Visual accommodation and defective vision

Visual accommodation

• Visual accommodation is the ability of eye to keep an object in focus on retina as its distance varies.
• It diminishes with age or during diseased condition
• During adolescence: 12-16 D of accommodation.

               Adult at 40: 4-8 D

                After 50      : 2 D

Defective vision

When focus is in front of or behind the retina, it is defective vision.

Some defective visions are:

a. Myopia

• It is short- sightedness
• Can see nearby object but unable to see distant object.

Causes:

• Eyeball being too long
• Excessive refraction of lens.

Correction :

• Concave lens of particular focal length.

b. Hypermetropia

• It is long sightedness
• Can see distant object but can’t see nearby object.

Causes :

• Eyeball being too small
• Insufficient refraction of lens.

Correction :

• Convex lens of suitable focal length.
• Muscle work which can change shape of lens.

1. Presbyopia

• It is also long sightedness occurs especially to old aged people.
• It is due to loss of elasticity of lens and stiffness of ciliary muscle.

c. Astigmatism

• This defective vision is unable to see both horizontal and vertical view with same clarity
• Blurred vision