topic 3: Refractive Error of the Eye & its Correction Flashcards
what is an eye without lens called?
An eye without the crystalline lens is aphakic (has aphakia).
what is emmetropia?
There is no refractive error because the eye’s total refractive power is just adequate to focus parallel light rays from a distant object precisely on the retina.
what is myopia? how does it happen?
In myopia, the eyeball is too powerful for its axial length, leading to light rays being focused in front of the retina.
Myopia can be due to a cornea or lens that is too powerful, or an axial length that is too long for the eye’s power.
what is hyperopia?
In hyperopia, the eyeball is not powerful enough for its axial length, leading to light rays being focused behind the retina.
what is far point?
point in space that is conjugate with the retina of the unaccommodated eye. It is denoted by the symbol, Mr
the position in space where, if an object were to be placed there, it will be seen clearly by the unaccommodated eye.
what happens when object is placed further than far point for myope, hyperope and emmetrope?
for the myope, the light rays will come to a focus in front of the retina (blur)
for the hyperope, assuming ‘further than the far point’ means not so far behind the retina as the far point, the light rays will come to a focus behind the retina. (blur)
for the emmetrope, the far point is at infinity so it is not possible for any object to be positioned further than the far point)
what happens when object is placed nearer than far point for myope, hyperope and emmetrope?
nearer than the far point will be blurry because the light rays from the object will be even more divergent / less convergent.
This will require more refractive power to focus them on the retina. The eye can obtain more refractive power by accommodating.
what happens in astigmatism?
In astigmatism, light rays are not focused to a single point. Instead, they are focused into 2 lines. This is because in astigmatism, there are 2 different ‘directions’ or meridians that light is being bent.
what is the difference between regular and irregular astigmatism?
In ‘regular astigmatism’, the 2 lines are perpendicular to each other and can be corrected by the standard spectacle prescription or soft contact lens with astigmatism correction.
In ‘irregular astigmatism’, the 2 lines are not perpendicular to each other and cannot be corrected fully using the normal spectacle or astigmatic soft contact lens.
Irregular astigmatism can be found in eyes with disease or distorted eyes
what are the 3 types of astigmatism?
- WTR (power of vertical M higher than horizontal meridian, axis of corrective cyl is 180 +/- 30)
- ATR (power of horizontal M higher than vertical meridian, axis of corrective cyl is 90 +/- 30)
- oblique (power of stronger m is neither hori nor vert, axis of corrective cyl is 45 +/- 15 or 135 +/- 15)
what is ocular refraction , K?
dioptric distance to the eye’s far point.
a measure of the eye’s refractive error.
what is the dioptric length of the eye?
kind of like image vergence
what is the formula for K’?
K’ = n’ / k’
n’ = refractive index of the reduced eye 4/3
k’= axial length in metres
what is the relationship between K and K’?
K’=K + F
how does lenses affect magnification?
Plus lenses make the object appear larger (magnified). Negative lenses make the object appear smaller.
