Hi prolibertate-ga,
Your problem is shared by many of us.
Because of that fact, there are a myriad of technologies designed to
address the problem. Before diving into specifics, you might want to
read a good overview of the problem at
http://www.actionfund.org/ohsay/saysee35.htm
The most important thing that site has to say is that, if possible,
it's a good idea to try out any solutions you're considering at a good
low-vision clinic before making a purchase. Because solutions are so
individualized, personally experimenting with them is the ideal way to
go.
Since you'll be looking at various reading technologies, it might be
useful to understand some relevant reading lens/correction terms. The
following is taken from Wemagnify.com at:
http://www.wemagnify.com/magdef-b.htm
--------------------------
"Power (X):
The number of times a lens multiplies the image.
Focal Length (FL):
The working distance between the lens and the object where the image
is sharply defined.
Field of View:
The maximum area seen through a magnifier.
Depth of Field:
The allowable variation in focal length in which a lens remains in
clear definition.
Aberration:
Spherical aberration distorts the image. Color aberration separates
the colors of the spectrum.
Corrected Lens:
Lenses which are ground with variable radii or a combination of lenses
or various types of glass spaced or stacked to form a lens system.
All of the above factors are variables that must be taken into
consideration in selection and use of a magnifier. Each of these
factors affect the other. The power of the lens determines the focal
length, the focal length limits the field and depth of view, (i.e.,
the shorter the focal length, the stronger the power). Aberrations are
more predominate when lenses are used at improper focal lengths,
especially in the stronger powered lenses.
How All magnification works:
Rays of light reflected from an object pass through the lens and are
bent toward the apex of the lens (thinner part) to produce a magnified
image. The smaller the radius on the surface of the lens, the more the
rays are bent. Therefore, the stronger the lens the larger the image.
How to use a magnifier:
The plane of the lens in a magnifier should always be held parallel to
the plane of the object being viewed. By holding the magnifier as
close to the eye as comfortably possible, the fullest number of rays
from the object being viewed are permitted to enter the eye and cuts
down reflected light hitting the surface of the lens, thus minimizing
reflection and glare.
Each Individual and the particular application of a magnifier will
dictate the method of use. By experimentation the individual may find
the best way to use the magnifier for his special task. It is best to
employ a magnifier in the least power sufficient to serve its purpose
with a comfortable focal length. Hand magnifiers are designed for use
of both eyes so it is only natural to hold them further away from the
eyes and closer to the object or reading material.
IRA:
Independent Reading Aid.
Diameter versus power:
Lenses are like sections cut from spheres. A section cut from a sphere
the size of a golf ball would be stronger in power than a lens cut
from a sphere the size of a basketball. It is apparent then that the
diameter of a lens is limited in size by the radii of its surface.
Hence, it is important to produce a large lens with a small radius to
effect strong power.
Lens materials:
Many transparent materials may be used to manufacture lenses. The most
common materials for magnifying lenses are glass and acrylic plastics.
The selection of the material for the lens is based on its use. A
glass lens would be harder and more scratch resistant, whereas a
plastic lens is lighter in weight but would require more care to avoid
abrasion.
Mountings:
The mounting used on a magnifier is most important to its utility. A
lens of specific power may be mounted in a number of fashions so as to
provide a convenient instrument for various uses.
Usage:
A person performing a task which requires the use of both hands
obviously must find a different method of holding the same lens that a
person would use to scan a surface. Lenses having strong powers and
short focal lengths should be mounted when possible in fixed focus
stands because a slight variation in focal length may cause image jump
and distortion.
Aberrations:
Spherical lenses (especially the stronger powers) produce a pin
cushion appearance with peripheral blurring. In the weaker powered
lenses this effect is tolerable. When lenses of a stronger power are
required, it is necessary that a system be developed by composing two
or more lenses stacked or space to correct the inherent distortion; or
a lens may be ground with an aspheric curvature to eliminate
distortions."
--------------------------
There are many devices designed to aid the reader with diminished
close-up vision (which describes almost everyone over 45 years old -
the condition is known medically as presbyopia, or "old vision".)
These technologies range from reading glasses, to magnifying rulers,
to various kinds of magnifying glasses, to closed circuit television
systems.
Because you asked specifically about larger lens solutions, I will
concentrate on that area in this answer. First of all, there are the
CCTV devices. These are units featuring monitors, lenses, lights, and
software to enhance and fine-tune viewing in different conditions.
These units (the best of which seem to be by Aladdin) are not
inexpensive. Westchester Low Vision offers a good overview of CCTV
devices on their website: http://lowvisiondoc.com/LVCCTV.htm
One of the things the Westchester site says is that CCTV systems
typically cost more than $2,000.00. If you want to check out such
devices, take a look at this site by A. T. Kratter:
http://www.atkratter.com/mags/ergo/ergo.htm
I'm guessing you're not interested in a system that elaborate.
Presuming that to be the case, let's quickly move to a less pricey
approach.
To some extent, what you've asked for is a contradiction in terms. The
very best magnified image requires an aspheric magnifier such as these
from Donegan Optical Company:
http://www.doneganoptical.com/catalog/aspheric/
Aspheric lenses are virtually distortion-free, but they are limited in
size because of the thickness and weight that would be required to
make them in the large dimensions you are seeking.
There are, however, lenses that meet your criteria although they may
not be of perfect image quality. Comfort House offers a Full Page
Magnifier that "is large, nearly weightless, and so flat that you can
store it for ready use right inside your phone book, atlas, or
dictionary. Unbreakable plastic "lens" measures 8.75" x 6.5". Vinyl
frame." that sells for $3.99. (I found the same lens elsewhere on the
Net at prices ranging up to $12.95) - Comfort House has, by far, the
best price on this lens:
http://www.comforthouse.com/fulmag.html
Next to consider is a "2X, full page Fresnel magnifier [which] is the
answer for looking at small print in phone books, instructions,
reading maps, or hobby work. This deluxe model has folding legs and
maintains the lens at the proper focal length from the paper to allow
you to read or write without holding the lens. It also comes with a
round-the-neck cord that enables you to hold the magnifier on your
chest so that your hands will fit underneath it for hobby work. The
lens measures 7" x 10"":
http://www.shopallenbeys.com/full-page-magnifying-magnifiers-magnify-p-ej-570284-12.html
Should you be interested in the workings of the fresnel lens
(pronounced Fray-nell, after Augustin Fresnel), here's a good, brief
explanation:
http://acept.la.asu.edu/PiN/rdg/fresnel/fresnel.shtml
The advantage of the fresnel design is that, while not optically
perfect, it allows a lens to be made virtually flat rather than
thickly concave and heavy. Fresnel lenses are also easily moulded in
acrylic plastic.
There is a lens from Magnifico - "Magnifying Sheet/Extra Strong" -
that is approximately 8.25" x 10.75" and is described as a "tough
injection-moulded sheet magnifier with a shatter-resistant 2mm thick
optical quality lens. For detailed viewing of large areas of text and
pictures. Rigid surface enables one-handed positioning above printed
matter without 'flexing'." Magnifico does not say that this lens is a
fresnel type, but I suspect it is since it's a uniform thickness. You
can see it here: http://www.magnifyingglasses.co.uk/acatalog/Online_Catalogue_Page___Sheet_Magnifiers_8.html
Lastly, there is this offering from Dynamic Living: "If you need an
extra boost in order to read, the full page magnifier is an easy
solution. This large (9.5" x10.5") full page magnifier will cover much
more area than other hand held magnifiers. It is made from an optical
grade acrylic that is 1/16" thick, rigid like glass and will not bend
or distort the image. It is the clearest 2X magnifier on the market
today! $39.99"
http://www.dynamic-living.com/full-page-magnifier.htm
Once again, the website does not mention the technical design of this
lens, but in order for it to be as large as it is, and a uniform 1/16"
thick, it is almost surely a fresnel design. However, as with any
product, there can be noticeable variations in design and quality, and
at the $39.99 price, presumably this lens is both larger and of
greater precision than similar lenses using the same technology.
So, there are some viable options. If I were trying these options, I
would first order the inexpensive ($3.95) lens from Comfort House and
see how I liked it. Based upon that experience, I would then consider
what upgrades might be necessary to assure my maximum reading comfort.
I would also consult the Yellow Pages for a "low-vision clinic" where
I might see some lens alternatives "up close and personal" - after
all, the only way to decide how a lens is going to satisfy my vision
requirements is to actually look through it.
I hope this answer satisfies your curiosity. If it's not clear (so to
speak :-), or if there is any problem with the links provided, please
use the "Answer Clarification" feature before rating the answer, and
I'll get right back to you.
Thanks for your question,
ericynot-ga
Google search terms used:
reading magnification -software -glasses
"full page magnifier" |
