3.2 - Emmetropia and Myopia Flashcards
What is emmetropia
Sharp optical image formed at retina
Second focal point of eye Fe’ coincides with the retina for an unaccommodated eye
Where is far point located in emmetropic eye
At infinity
Where does 2nd focal point in emmetropic eye fall on
Retina
What is meant by unaccommodated
Distance vision - eye hasn’t increased power in an attempt to focus at something that’s close
What happens in ray diagram for emmetropic eye
Parallel rays entering eye = relate to infinitely distant object = converged by eye = single surface reduced eye to a focal point which is at retina itself
Can VA be improved with optical aids
Visual acuity can’t be improved with optical aids e.g. contact lenses or glasses but it doesn’t have to be 6/6
Does optical correction stop at 6/6
NO..
You can use optical correction and create a VA of 6/5 or 6/4 – becomes end point or point where refracted error fully corrected
Shouldn’t stop at 6/6 – try correct all of refractive error – the point at which no optical correction can make any further improvement ( no such thing as Snellen standard observer 6/6 )
What is far point
Object point conjugate with the retina in the unaccommodated eye
Anything further than far point will not be in focus
What is conjugate pairs - object and image
Object point will form image = object and image points = points conjugate are in pairs
If at object point is at infinity where is image formed
At 2nd focal point = object conjugate with 2nd focal point
What is standard emmetropic reduced eye
Single surface model of eye - reduced eye
What is power of standard emmetropic reduced eye
+60D
What is n of standard emmetropic reduced eye
1
What is n’ of standard emmetropic reduced eye
4/3
What is k
Far point distance measured from the surface of the reduced eye
What is K
Ocular refraction.
It is the vergence of light striking before eye
What is k’
Axial length of the reduced eye - distance from single surface of eye to retina
What is K’
Dioptric length of eye
The vergence of light leaving the surface of the reduced eye that forms an image at the retina.
What is Fe
(Equivalent) power of the reduced eye
What is Fsp
Thin lens power of a spectacle lens
Where is far point distance k for emmetropic reduced eye
At infinity - infinitely distant for emmetropic reduced eye
What is ocular refraction K equation
K = n/k
What is dioptric length K’ equation
K'= n'/k' K' = K + Fe
Why is ocular refraction K 0 for emmetropic reduced eye
K = n/k n is 0 k is infinity Anything divided by infinity is 0 So K is 0
What is vergence of light travelling from infinitely distant object
0
What is myopia
Second focal point Fe’ falls in front of the retina for an unaccommodated eye
Retinal image blurred – vision reduced as light spreads out by the time it hits retina
Is myopia long or short sightedness
Short sightedness
Where is far point located in myopic eye
In front of the eye – a real point
Far point - object point conjugate with retina
What happens in myopic eye
Objects closer to the unaccommodated eye are seen clearly
Distant objects might focus short to retina but its possible to find distance closer to eye - produce sharp image on retina - furthest distant this can be is far point
Where is 2nd focal point for myopic eye
Infront of retina
What happens too axial length in myopic eye
Its too long
What is far point - more detail
Distant objects might focus short to retina but its possible to find distance closer to eye - produce sharp image on retina - furthest distant this can be is far point
e.g far point was 50cm in front of eye - can find object position at 50cm = produce sharp image on retina - anything beyond 50cm wouldn’t be sharply focused
What happens if far point was 50cm in front of eye i.e. object position at 50cm
Produce sharp image on retina - anything beyond 50cm wouldn’t be sharply focused BECAUSE
Anything further than far point will not be in focus
How to find axial length k’
- k = given in question
- K: K = n/k
- K’: K’ = K + Fe
- k’: K’ = n’/k’ SO k’ = n’/K’
What does it mean if k is -0.2m
Far point 0.2m to left of eye ( left because its negative )
What is ocular refraction, K
- Incident vergence required for a sharp optical image to be formed on the retina
- The thin lens power of a contact lens needed to correct ametropia
- Change in power of cornea if refractive surgery
What is K equal to
The power of a thin lens placed in contact with the eye to fully correct its ametropia
Where will object be at distance k
At far point location
What are K and K’
- K and K’ are parameters/properties of reduced eye and not always vergences from ray tracing
- They are not necessarily object or image locations
When might vergences L and L’ equal K and K’
If the image is formed on the retina and the object is at the far point
