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Bifocals,

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Alice Tong

on 21 November 2016

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Transcript of Bifocals,

Bifocals
,
Trifocals
,
and
Progressive Lenses

Alice Tong, SNC2D
Presbyopia

Myopia
(nearsightedness) is a
condition in which the curve of a person's
cornea bulges outwards.

This causes light rays to, when coming through the cornea, be
unable to meet at one focal point
on the retina, which causes the light rays to be
unable to meet the retina
at all.

A person who is
nearsighted has
diverging lenses
which
redirect the
light rays
so that
they can meet at
one focal point.
Hyperopia
(farsightedness) is a similar
condition, but the cornea is flat
instead of bulging. A
person who is farsighted
uses converging
lenses instead of
diverging.

Someone who is
presbyopic
has both hyperopia
and
myopia. Presbyopia is often caused by
the eye's
lens stiffening
in old age, causing
light rays to not meet on the retina.
Someone with presbyopia cannot
see especially far or near.


Bifocals
[
An executive bifocal lens.
]
[
A round seg bifocal lens.
]
- for far objects
- for near objects
Before the invention of bifocals,
people with presbyopia would have to
carry two pairs of glasses: one pair to see
distant objects, and one to see near objects.

Around 1780, Benjamin Franklin split the lenses
of two pairs of glasses in half, then
glued
the
top half
of the distance correction lens to the
bottom half
of the reading glasses. When
Franklin looked down to read, his eyes would
focus in the bottom half of the glasses, and
when looking up into the distance, his
eyes would focus in the top half.

Today, bifocals come in many designs-
all with a converging lens section in the
bottom half, and a diverging lens section in the
top half. Bifocals are typically adjusted so that
the line separating the two sections is placed near the lower eyelid.
Trifocals
[
An E-D Trifocal lens.
]
[
A trifocal lens.
]
- for far objects
- for near objects
If a person's presbyopia is more
advanced, they may have to wear trifocals.

Trifocals are similar to bifocals; however, there is
a third region- the
intermediate zone
- above the
near-object zone.

Wearers of trifocals often find that their eyes
are constantly switching between focusing in
each region; for this reason, they may
develop headaches.
- intermediate zone
intermediate zone
18-24 inches away
for far objects
24+ inches away
for near objects
less than 18 inches
away
Progressive Lenses
[
Progressive lenses have
no visible lines.
]
[
General areas of a progressive lens.
]
- for far objects
- for near objects
Bifocals and trifocals have recently
given way to progressive lenses.

Progressive lenses work similarly to bifocals
and trifocals, but there are
no lines in the lenses
.
Instead, there is a
smooth transition
between
each prescription area.

Progressive lenses are also called 'multifocal'
lenses.

- intermediate zone
- no prescription
Which lenses are better:
bifocals
and
trifocals
, or
progressive lenses
?
Bifocals
and
Trifocals
Progressive Lenses
- erase old-age stigma that comes with visible lens lines

- erase problem of 'image jump', where images change quickly between areas

- no distracting line that can obstruct vision

- reduce risk of headaches due to eyes trying to focus in a certain area
- have a wider range of vision; progressive lenses have a blurry section obstructing peripheral vision

- easier to learn to use; have lines that distinguish between areas

- less expensive than progressive lenses
How are these lenses made?
What is the average cost for glasses with each type of lens?
Bibliography
"Consumers' Guide To Buying Glasses 2016". Laramy-K. November
13, 2016.

Heiting, Gary, OD. "Bifocals & Trifocals: Solutions For 'Short Arms'."
November 13, 2016.

Heiting, Gary, OD. "Progressive Lenses Replace Bifocals For Age
Defying Appearance". All About Vision. n.d. Web. November 13,
2016.

"How Lenses Are Made". Shamir. n.d. Web. November 15, 2016.

"Progressive Lenses and No Line Bifocals". Master Eye Associates.
n.d. Web. November 13, 2016.

"Progressives: The Perfect Prescription". VSP. n.d. Web. November
15, 2016.
As with most single-vision lenses, a mold is made for the lens. With single-vision lenses, there is a single, smooth cut made to create the mold for the top side of the lens. With
bifocals, trifocals
and
progressive lenses
,
multiple cuts
are made into the mold.

The mold is then filled with the materials for the lens (liquid plastic); if it has multiple cuts, the materials will fill those areas to form curves that can cause the lens to be
converging
in one area and
diverging
in the next.

The back side of the lens is then surfaced- which means that curves are cut into it to meet the needs of the prescription. A deeper curve will be created for a higher prescription.
Single-vision
Bifocals
Trifocals
Progressive
Lenses
150 - 400
150 - 400
100 - 300
200 - 600
Full transcript