There are many different types of lens available all of which have different properties. Listed here are the different lens types and information on their uses and limitations.
This is a lens of one focal length, giving a specific visual range i.e. Distance, Computer or reading. If more than one distance needs to be viewed it may be necessary to have different spectacles for different tasks.
This is a lens with two focal lengths, usually distance and close (reading). There is a clearly visible dividing line between the two zones. It is possible to do an intermediate and close vision combination in this lens type.
This is a lens with three focal lengths with clearly visible dividing lines between the zones. They can be a simple Distance, intermediate & close combination or something more complex for occupational use (see below).
This is a lens with a gradual shift in power from distance to close with no visible dividing line. There are lenses of this nature for general wear and task specific lens i.e. for driving or office use. All lens of this type have a certain amount of peripheral distortion and require time (usually about 1-2 weeks) to adapt too.
It is possible to get lenses for specific occupational tasks from office lenses which give a range of 3M to a 1/3M to trifocals with a reading or intermediate zone above and below the pupil ( ideal for people who need to see something close but positioned above head height i.e. electricians, plumbers or pilots). It is even possible to get a varifocal lens which has distance vision in the centre and graduates to close both above and below the pupil.
There are many different types of lenses made in a variety of different plastic & glass materials, this list aims to give you an idea of what is available, it’s properties and uses.
Standard CR39 plastic lens are the commonest type in use in the UK. It is fine in most full rim and Supra (half rim) frames. However it is not the best option for rimless frames.
Trivex (Also known as Trilogy or PNX) this material is very good for rimless frames as it is very hard to break. It is also thinner and lighter than CR39 Mid and High index plastic lenses n=1.6 up to n=1.74 are thinner and lighter for higher prescriptions most come with a hard reflection free coating which can incorporate an antistatic and or a grease/water repellent coating. Plastic lenses with refractive index of n=1.6 or higher give full protection from Ultraviolet light entering the eye through the lens.
Glass lenses are less widely used as they break more easily and are generally heavier than plastic. That said there are advantages for very high minus prescriptions and some specialist uses.