vision – single-vision lenses are the right solution.
• Multifocal lenses, which include both bifocal and progressive lenses, are designed to correct vision at multiple distances within the same eyeglass lens. Bifocal lenses have two distinct strengths, typically one for distance and one for reading, separated by a visible line. Progressive lenses offer a smooth transition between multiple strengths (distance, intermediate, and reading) without visible dividing lines, providing a more natural field of vision. If you experience difficulty shifting focus between distances, multifocal lenses may be the best solution.
When should you seek help?
If you experience frequent headaches, blurred vision, or eye fatigue, it may be time for an eye exam. Your vision change may be small, so we don't always notice that our vision is deteriorating. We slowly get used to weaker vision and only notice it when it becomes a problem. Therefore, it is a good idea to have an eye check-up every year or every other year.
