10.3 AO and As in emmetropes Flashcards
What is ocular accomodattion (Ao) ?
accommodation measured at the first principal plane of the eye - so it is at the surface of the reduced eye
What is Spectacle Accommodation (As) ?
accommodation measured at the spectacle plane (equivalently the spectacle lens power required for unaccommodated viewing of a near object)
-therefore. consider it equivalently as the poisitve spectacle lens power required to compensate for the divergence of light coming from a near object - so this positive power would removed the divergence off the object for light travelling onto the eye
What are Ao and As not?
not necessarily the maximum values of accommodation but the amount of accommodation required to view a near object. (AMPLITUDE)
What does the diagram for ocular accomodation show ?
Ao - light diverging from near object from left - and strikes the reduced eye with vergence L
-power of eye to focus on retina is Fe + Ao
What does the diagram for Spectacle accomodation show ?
- Have near object on left
- light diverges form it until it hits the spectacle plane
- as this is defined as the positive power of the spectacle plane that negates the divergence of light from object
- positive power in spectacle plane is the As
- if it is correct light leaving the spectalce lens is parallel to optical axis
- power As has compensated for that divergence and now the vergence value is 0 as it travels to eye
- it is now an emmetropic eye - the power of eye is still Fe because its the power of the spectacle plane thats provided the accomodation and focuses it on the retina.
Why is spectacle accomodation useful to us?
- as we are measuring things from the spectacle plane
Why is the power required in Ao less than As?
due to the spectacle plane is closer to the object, by the vertex distance d, therefore needs a more positive power is needed to cancel /overcome the greater divergence of the light so light can focus on the retina.
What is the formula for when tracing the vergences (object distance l from the eye) in Ao in emmetropes ?
K’ (As image is formed on retina)= L + Fe + Ao
What about the formula for tracing for an emmertrope in Ao ?
K’ = Fe since K = 0
Therefore the Ao is the negative value fo the object vergence
-as it is a real object, the light is diverging from it and therefore the negative sign means that this value will be positive
Ao = -L
-accomodation - increase in positve power in eye hence positive value
What is the formula if the distances were measured for the spectacle plane in Ao ?
In terms of ls if the object distance is measured from spectacle plane
Ao = -Ls / 1 - dLs
What is the formula for tracing vergences forwards in As in emmetropes?
Ls’ = Ls + As
What is the value of the spectacle plane ?
it is the As - spectacle acoomodation
What is the formula for when L’ = 0 in As emmetropes?
- since the eye is unaccommodated
- and is also emmetropic - (so if light travels into the eye parallelt ot he axis wiht the vergence of 0 it will be accurately focused on the retina)
As = -Ls
What is the formula when the object location is given from the eye ?
From the figure ls = l + d
Therefore
As = -1/ l + d = -L/ 1 + dL
What is the difference between ocular and spectacle accomodation in emmetropes?
- For different object positions ( x axis)
- accommodation (y axis)
- As object becomes closer accommodation is higher
- spectacle accommodation is higher than Ao
