Optics of Ametropia Flashcards
What is ametropia
In contrast to emmetropia the ametropic eye fails to bring parallel light to a focus on the retina, i.e. the second principal focus of the eye does not fall on the retina
What is myopia
the second principal focus lies in front of the retina.
Causes of myopia
1) This may be because the eye is abnormally long. This is called axial myopia and includes high myopia in which there may be a posterior staphyloma.
2) Alternatively, the eye may be of normal length, but the dioptric power may be increased. This is called refractive or index myopia. Examples of this are keratoconus, where the corneal refractive power is increased, and nucleosclerosis, where the refractive power of the lens increases as the nucleus becomes more dense
What is hypermetropia
the second principal focus lies behind the retina
Causes of hypermetropia
1) If the eye is short relative to its focal power, then axial hypermetropia results. 2) Alternatively, if the refractive power of the eye is inadequate, then refractive hypermetropia results. Aphakia is an extreme example of refractive hypermetropia
How do phakic patients overcome some or all of their hypermetropia
by using accommodation for distance vision. They then have to exercise extra accommodation for near vision.
How to classify hypermetropia
manifest and latent hypermetropia
What is manifest hypermetropia
the strongest convex lens correction accepted for clear distance vision
What is latent hypermetropia
the remainder of the hypermetropia which is masked by ciliary tone and involuntary accommodation
What is facultative hypermetropia
Hypermetropia which can be overcome by accommodation
What is absolute hypermetropia
hypermetropia in excess of the amplitude of accommodation is called absolute
What is regular astigmatism
If the principal meridians are at 90° to each other
What is oblique astigmatism
If the principal meridians are at 90° to each other but do not lie at or near 90° and 180°
What is irregular astigmatism
If the principal meridians are not at 90° to each other
What is compound hypermetropic astigmatism
rays in all meridians come to a focus behind the retina
What is simple hypermetropic astigmatism
rays in one meridian focus on the retina, the other focus lies behind the retina
What is mixed astigmatism
one line focus lies in front of the retina, the other behind the retina
What is simple myopic astigmatism
one line focus lies on the retina, the other focus lies in front of the retina
What is compound myopic astigmatism
rays in all meridians come to a focus in front of the retina
What is anisometropia
When the refraction of the two eyes is different.
Larger degrees are a significant cause of amblyopia
How does the stenopaeic slit work
when the slit lies in one principal axis of the astigmatic eye, the second line focus is eliminated and the blur of Sturm’s conoid reduced thus allowing a clearer image to be formed
What is the far point
is the position of an object such that its image falls on the retina of the relaxed eye, i.e. in the absence of accommodation
The distance of the far point from the principal plane of the eye is denoted by r, which according to sign convention carries a negative sign in front of the principal plane and a positive sign behind the principal plane
What is the static refraction/ametropic error
The reciprocal of the far point distance r, in metres, is symbolised by R, expressed in dioptres
What happens when the correcting lens is moved further from the eye in hypermetropia
the image is brought still further forward.
The effectivity of the lens is said to be increased. Therefore in this position a weaker convex lens throws the image onto the retina and corrects the hypermetropia