LET US DISCUSS MAGNIFICTION

Ayomikun Dahunsi (OD)

 

To magnify means to increase, to enlarge, to enhance, to expand and a lot more. In the context of low vision, Magnification means to increase the retinal image size. This makes seeing an object easier for people with visual impairment.

There are several ways of magnification for patients with low vision;

●     Relative Size Magnification

●     Relative Distance Magnification

●     Angular Magnification

 

RELATIVE SIZE MAGNIFICATION

In this case, the object is made twice the size thereby making the image on the retina twice as large hence creates X2 magnification. For example, large prints book or television with large screen size. This type of magnification because of the physical limitations of how large you can make an object such as book look like, it is often limited to about X2.5

 

          M = New Object Size

                  Old Object Size

 

RELATIVE DISTANCE MAGNIFICATION

In this type of magnification, we reduce the distance of the object. That is half the distance of the object and the retinal image becomes twice as large creating a X2 magnification.

When you view a television from 3m instead of 6m, you get a X2 magnification. It is also applicable for near distance where a print placed at 40cm is moved closer to 10cm to give a X4 magnification. This magnification is common among many children and young adults. They use accommodation as a brand for this type of magnification. For older adults with little or no accommodation (presbyopia), or young individuals with the inability to sustain their accommodative need, though image on the retinal will be magnified when object is closer, image will unfortunately be blurred.

 

             M = Old Object Distance

                  New Object Distance

 

ANGULAR MAGNIFICATION

This is also known as TELESCOPIC MAGNIFICATION. Telescopes are the only optical aid for distance magnification. However, when focused on a near target, they can also increase working distance. Telescopes are very efficient In producing magnification and also allowing the patient to stay at their chosen distance from a task like viewing a blackboard or a large publicity banner. The limitation is a restricted field of view and because of the space distortion and perception of movement, to walk around while using it is quite difficult. To follow moving objects particularly, their use needs quite a lot of manual practice, skill and handiness. This is why distance telescopes are prescribed mostly at a follow up appointment since it gives room for assessment of ability and motivation.