4: impact of uncorrected refractive error on vision intermediate and near viewing distances ; impact of pupil size

unaided vision at near, intermediate and distance

• range of clear vision; from far point ( no accom in use) to the near ppint ( accom max exterted)

far point and near point in an uncorrected myopic eye

-4.00DS myopic eye with 10D of accom

the far point at 25cm is where there is no accomodation

at the near point of 7.14cm all 10D of accomodation is being used

clear vision is between the far point and the near point

myopia of -4.00 with 10D accom

unaided vision remains poor until object is very close to the far point

inside the far point, the eye retains clear vision up to the near point

gap between near and far dictates the range of clear vision and depends on the amplitude of accom

MAR= kDE

MAR = 0.83 × 3mm pupil x 4

hyperopia with +4.00DS accom amplitude 10D

unaided vision remains good until the object is insed the near point

the eye is capable of achieving clear vision up to the near point but it requires lots of effort ; more effort needed for nearer target

range of clear vision for this eye would be from infinity to the near point

4D of mixed astigmatism with an eye with 10D accom

+2.00/-4.00 × 90

there is no far or near point because, unaided, there is no viewing distance for which a point image is formed on the retina, until the astig is corrected vision is blurred at all distances

for this eye the CLC on the retina and the eye has enough accom to keep the CLC on the retina as the target gets closer and closer , up to 10cms away from cornea

variability comes from the eye altering its accom to place diff parts of the interval of sturm on the retina

as target gets closer vision remains poor, doesnt improve or fall dramatically

if there is myopia alonside astig them unaided worse at distance, if hyperopia alogside astig then near worse

impact of astigmatism

evn 1D of uncorrected astigmatis causes a reduction in distance vision

more astig means more impact on unaided vision

near and reading also decrease with increasing astig

impact of with/against /oblique

impact of with and against the rule astigmatism on unaided distance vision is similar

eyes with oblique astig have significantly lower visual performance than eyes with WTR or ATR astig

impact of uncorrected ast depends on what

depends on the amount and axis of astigmatism but is also curcially dependent on the pupil size

blue and red at the start where most blurry, the sharp drop is due to finite amount of accomodation

green drop at the near where blurred is caused by accomodation being used up to see clearly in the distance , gives incorrect impression that the eye has insufficient accom

variable vision is signalled by the bumpy lines

what happens as the object being viewed gets closer

1. light leaving the object becomes increasingly divergent, this improves vision in uncorrected myopic eyes, but it places a bigger demand on accom in uncorrected hyperopic eyes and in uncorrected astigmatic eyes

2. the angular subtense of the object increases so the unaided vision of the eye does not need to be s good to see the target

hence if unaided vision at near/intermediate distance is only 6/12 at that distance, this is easily enough to perform that task

what happens when object gets closer for near and intermediate viewing

prediction vision in eyes with various types and amounts of uncorrected refractive error is complicated becase it depends on the balance between accom demand , the accom availability

the acuity required for a given task is less when viewing from an intermed or near distance

impact on pupil size

whenever vision is reduced by uncorrected refractive error, vision will be better when the pupil is smaller, and poorer when the pupil size is larger

myopic unaided

eyes remains blurred until the object of interest gets close to the far point

vision becomes clear when object is at the far point where no accom is needed for clear vision

inside the far point vision in myopic eyes remains clear provided the eye has accom and it remains clear as the object gets closer still until the eye runs out of accom

hence uncorrected myopes complain that at intermed and far distance vision is blurry but near is clear

hyperopia unaided

goof at distance dependind on size of amplitudue of accom

closer the object the more likely vision is to e variable or blurred or clear but uncomfortable

just ecause the eye has 6D of accom doesnt mean the 6D is continously available

how to distnguish between hyperopia and presbyopia

presbyopia is a problem for insufficient accomodation

hyperopia is a problem of an underpowered eye