The first thing we need to cover is visual aid prescriptions. Understanding what the specifications mean helps to guide a persons best match. Terms relating to power and diopter need to be covered.
Magnifying or reading glasses usually have their magnification power listed as a number followed by an X, such as 2X. The "X" means that is the power rating, or the size of enlargement. A 2X magnification makes items appear to be twice as large, 4X yields four times, etc.
When choosing visual aids, keep in mind that the trade off for the enlarged object is a loss of total visual area. It is best to pick the lowest amount of magnification needed to comfortably view an object so that more area of the object is available at any given time.
A diopter is the measurement of refractive strength, usually marked as a number followed by a D, such as 10D. A plus or minus sign before the number indicates a positive or negative property.
A lens rated at one diopter (+1D) will focus at a distance of about 40 inches (one meter) from an object. The distance (in inches) of best focus (or focal length) can be calculated by dividing 40 by the diopter value. A +60D lens (such as the average human eye) will focus at 40/60 or 2/3 of an inch (just over 22 millimeters).
Focal length is an important factor because it determines how close an object must be to the lens. A person would want to choose a visual aid with a realistic working distance in mind. Many visual aids have two focal length ratings, one for the distance between the object being viewed and the aid and one for the distance between the aid and the eye. The second value is usually not posted for glasses and contact lenses as it is usually a standard measurement.
Binoculars and some visual aids are marked with specifications similar to 8x21. In this example, 8 is the power magnification and 21 is the size of the front lens in millimeters. The size of the lens determines the amount of light which can enter into the system.
Now that the properties are defined, we shall look at some of the available visual aids. There is a huge variety of styles and options in each one of these groups. The types and abilities are also always changing.
Here we will cover the more common visual aids. Glasses and contacts, magnifiers and binoculars, sunglasses, and finally surgical options to improve vision.
When the eyes are not able to clearly see objects, glasses might be a good solution. Glasses contain convex or concave lenses depending on whether they are treating a near-sighted or far-sighted condition respectively. The power level of the glasses is determined by the severity of the bend of the lens. The lens in glasses are designed to perform the same basic function as the lens of the eye, but the lens of the glasses help to start the focus process to make up for what the eye cannot do.
Bifocals are glasses with two different prescription values on one lens. Usually one part is for help with distance viewing and the other for close work such as reading. It is possible to need different prescriptions for each eye as well.
Serving the same basic function as regular glasses above, contacts are popular with people who don't want to wear glasses due to appearance or possibility of the glasses falling off. Contacts are placed on the eyes surface and held in place by the eyelids and the eye fluids. Contacts are made to meet prescription specifications just like glasses.
Contacts need to be kept very clean since they rest on the surface of the eyes. Some people may experience allergic reactions or sensitivity to the chemicals used to clean contacts or problems with the eye adjusting to the new addition.
Contacts require much more maintenance than glasses. Most contacts are worn through the day but must be removed during sleep periods. Extended use contacts can be worn longer, but still need regular removal for cleaning. Some people have difficulty putting in contacts and it is possible for contact lenses to fall out accidentally.
Rigid contacts are made of shaped solid transparent material. Since they are not flexible, they can take some time to adjust to. They can also cause eye fatigue and may even scratch the eye or eyelid if they move around. These also inhibit the proper flow of oxygen to the conjuctiva, which can become a problem.
Gas-Permeable contacts are much like the rigid contacts, but are designed to allow oxygen to flow through them. They are also a bit more flexible which allows for a better fit.
Soft contacts are even more flexible and are designed for a more comfortable fit.
Giant papillary conjunctivitis (GPC) can occur with soft contacts. It is an allergic reaction which may involve discharge from the eye and irritation. People affected by this should switch to the gas-permeable contacts or discontinue using contacts altogether. Episcleritis is a sensitivity reaction where the eye becomes red in one area. It occurs with rigid or soft contact lenses.
Magnifiers come in a variety of shapes, sizes, and powers. These are made up of a convex lens (middle thicker than the edges) whose strength (magnification power) is determined by the severity of lens curve. A magnifier is often used for limited periods of time to do a specific task, such as reading fine print in a phone book.
Like magnifiers, binoculars and telescopes come in a variety of shapes, sizes, and powers. They are different though in that magnifiers are designed for enlarging close objects while binoculars are designed for enlarging distant objects. Most binoculars have magnification powers of 8x to 15x with some models having a range of adjustable power (called zooming). Personal telescopes have magnifications of 250x to 750x and are used for even farther distant objects, such as astronomy viewing.
Most binoculars are hand held devices that require both of your hands to control (one hand needed for focusing or zooming). Now you can also get binoculars that have head-mounting ability or even some which clip on to the frames of glasses and flip out of the way when not in use.
Believe it or not, sunglasses are considered to be a visual aid as well as eye protection. Sunglasses have shading material that limits the amount of light which is allowed to enter the eye. This provides relief from bright sunlight causing glare or eye fatigue. In addition to help with shielding of bright visible light, many sunglasses now protect from harmful ultraviolet (UV) light which can be present even on overcast or cloudy days. Protecting the eye from ultraviolet light can increase its life expectancy normal functions. Always read the labels on sunglasses to determine the level of UV protection they offer.
Also available are glasses that combine glasses prescription with shading qualities. There is also a style of sunglasses available which changes its shading as the light intensity changes. They provide less shading in lower light environments and more shading in bright light. Be sure to inquire about the reaction time of these before purchase.
While surgical correction methods offer the best cure for vision problems, the subject is far to complex to cover all of them here. Surgery is an issue best left discussed between a patient and doctor.
There are two main types of surgery, incision and laser. Incision surgery is where a doctor operates by hand and uses tools within the eye. Laser surgery involves a machine which uses a laser beam to perform the operation. Laser surgery often requires very little incisions, usually just the conjunctiva and cornea is peeled back for this.
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