3.4 - Causes and correction of ametropia Flashcards
What can spherical ametropia be split into
Myopia and hypermetropia
What happens in spherical ammetropia
Image formed in wrong place – front or behind retina
What is spherical ametropia caused by
- Axial length of eye incorrect ( myopia - too long, hypermetropia - too short )
- Refractive Power of eye incorrect – due to surface curvature and/ or refractive index have values not compatible with producing images on retina
Causes of ametropia
- Failure in coordination during growth of the optical components of the eye
- For high levels of ametropia larger than ±4D, eye is usually too long or too short – axial length
- Sometimes pathology e.g. nuclear cataract - eye changes its refractive state causes scatter of light – refractive index changes – myopic shift
What happens when axial length of eye increases by 1mm
Eye becomes -2.7 dioptres myopic
Small changes of axial length even 1mm have significant effect on ocular refraction as eye is powerful
What is length of cone photoreceptor
0.07mm
Where is far point in myopic eye
Real point infront of eye
What does light from object point do
Will strike eye at correct vergence K and form sharp image on retina
What happens when px looking at faraway object
Light changes so that it diverges at K at eye and so will form a sharp image on retina
What lens is needed to correct myopia
Negative
Why is negative lens needed to correct myopia
FOR CORRECTION OF MYOPES NEED A NEGATIVE LENS TO DIVERGE THE INFINITELY INCOMING LIGHT
– Light travelling parallel to axis from infinitely distant object, is diverged by spectacle lens and strikes eye with a divergence of K – value after refraction leaving eye will be K’ – sharp image formed on retina
- Second focal point of spectacle lens, Fsp’ coincides with MR – NEGATIVE LENS REQUIRED
- Rays incident on the eye appear to come from MR and thus form a sharp image on the retina
Spectacle correction of myopia - ray diagram
- Light coming into the spectacle enters parallel to the axis and it diverges leaving it
- Extrapolate that ray that leaves the spectacle lens backwards – 2nd focal point
- As that light coming from 2nd focal point of spectacle must diverge at the correct rate so that hits eye with a vergence of K then that point is also far point
How to fully correct myopes with negative lens
- The 2nd focal point of spectacle lens must coincide with far point of that eye
- Lens is put in place to make the light appear to come from far point – sharp image formed on retina
What lens is needed for spectacle correction of hypermetropia
Positive
Spectacle correction of hypermetropia - ray diagram
- Take parallel rays of incoming light from infinitely distant object and converges it
- Follow converging ray and extrapolate it forward, where it crosses the axis is the 2nd focal point of the spectacle lens
What happens in fully corrected eye
- 2nd focal point of the spectacle lens must coincide with far point
- If spectace lens achieves that, what eye perceives will come from far point so from sharp image on retina
When will a sharp image be formed on retina
Vergence of eye has to be equal to K – ocular refraction
In hypermetropia where is far point
Behind eye
What is ocular refraction
The thin lens power of a contact lens needed to correct ametropia
What happens to vergence as light travels through space
Vergence changes as light travels through space
When does vergence not change
When parallel rays of light coming from infinity – vergence 0
Where is spectacles placed
At the vertex distance, d, from the eye
True or false - spectacle plane refraction will not in general be the same as ocular refraction
True
What does spectacle lens produce
An ‘effective power’ that varies at different distances from the eye
