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Mostly ‘low vision aids’ are designed for use with reading Some are designed for distance All work by presenting the patient with a magnified view of the object Because these aids are optical systems they work by increasing the angle subtended by an object thereby creating a larger retinal image
When working with optical aids, it’s important to think about accommodation & how it impacts on the patients ability to use the magnifier
To determine if they need to accommodate, you have to consider several things including working distance The closer the object is, the more accommodation required
Need for accommodation is deter Accommodation demand = 100/working distance (cm)
Example 1: Reading material held at 40cm Accommodation demand = 100/40 =2.50D Example 2: Reading material held at 10cm Accommodation demand = 100/10 = 10D
Closer WD = much more accommodative effort mined by a formula:
Manufacturers use 2 methods to label the power of a magnifier Actual power Described in dioptres Magnification Labelled as 5x, 10x etc Common formula to relate the magnification to the dioptric power is: Magnification = dioptres/4 So 50d mag = 12.5x magnification
The number 40, yellow arrow for this brand of magnifier, refers to the size of the lens in mm.
Whichever way the magnification is expressed, this information tells you how to position the magnifier so you need to understand Focusing power Focal distance Magnification
As I just said, the unit of measurement of the strength of a magnifier is expressed in dioptres.
As the power of a lens increase, the parallel rays of light are focused closer to the rear surface of the lens So, as the lens becomes more powerful, the focusing distance is decreased.
Focal distance of a magnifier is determined by dividing 100 by the dioptric power FD (cm) = 100/D
The focal distance of a lens is the reciprocal of the dioptric power The greater the power of the lens, the closer the image needs to be focused on the back of the lens
For example, the strength of the hand magnifier shown here is 3.5x or 12d.
This magnifier has a strength of 24 dioptres so if we apply the formula, the focal distance is 4cm
If you forget the formula, don’t worry, an easy way to work out how to use a magnifier is to place it flat on the page and move it towards you until it comes into focus. Correct focus is shown in the left image. If you place it too far away from the page (i.e. incorrect focal distance) the image will invert and therefore this is focussed incorrectly, as shown on the right side.
Not all magnifiers need focusing. There are some that we call stand magnifiers and the focal distance is pre-determined because the optical component is mounted on a stand, as shown here.
Don’t forget about the limitation of the size of the area that can be seen through a lens, magnifier or telescope Patients will always ask for a strong magnifier with a big field of view
Here is a comparison of two different magnifiers on the same text. The mag on the left is 24 dioptres and is 4.5cm wide and on the right is 12 dioptres 10 cm wide. Look at the word “much” as your reference point. You can see that with the stronger magnifier with the smaller field of view you only get to see 1-2 words but with the weaker magnifier which has the larger field of view you can see 3-4 words.
The other factor that will impact on field of view is distance between the magnifier and the eye. In the left image the eye is 20 cm from the magnifier and you can see that the field of view is restricted. In the image on the right, the eye is 3 cm from the magnifier and all of a sudden this opens up the field of view greatly.
In conclusion, here is a summary of the concepts introduced in this lectorial.
Optical low vision aids
Low vision rehabilitation
Optical low vision aids – part 1
Prepared and narrated by Dr Meri Vukicevic
All images used in this presentation
are the author’s own unless
Low vision aids
Images from Schweizer Products for Improved Vision October 2012
Low vision aids
Magnifying power (MP) of an optical
system is defined as:
MP = retinal image size with aid
retinal image size without aid
Don’t forget about
Consider whether your patient needs
This will affect working distance
◦ The closer the object is, the more
Amount of accommodation
Accommodation demand =
100/working distance (cm)
Example 1: Reading material held at
◦ Accommodation demand = 100/40
Example 2: Reading material held at
◦ Accommodation demand = 100/10 = 10D
Focusing power of a lens
Unit of measurement = dioptre (D)
Increased power = decreased focal
Focal distance of a magnifier is determined
by dividing 100 by the dioptric power
FD (cm) = 100/D
Factor Effect on field of view
Diameter Wide diameter = wide FOV
Related to lens power
Strong lens = smaller diameter
Power Greater the power = smaller field of
eye & lens
Field of view is larger when it’s closer to