At Stingy Specs we've
taken the guess-work out for you when it comes to choosing lenses for
your prescription glasses.
Our Stingy Specs website
is especially designed to make things really easy for you. With just a
few clicks you can figure out your lens needs based on your
prescription details.
Below is information about
spectacle lenses which may help to understand differences between
different ophthalmic lens types.
Glass Spectacle Lenses
Originally spectacle
lenses were made from glass. Glass has the benefit of being very
hard so it doesn't scratch easily. However, glass has the disadvantage
of being heavy and also easy to break. Serious eye injuries have resulted from glass lens breakage. Many people do not like heavy glasses pushing on their nose. Glass is difficult to work with for opticians, as it can break whilst glasses were being made. Simply dropping spectacles cause glass lenses to break. Glass lenses offer very little protection against ultra-violet light. Glass is more expensive to manufacture than plastic. For these reasons glass lenses are rarely used these days. Stingy Specs does not sell glass lenses. See the History of Spectacles ezine for more about the history of lenses.
CR-39 Plastic Spectacle
Lenses
During the early 1960s a
lightweight plastic spectacle lens called CR-39 (Columbia Resin #39) was developed. CR-39
lenses have a similar thickness to glass lenses. They don't break as easily, and they are lighter than glass. However, CR-39 lenses were originally easy to scratch. This initial problem was reduced with the
development of Hard Coatings. A hard coating is a thin coating of a hard plastic on the lens surface which does not scratch as easily as the CR-39 lens. All lenses sold by Stingy Specs are hard coated.
CR-39 is the main lens material used
today in ophthalmic quality plastic lenses. Ready-made reading glasses
such as what you may buy from the chemist do not use CR-39 for the
lenses. All standard plastic lenses sold by Stingy Specs are hard
coated CR-39.
The refractive index of CR-39 is 1.5 (sometimes written more accurately as 1.49) The higher the refractive index, the thinner the lens can be.
Mid-Index Spectacle
Lenses
With the constant advance
of technology, newer plastic materials have been developed that are
thinner and therefore lighter than CR-39.
The higher the refractive index, the thinner the lens can be. Mid-index lenses usually have a refractive
index between 1.54 and 1.56, making them thinner than standard CR-39,
but not as thin as high index lenses. Mid index lenses are appropriate to keep the lens thickness down for people who have a slightly higher than average refractive error.
High Index Lenses
For higher prescriptions
even 1.56 lenses may look too thick and heavy. Lenses are available with a
refractive index of 1.6 and 1.67. These lenses are referred to as high
index lenses. The higher the index, the higher the price also. At
Stingy Specs we sell those high index lenses that we can offer significantly cheaper
than the retail price.
Aspheric Lenses
Aspheric lenses are made
with a non-spherical curvature to make them flatter and therefore
thinner. Basically the lenses have a less curvature and are flatter. It can take a few days to get used to wearing aspheric lenses
for the first time but once you are used to
them you will see that they give clearer and less distorted vision. All
high index lenses sold by Stingy Specs are aspheric.
Photochromatic Lenses
Photochromatic lenses
change colour in response to UV light.
This means that they
become dark in the sunlight and lighten up indoors. They become as dark
as sunglasses outdoors and about half as dark as sunglasses when inside
a car. Some examples of brands of photochromatic lenses are Transitions®, Sunsensors®, and InstaShades®.
Photochromatic
lenses block 100% of UV rays to protect your eyes from UVA and UVB
radiation which could damage your eyes.
Anti-Reflective Coatings
An anti-reflective coating
(also called multi-coat) reduces the reflections visible on the lens
surface.
Lenses with
anti-reflective coatings are excellent for night vision as they reduce
reflections caused by light reflecting off the lens surface. However,
contrary to popular opinion, they cannot reduce glare.
Anti-reflective coatings
make glasses look better as the lens looks clear, rather
than having visible reflections on it. This means people
can see your eyes rather than the lens in front of your eyes.
Anti-reflective coatings
consist of a very thin coating of material that absorbs light that
would otherwise have been reflected. It is not possible to make lenses
completely clear so often there is a
very slight green or blue reflection visible.
Anti-reflective coatings
used in the past were difficult to keep clean as dust and grease and
fingerprints showed up very easily on them. Advanced hydrophobic
anti-reflective coatings, as used by Stingy Specs, are easier to keep
clean.
Please refer to the How to Clean your Glasses ezine for
more info on keeping your glasses clean.
UV Protection
All lenses sold by Stingy
Specs filter out most UV light. Prescription sunglasses are coated for
FREE with UV filters to block all UV light.
Single Vision Lenses
Lenses with only one
optical power. These are often reading glasses or glasses worn all the
time.
Bifocal Lenses
Lenses with two optical
powers. There is a line separating the two powers. The top section
usually is for distance vision and the bottom section is usually for near
vision.
Multifocal Lenses
(also called Graduated Lenses, Progressive Lenses or PAL - Progressive Addition Lenses)
Lenses where the optical power gradually changes to give a reading power at the bottom of the lens, and the distance prescription at the top of the lens.
The fitting position of multifocal lenses needs to be accurately measured when the lenses are ordered. These measurements are different for every frame. For this reason we do not sell multificals at Stingy Specs.
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