10.4 - AO and AS in ametropes Flashcards
What is the formula for Ao for ametropes ?
Refraction at the eye
K’ = L2 + Fe (power of eye at unaccommodated) +Ao
Therefore
Ao = K – L2
What is the Ao for ametropes using diagram in slide 3?
- consider near object
- light from near object diverged by spectalce lens
- strikes eye diverging
- eye has to exert accommodation and an Ao value to focus on retina
- can see vergence being diverged as it hits the eye and then converging on retina in order to be seen clearly.
What is the formula of Ao for ametropes by tracing vergences ?
it can be shown that
L2 = Ls + Fsp / 1-d (Ls + Fsp)
Therefore:
Ao= K - Ls + Fsp / 1-d (Ls+ Fsp)
How do we calculate L2?
need to trace the vergences from the object through the spectacle lens to the eye
-vergence at eye.
What is Ls?
vergence of light at spectacle plane of object
What must you remember about this?
Ao = K – L2, you must also remember that “L2” is the vergence at the eye from the image produced by the spectacle lens
example to calculate the Ao ?
A myope is fully corrected by a thin spectacle lens of power −3D at a vertex distance of 12mm. What is the ocular accommodation when viewing an object -25cm from the spectacle plane?
- distance vision
K = Fsp/ 1-dFsp
K = -2.896D
For near vision
Ls’ = Ls + Fsp
L2= Ls’ / 1- dL’s
Ao= K - L2
What is the Fsp?
spectacle refraction
What is the variation of Ao
in Slide 6 ? - emmetrope
y axis - Ao (D)
x axis - Fsp (D)
with objects at 3 different positions ( 3 colours lines)
-horizontal lines - represent the required accomodation in spectacle plane that would be requried to overcome the vergence from object positon
-So for the red line ( object is at 25cm ) and so should expect an Ao of 4D - but shows that accommodation for a spectacle refraction of 0 (emmetrope) ocular refraction is less than expected .
- As Ao lower than the horizontal line
Increase in divergence between spectacle and Ao that we have seen in emmetropes already
-The (di)vergence is less at the eye so the curves lie below the reference lines for an emmetrope
What is Ao like in myopes and hypermetropes?
In myopia- there is reduced Ao
In hypermetrope there is increased Ao
For example if you take a myope what happens to the Ao ?
A myope and near object -0 the light is diverging from object as it hits the spectacle plane , the spectacle plane diverges more so again and
increasingly so as we go to increased levels of myopia- that divergence when it travels between spectacle plane and eye gets less and so reduces accommodative demand at the eye.
Why does the difference in vergence between spectacle and ocular plane increases as the divergence increases ?
take a -5 myope
-light comes from object and is diverged further at the -5D at the specatcle lens and ends up at -9D
-When the vergence transfers to the eye the vergence reduces a reasonable amount
If specatcle correction is -10D then the light leaving the spectacle plane could be -14D and the difference of transferring across vertex distance becomes even higher which means an even greater reduction in the Ao at the eye.
The change in vergence lessens the Ao in myopes and converse is true for hypermetropes
What are the consequences ?
Spectacle corrected myopes have a reduced accommodative demand.
This is not the same if corrected with contact lenses or refractive surgery (see later).
Hypermetropes will have a reduced accommodative load if corrected with CLs or refractive surgery
What is the As for ametrope expression ?
As spectacle lens power required for unaccommodated viewing of a near object
Vergence at eye must be K hence Ls’ = K/(1+dK)
and Ls’ = Ls + As + Fsp
Therefore As = Ls’– Fsp– Ls
Summary
In spectacle corrected ametropes, the ocular accommodation depends on the change in vergence from spectacle to ocular plane.
Spectacle corrected myopes need to exert less accommodation than emmetropes whereas hypermetropes exert more accommodative effort.
