Principles of Magnification

When conventional spectacles can no longer help and surgery or medical treatment is not appropriate, or if the result still does not allow you to see better,  then it is time to consider low vision aids. These come in a bewildering array of strengths and designs. They range from simple hand-held lenses to electronic devices. Different ones are needed for different visual aids.

The use of a low vision aid is dependent on many factors, the type of vision loss, the degree of loss, the patient’s light and glare needs, the ability of the patient to handle and operate the aid plus many other factors.  The best place to obtain a low vision aid is from your low vision specialist following a comprehensive low vision examination Depending on your state, Medicare may cover much of the cost of the evaluation and training/therapy costs, but Medicare does not cover the aids.

Your care should start with a low vision evaluation by a doctor of optometry or an ophthalmologist who specializes in low vision rehabilitation.  The low vision examination is not the same as the retinal or general eye examinations you have experienced.

Magnification is a major

Principles of Magnification

 

Definitions:

Focal Length: The distance from the center of the lens (or lens system) to the viewed object. Working distance is the distance from the bottom of the lens, or the lens housing to the object when the object appears at maximum magnification without distortion. It is important to remember that as magnification increases, the focal length decreases.

 

Diopter (d): A term used to identify the refractive (light bending) capacity of a lens. In magnifiers, there is a direct correlation between focal length and diopter. To find the diopter of a magnification lens, follow these steps. With the eyes 10" above the lens, move the object to be viewed to the point the greatest distance below the lens where it remains in sharp focus. Measure this distance and divide into 1 meter (39.37"). The result is the diopter of the lens - e.g., if the object is at a 13" distance than it is a 3-diopter lens (39.37/13 = 3d). Each diopter increases the size of the viewed object by 1/4 (25%) when the object is at its full focal length from the lens.

 

 

Field of View: The distance across the lens surface to which the viewer brings both his eyes (note: eyes should be 10" above the lens). It is important to note that as magnification increases, meaning the lenses used are stronger, viewing areas and focal length decrease.

Magnification: The degree to which the viewed object is enlarged. Magnification is usually expressed by a number followed by an "x", the symbol used to express power or the size of the object in relationship to its actual size.

 

 

Selecting The Right Magnifier:

1. Determine the desired magnification for your needs. Remember, as you increase magnification, you decrease both the focal length and the viewing area.

2. Check to find out the correct diopter you need to achieve that magnification. The vast majority start with the standard 3 diopter lens and add secondary lenses or change primary lenses as needed.

3. Note the focal length and lens diameter that correspond to the magnification and lens diopter you have chosen, and make sure they are suitable for your task.

4. Select the primary and secondary lens combination required to achieve the desired magnification. 5. As a general rule, because the working distance will be less than 8", lenses or lens combinations above 5 diopter are recommended for inspection purposes only.

 

Tips on Proper use of a Magnifier

To take best advantage of the comforts built into illuminated magnifiers, please keep these points in mind:

1. Use both eyes. Magnifiers are designed as "working tools". They can be used as comfortably as a pair of glasses.

2. Position the lens so that it is a proper distance from the work area, yet close enough for your eyes (8" to 10") so that you have the maximum magnification without distortion. Do

not lean back away from the lens to increase magnification.

3. Chair height and work surface should be positioned so the operator can maintain good posture while working.

 

Types of Magnifiers

Magnifiers, often called magnifying glasses, fall into several categories. They can be either

1.    Spectacle mounted
2.    Held in the hand
3.    Hung around the neck so the hands are free.

 

They can have a light inside or on the outside; some are provided with a stand to keep the magnifier still to help produce a large still image.

Use the good eye

With magnification it can become hard to use both eyes together because your nose gets in the way. Therefore with more magnification, put the lens in front of your best eye. If you use the right eye only for instance, hold the object directly in front of your right eye. When you use both eyes hold the object in front of your nose.

Is it tiring?

Holding a book or sewing 5-6" (15cm) away from the eyes becomes extremely tiring. If you use a "ribbon sling" your hands care supported and it can be more comfortable. This is a piece of ribbon or scarf that is tied in a loop. Tie the ribbon, and thread each hand through the loops. This takes the weight off your arms and keeps the right  working distance. You need to experiment to find the correct length of ribbon.

 

Hobbies & Interests

 

Different tasks require different degrees of magnification, and also different types of magnifier. It is therefore important that your optometrist (or whoever provides your magnifying aids) knows what you need to do and what your hobbies are.

 

If you have an unusual hobby, show your optometrist how you like to work. Your optometrist must know the ‘working distance’: this is the distance from the eyes to the book, newspaper, painting, piano music, or sewing held in your hands etc.

Stand Magnifiers

Stand magnifiers that are internally illuminated but available without lighting.  You should use your standard reading glasses.
These usually produce the best results when held near your reading glasses and resting on whatever you are reading.

 

How to use a  magnifier

There are many way of using a hand held magnifier, whether illuminated or not. There are no particular rules, but lighting is critical, even with an internal light.

As mentioned, it is important that you hold things in front of your better eye. This may mean changing habits of a lifetime, which can be very frustrating. A fixed distance magnifier that is easy to carry around is great!

Large supported magnifier (table or wall mounted)

 An illuminated lamp with magnifying glass on a stand attached to a table or a floor stand.  These are very useful for sewing, reading and hobbies for many people.

 

Spectacle aids

There are many different types of spectacle aids. They have to be fitted by a specialist Low Vision Optometrist or expert. Some use one eye, some use both eyes. Some newer devices use a type of television camera and are electronic and very expensive. If you are ages 85 years, you may not be able to get used to such a device. They can be extremely effective.

 

Monoculars

These Monoculars and Telescopes can be helpful for distance sight, perhaps seeing a lecturer.

Monocular telescopes may help in and out of the house. Use with the good eye.

 

Good lighting is essential

When reading, the light should come from behind. A very bright room light may not be helpful. It is better to have a light shining from behind. Clear light is the most helpful.  Today’s LED devices seem to provide very clear light as well as the lights that are imitating the light from the sun.

For walking round the house, good lighting helps, but should not be too bright.

Electronic Aids

As with everything electronic, there are new versions each month, and they are getting less expensive. You really need expert advice from a low vision expert, many optometrists can help. One very clear rule applies here.  Make sure that you are dealing with an expert not just a single line vendor that sells only one brand of electronic magnifier.  Sales prejudice comes into play here and you may not find the perfect device for you or for your budget.

 

 CCTVs / Screen Readers

A CCTV is a stand alone electronic magnification device.  The different brands have different options and there are clearly vast differences between the quality of the device, the quality of the image produced and the ease of use.  ALL factors should be considered before purchasing!  A buyer should look at different brands at the same time to compare the quality. We would be happy to send you our full report on the variety of CCTVs and manufacturers of these devices that will give you a true comparison and list of things to look for when purchasing.  ALL CCTVS are NOT created equal but they are ALL priced the same!

 

A CCTV is a device that has a sliding tray/table, called an X-Y table.  The document is placed onto this tray.  A camera is mounted above this table and “sees” the text and displays it onto a monitor/screen in front of you.  The size, color and quality of the image, is controlled by using the control panel on the CCTV.  These devices will normally produce magnification ranges of 3X to 60X and the best devices provide for even higher levels of magnification.

 

Scan and  Read Device

A Scan and Read Machine is one unit that integrates a scanner, Optical Character Recognition (OCR) Software and speech software. The printed document can then be scanned and read by the same machine. This option is most popular with those people who don't have a PC or don't want to use a computer. Some of these machines allow document storage. If you already have a PC with speech output software, then usually a cheaper option for reading documents is to buy a scanner and some mainstream or specialist OCR software.

 

Optical Character Recognition (OCR) software

This software is used in conjunction with a PC and scanner to copy printed text to the computer and hold it electronically so it can be read by a screen reader or magnified with software. There is mainstream OCR software available that just copies the text.

 

Speech Output software or Screen Reader software

A screen reading program sends screen text displayed on the screen to be spoken by a speech synthesizer. Common features include the ability to speak the full screen, a user defined area of the screen, a line, a word, individual letters or the phonetic equivalent of a letter and punctuation. A screen reader allows menus, dialog boxes, tool tips and system messages to be read back. A screen reader is designed to give a blind person access to the computer.

 

Screen magnification software

This software can magnify the text, menus and icons on the computer screen up to 32 times. Because screen magnification software increases the size of the image displayed on the screen, only a portion of the original screen image can be seen at one time. Normally the magnification will automatically follow the area of attention, for example the cursor. Because of the restriction on the amount of viewable area of the screen, a large monitor is usually used in conjunction with screen magnification software to effectively increase the viewable area on the screen.