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Understanding Your Spectacle Prescription

At our practice, we are commonly asked by our patients exactly what their prescription means, both in terms of the quality of their vision and the implications it may have on their choice of spectacles and lenses.

When you purchase spectacles from us, we want you to be completely confident in our advice and recommendations. To help achieve this, we have posted this blog to explain exactly what your prescription is and what it means for you.

The Basics

In this blog post, we will start by explaining the three most common features of a spectacle prescription – Sphere (Sph), Cylinder (Cyl), and Axis.

The sphere power is denoted in plus (+) or minus (-) form and is measured in dioptres. Plus-powered lenses are used to correct vision for long-sighted patients, and minus lenses are used to correct vision for short-sighted patients.

The cylindrical power and axis of your prescription are directly related to one another, and these are used when astigmatism is present. Astigmatism is very common, and an eye that is ‘astigmatic’ can be described as an eye that is not perfectly spherical. The cylinder represents the power required to correct the astigmatism, and the axis specifies the orientation of that power within the lens.

Lens Thickness

There are a number of factors that determine how your finished pair of lenses will look, but your dispenser will encourage you to consider the power of your lenses as well as your choice of frame so that your finished pair of spectacles offers the very best result.

If your lenses are of a certain power, your dispenser may recommend that you opt for a high index lens, as these are ideal for reducing thickness and weight. This improves the appearance and comfort of your spectacles.

In some instances, you may also be advised to consider frames which are of a shape that will compliment your prescription. High prescription lenses are often thicker and heavier when fitted into larger frames.

As always, our dispensers at the practice are here to help you select the frame and lens options that are right for you and your prescription.

The Reading Addition

When reading your spectacle prescription, you are likely to see a column headed ‘Add’. If you’ve ever wondered what this part of your prescription means, it is the required power of your ‘reading addition’. If a power is present in the column, it is there as a means of improving your near vision.

The reading addition is used to make reading glasses, giving the patient a superior and more accurate option than ‘off-the-shelf’ reading spectacles. The reading addition can also be adjusted in cases where intermediate spectacles are required. In bifocals and varifocals, the reading addition determines the reading area at the bottom of the lenses.

The reason this power is called an ‘addition’ is because it is added to the distance prescription in order to produce spectacles for high quality near vision. The reading addition is most commonly required for presbyopic patients, and its power is likely to increase over time.

In a pair of spectacles, the reading addition is usually the same in both lenses. It is important to make it clear that not all spectacle prescriptions will include a reading addition, so do not be concerned if this part of your prescription is blank.

If you would like further advice on your reading addition or your prescription in general, please contact us here.