Your contact lenses prescription

The base curve of a contact lens is the curvature of the back surface of the lens. It determines the type of fit the lens must have to match the natural curvature of your eye. It is usually expressed in millimetres and may be further characterised as steep, median, or flat. Typical base curve values range between 8.0 and 10.0 mm, though it can be flatter (from 7.0mm) if you have a rigid gas-permeable lens. A person with a higher base curve number has a flatter cornea (the clear, front surface of the eye) compared to someone with a lower base curve number, which indicates a steeper cornea.

The diameter of a contact lens is the width of the lens from edge to edge. It is also expressed in millimetres. This number is usually between 13 mm and 15 mm, though it can be as small as 9mm if a rigid gas-permeable lens, and it determines where the lens will sit in your eye. When you wear a contact lens with the appropriate diameter, the lens will remain stable in your eye, i.e. hold its position. If you wear a contact lens with the wrong diameter, it can cause discomfort and may even fall out.

Having the right fit of contact lenses is essential for clear vision and long-term comfort and satisfaction with your lenses. The diameter and base curve are important factors in determining what the optimum fit is for you. A proper fit ensures full coverage of the cornea, optimum edge alignment, and adequate movement of the lens for tear exchange.¹ Tear exchange describes the flow of tears at the edge of the lens, that is triggered when we blink.

A well-fitted lens covers the cornea properly and prevents dryness due to an exposed cornea.¹ Dry eye is a common cause of eye discomfort with contact lens use, and a well-aligned lens does not produce any edge strain (lens tightness) which can lead to discomfort.¹ When the base curve and contact lens diameter are appropriate, there is adequate tear exchange from under the lens surface, which helps clear out debris.¹ All these factors contribute to your lenses being comfortable to wear and also to preventing eye damage and strain, thereby promoting overall eye health.

Your contact lens prescription is made up of different numbers with positive (+) or negative (-) values that define the ‘settings’ of your lenses. These settings, which include power, sphere, and cylinder, are those that your optician has defined as being the most effective for correcting your vision.

The figure in front of ‘power/sphere’ (PWR/SPH) or ‘D’ (dioptres — a unit that measures a lens’ refractive power) indicates the strength of your prescription. This describes the amount of vision correction you need. The number goes up from 0 in increments of 0.25, for example: +1.00, +1.25, +2.00, +2.25, etc. The higher the number, the stronger the vision correction you need.

Does this differ for short-sightedness, long-sightedness and presbyopia?

A negative number, for example, D -1.00, indicates myopia (short-sightedness). A positive number, for example, PWR/SPH +2.00, indicates hyperopia (long-sightedness). The figure in front of ‘Addition’ (ADD) determines the correction you need for presbyopia to obtain near-clear vision.

Cylinder (CYL) is always a minus number, such as -1.25, -1.50, -1.75, etc. It is measured in increments of 0.25, in the same way as the power/sphere figure. CYL denotes the extra power you need to correct your astigmatism (an irregular curvature of the eye that causes blurry vision). The higher the number, the more severe your astigmatism. Axis (AX) describes the orientation of your astigmatism and the angle needed to correct it. It ranges between 0 and 180 degrees.

People with high-power prescriptions can wear contact lenses safely and comfortably, so if you want to get rid of your glasses, this is certainly an option. Advances in contact lens technology have made it possible for people with strong prescriptions to wear them. Standard contact lenses are readily available off-the-shelf in prescriptions up to -12.00D for short-sightedness, +8.00D for long-sightedness, and -2.75D cylindrical power for astigmatism.2 Extended-range contact lenses are available in spherical powers up to -20.00D for short-sightedness and +15.00D for long-sightedness, as well as up to cylinder power -5.75D for astigmatism.²

However, it’s important to know that soft contact lenses with high powers have thicker edges compared to low-power ones.3 For this reason, they are more likely to be discernible in the eye and less comfortable to wear. The thicker material also means that soft hydrogel contact lenses may cause dry eye in people with high-power prescriptions.3 Many people with high-power prescriptions choose rigid gas permeable (RGP or ‘hard’) contact lenses as they are smaller and thinner. RGP lenses take a little longer to adapt to, but they allow more oxygen to pass through to the eyes, making them the healthier option for those with high-power prescriptions.

If the power in your contact lenses is not strong enough, you will not be able to see clearly at near and/or far distances, depending on your prescription. You may also experience symptoms such as blurred vision, headaches, and eye strain. If you think you need a new contact lens prescription, see an optician for a contact lens health check.

Alongside the numbers on your glasses prescription, your optician will need to make some additional measurements. Your contact lens prescription will contain two numbers called the ‘base curve’ and ‘diameter’, which determine the fit of the contact lens on your eye. Most disposable contact lenses come in a selection of base curves and diameters, and it’s important to find the one which will best fit your eyes.

If you have astigmatism, you’ll have to wear toric contact lenses and your prescription will include measurements of ‘cylinder power’ and ‘axis’, which are needed to correct this condition.