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As with most things making spectacles or eyeglasses is a far more complex process than one would first imagine. The difficulty is that at first glance to the untrained eye, all lenses look pretty much the same which of course they are not! Each pair of lenses must be custom made to each individual’s needs to be truly effective. The common abundance of ready made reading glasses has probably altered the pulic perception of the complexity of glasses and the degree to which prescription glasses are customized for each and every wearer.
There are a number of areas in which ready made glasses fall short of the mark for providing optimum corrected vision.
1. They cannot take into account a difference in required lens power between the right and left eye.
2. They cannot correct for astigmatism.
3. They cannot account for accurate optical centre centration.
4. Lenses are generally made of inferior lens materials which even if were correct in power for the wearer would only give a second rate result due to the impurity of the material (low abbe number)
I will explain the spectacle making process from the manufacturing standpoint, that is everything after the eye examination and frame and lens dispensing process has been completed.
There are two main categories of spectacle lens available. They are stock lenses and grind lenses.
Stock lenses are usually injection moulded lenses which are mass produced in varying powers in an enormous range of materials. There are hundreds of lens manufacturers worldwide producing millions of stock lenses annually. Most lines of stock lens are available in at least 300 different power combinations, of which 266 would be cylinder lenses that correct for astigmatism. Of that 266 lenses they can be set at nearly any angle (axis) from zero to 180 degrees. Whether you can have stock lenses or not will depend on many varibles, and only your dispensing eyecare professional can decide. Progressive and bifocal lenses can only be done as grind lenses. Other factors include your prescription power, frame size, interpupillary distance, frame type, direction of astigmatic correction etc. Stock lenses come in a range of diameters with the most common being from 65mm to 75mm. These large stock lenses are then cut to the right shape by a machine called an edging machine. Often much of the original stock lens area just becomes waste after being ground down by the machine.
Grind lenses are lenses which are specially surfaced from a thick blank. Generally the front surface is finished with the prescrption being ground in the back surface only. These lenses include all multifocal and progressive, and polarised lenses. These days the surfacing is done on computer controlled machinery called a generator, which very accurately grinds the right curves into the surface of the lens. Traditionally the lens is then fined and polished on separate machinery, however recent innovation has meant that the fining/polishing process is completed on the same machine that cuts the lens material. This is something that could have only been dreamt about only a few years ago.