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Contact Lens Infomations
When choosing contact lenses for vision correction, there are a number of important factors to consider. One of them is optimum fit, which is helped largely by making sure you order the exact prescription your optician has given you: this will ensure that the contact lens fits well on your eye. Wearing poorly-fitting contact lenses can damage the eye and cause problems such as blurred vision, headaches, and eye strain. The diameter and base curve of a contact lens are two measurements that affect how well they fit your eye.



Diopter

Visual acuity tests show that the vision is 1.5, 0.2, or 0.1. However, if you wear contact lenses, you have probably heard of prescribed lenses with -0.75D, -3.00D, etc. How are different?
The visual acuity is a subjective figure that determines how are you can see while diopter is an objective measurement of refracting power. This means that diopters can be completely different depending on the health of your eyes even though the results of visual acuity are the same.
Diopter is represented by D and refers to the reciprocal of the focal length (m, in meters). In other words, -1.00D means a lens with a focal length of 1m, and -4.00D means a lens with a focal length of 25cm (1/4m). The concave lens required for myopic correction is indicated by (-), the convex lens required for the hyperopia correction is represented by (+).

Lens Size : Diameter

The diameter of a contact lens (DIA) 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. Sometimes, Graphic Diameter(G.DIA) is used either, it is the pattern width on the lens surface which is 0.5~1mm shorther than a diameter.

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.

Base Curve

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.