Facts About your Lenses
Lenses come in various shapes, sizes and forms and it would be impossible to explain each and every lens combination in this series of fact sheets. However we can split the different lens types into groups for the purpose of easy explanations.
SINGLE VISION LENSES
This is a lens of one power only and can be a plus power or a minus power. These are widely available from stock and come in a range of various diameters. The most commonly used lenses are 60mm or 65mm in diameter.
The power of the lens will be prescribed by the optician but the size of the lens required is determined by the size of frame dispensed and certain other measurement considerations which will be explained in greater detail later. A further factor to be considered is the weight factor, the more powerful the lens, the more substance in the lens, so consequently the weight will increase as the power increases. Another factor to consider is the lens thickness in relation to the power, remember a plus lens is thick at the centre and a minus lens is thick at the outside edge.
This is a lens with two different areas of different powers. The main area is generally for distance work and the smaller lens area is generally for reading work. The lower portion can be circular semicircular. D shaped, or the whole of the lower area of the lens.
The most common types of Bifocal lens used in the UK are the circular type generally prescribed to NHS specifications. For identification purposes these are available in two forms, Fused rounds and Solid rounds, The Fused rounds are the most common because they are cheaper to supply on the NHS and can be identified by feeling the lens. If the Bifocal area of the lens cannot be felt to be different from the main area of the lens then it is most certain to be a fused Bifocal. A solid Bifocal although of similar appearance, when felt, a definite change in the curve at the back of the lens can be determined and is therefore identified as being a solid Bifocal lens.
The other types of Bifocal are not available on the NHS and are easily identified just by looking at them. A 'D'.seg looks like a letter D laying on Its tummy, a curved top 'D' looks similar to a 'D' seg but with a slight curve Across the top, An executive Bifocal is where the whole of the bottom portion of the lens forms the Bifocal area.
Trifocals are lenses with three different power areas and are not very common in the UK They are mainly prescribed to patients who need an extra power correction in the middle distance, usually for occupational purposes. These lenses can be easily identified just by looking at them. They are generally of a D seg type or an Executive type as described in the Bifocal section.
These are special lenses and are mainly prescribed in the UK for cosmetic purposes. They are lenses of an infinite number of focussing distances throughout the area of the lens. The top, centre and outer area being for distance work and the central lower and bottom areas for middle and close work.
Theoretically if the powers of the prescribed lenses are determined accurately by the Optician and then the lenses measured and fitted accurately into a suitable frame. Then they should be the nearest one can get to natural vision with spectacles. Unfortunately not everyone is able to adjust to wearing them, and they are very difficult to measure to determine the correct fitting into a lens, and if this is done incorrectly, it only adds to the problem. Fitting of these lenses is beet left to an experienced person, and your area or regional manager should be consulted. To identify these lenses place them down on a newspaper print, and the print can be seen to be of different sizes throughout the lens area, Available in glass and plastic form.
Prisms are sometimes quoted by opticians on prescriptions to correct an imbalance in the focussing power of the eye at certain angles. These prescribed prisms are usually quite small and need not worry us too much at this stage, as the additional cost to us will not significantly increase the cost of lenses. The remaining conditions and defects of the eye are not normally improved by the wearing of normal Prescriptions lenses and therefore are not important in the content of our project.