The bifocals have a shape at bottom portion of the lens called a segment for reading.
Varifocals do not have a shape or line - look like a normal lens.
Common bifocal shapes are crescent or semi circle. Some can have a line going all
the way across lower portion called an executive bifocal.
The near prescription is in the lower segment shape of the bifocal. The top area
above has the distance prescription.
Whilst the reading portion of bifocal is not as large as separate single vision reading
glasses, it is of generous size and larger than a varifocal.
However the reading segment shape is quite visible and so can be noticed by other
people as well the bifocal wearer. So its cosmetic appearance is not as nice as
It can show away the age of someone over 40 as that is when reading glasses are often
There is a jump in vision between the segment shape and distance portion of the bifocal
and so have to be careful going up and down any steps or kerbs.
There is no intermediate prescription for computer users.
The position of the lower segment of bifocal is designed for reading books or paperwork
lower down than a computer screen which tends to be bit higher up.
It may be possible to use the computer screen with a bifocal when stood up in certain
situations like a sales assistant stood at the tills and so the screen is relatively
Some people are very happy to use bifocals despite the limitations as the larger
area and convenience suits them better than varifocals. The lower cost is a benefit
too. For others, solutions is to use a varifocal :
The varifocal is a multifocal lens that does not have any visible lines - and so
looks like a normal lens - no tell tale signs of age and cosmetically nicer.
The varifocal works by gradually blending distance prescription into near prescriptions
- from the top of the lens towards the bottom of the lenses - and so the prescription
changes gradually from top to bottom - rather than an abrupt change like with bifocals.
This means the varifocal has intermediate prescriptions - and so its ideal to view
the computer screens and music sheets on piano.
The prescription tends to be near the middle portions of the varifocal lenses called
the corridor of progression.
Since the lens has been blended, the areas to sides have some distortion if try to
view through the varifocal. Therefore have to use the central areas of the lens in
order to get the best vision
It can seem strange at first - as have to point nose to look at objects - and then
perhaps move chin up or down slightly to get the area into focus.
The disadvantage of varifocal is the smaller reading area than a bifocals.
Sometimes this can cause difficulties if need to have wide area to read - like with
drawing plans - but it does not mean that they wont do the job.
There are limits to varifocals - like with edge distortion if try to look through
sides of the lenses - as there are blended areas here.
So if aware of varifocal limits. then most people can adapt to it - and learn how
to use them.
Of course, there are occasions that varifocals are not suitable - e.g. a person may
feel dizzy with them or needs a large area for close work.
Also if have been wearing bifocals for many years, it can be difficult to adapt to
varifocals - and vice versa.
Varifocals can be that more expensive than bifocals but offer more features.
There are many different brands of varifocals - some give less side distortion or
perhaps give more reading area
Some varifocals are designed to maximise intermediate areas to help with computer
but there may be a trade off with less reading or distance portions.
Varilux is a brand of varifocal and sometimes varifocals may be referred to as varilux
as it was a popular brand in the early days.