Spherical Ametropia Flashcards
What is Ametropia
It is the inability of the eye to produce a sharp image on the retina
Measured in dioptres
Refractive Error
Spectacle Refraction = measured at the spectacle plain for specs
Ocular Refraction = measure at the eye for for laser surgery and contact lenses
What are the two types of refractive error
Spherical ametropia and astigmatism
Emmetropia
Sharp optical image formed at retina
Second focal point, Fe’, coincides with the retina for an unaccommodated eye
The far point, Mr is at infinity
What is the Far point of Distinct Vision, Mr
Object point is conjugate - as in pairs - with retina in the unaccommodated eye
Eg for emmetropes, the object is at infinity and the image falls on the retina
Emmetropic reduced eye
K’ = K + Fe
K= ocular refraction - the incident vergence - vergence striking the lens/ eye required to form sharp image on the retina
So K is the thin lens power of contact lens needed to correct Ametropia
What is myopia?
Short sighted
Second focal point falls in front of the retina for an unaccommodated eye
Far point, Mr is in front of the eye - a real point
Objects closer to unaccommodated eye are seen clearly
What is Hypermetropia?
Second focal point falls behind the retina for an unaccommodated eye
Far point is behind the eye - a VIRTUAL point
The retinal image is blurred but not always - doesn’t always cause blurring as the eye can accommodate. The eye can increase its focal point. Increasing dioptric power of the eye places optical image on retina
What is Accommodation?
Ability of the eye to increase its optical power so that it is able to focus at near
Lens does this by increasing its dioptric power
Unaccommodated eye has weakest optical power
Presbyopia
= Condition where accommodation is insufficient for near work
Accommodation declines with age
Very little ability after 60 years
Ability to focus on near objects is affected unless myopic
Causes of Ametropia
- Failure in coordination during growth of optical components of the eye
- For ametropia larger than +-4D, eye is usually too long or too short
- Sometimes due to pathology eg nuclear cataract
Spectacle vs Ocular Refraction
- Vergence changes as light travels through space
- Spectacles placed at the vertex distance, d from the eye
- Spectacle plane refraction will, in general, not be the same as ocular refraction
What is Effectivity?
= The vergence of light leaving a spectacle lens is different to the vergence striking the eye
- A spectacle lens produces an effective power that varies at different distance from the eye
- Negative spectacle lenses need more power = higher than ocular refraction
- Positive spectacle lenses need less power = lower than ocular refraction
