Not all types of sunglasses can be fitted with prescription lenses so it is very important to know what to look for to suit the prescription, PD and lens type when selecting a frame.
Some important things to take into account before choosing a frame are:
• The spectacle prescription.
• The type of lenses required (single vision, bifocals or progressives).
• The PD measurement (pupillary distance) This usually ranges from 54-70 but most commonly is in between 58-64.
• Face shape, size and nose and cheek shape.
• If the prescription is more than 2.00D, avoid a wrap around frame.
• If the prescription is more than 3.00D, choose a flat frame.
• If the prescription is a high minus (over 4.00), choose a small, flat frame.
• If progressive lenses are needed, avoid a wrap around frame.
• If the PD is smaller than 60mm, choose a small frame.
Here are some examples of frames that cannot be fitted with a prescription:
• There is only one lens/shield/blade. Two separate lenses are needed for prescription use.
• The lenses are cylindrical (shaped like a cylinder or a tube), rather than spherical (shaped like a sphere or a ball).
• The frame is extremely large in size. Example: 70-18.
EXAMPLE OF A SPECTACLE PRESCRIPTION:
PD 62 Sphere Cylinder Axis
RIGHT -4.75 -0.75 45
LEFT -3.00 -2.00 135
If the sphere or cylinder power is higher than -2.00 or +2.00, the wearer will be more comfortable in a flatter frame. When a high prescription lens is fitted to a wrap around frame it causes optical errors which are perceived by the wearer as distortions and blurring. The higher the power of the prescription the more distortions will be experienced. In some cases this can cause headaches and nausea.
Another problem with a lot of wrap around and non wrapped fashion sunglasses is the large size of the lenses. Ideally, a person with a PD of 68 would suit an optical frame size close to a 50-18. This is because if you add the eye size to the DBL it equals the patient PD. This rarely happens in reality, usually the frame PD will be 5-10mm larger than the patient PD as it is often hard to find a frame that fits perfectly.
Patient PD = 68
Frame PD = 50 + 18 = 68
This means that their eyes will be in the exact centre of each lens. When this happens, the lenses will be perfectly centered within the frame and if the prescription is a little higher, the lenses won’t come out too thick or heavy.
If a person with a PD of 58 chooses a frame with a size of 62-18, their lenses will need to be decentered (moved away from the centre of the frame) a lot. Most lens laboratories won’t be able to produce a prescription lens with a large enough diameter to suit this PD and frame size combination but if they can, the end result will often be a pair of very thick and heavy lenses unless the prescription is very low.
Patient PD = 58
Frame PD = 62 + 18 = 80
The patient PD should be as close to the frame PD as possible. If there is a difference of more than 10mm, this means that the frame is too big and this will most likely result in thick lenses even with a lower powered prescription especially when combined with a wrap around lens.
For minus powered lenses to give the best vision, the front surface of the lens needs to be quite flat. With powers such as -5.00 or higher, the front surface may be almost completely flat and the back surface (which is closest to the eye) will be very curved and concave in shape. If this type of prescription is fitted to a wrap around frame, the front surface will need to be ground on a high convex curve and to compensate for the steep front curve, the back surface will need to be ground on an even steeper concave curve. This will produce extra lens thickness and weight and also distorted vision.
When choosing a style and buying online, it is best to know what type of frame usually fits as not every frame will fit every face. If the patient as a high bridge or a very thick or thin nose or a very flat nose, the frames may either sit too high or too low on the face. Sometimes, if the nose is very flat, the frames will sit on the cheeks and won’t touch the nose at all. Different shape noses and/or high cheek bones are the most common problem when trying to find a frame to fit. If the face is very wide or very narrow, this could also cause problems when selecting a frame so it’s important to understand how a frame should fit.
Choosing the correct frame for prescription sunglasses
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1 October 2009
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