Chapter 10: Optics of Ametropia Flashcards
What is the definition of absolute hypermetropia?
the least amount of plus lenses needed for clear distance vision without cycloplegia
What is the definition of manifest hypermetropia?
the most plus that can be tolerated without blurring of vision and without cycloplegia
What is the definition of facultative hypermetropia?
the difference between the absolute and manifest hypermetropia
What is the defintion of latent hypermetropia?
the difference between the manifest hypermetropia and the hypermetropia measured with cycloplegia
Calculate the following for the following scenario:
A patient requires +3.00D to see in the distance clearly and can tolerate up to +4.50D without getting blurred. A cycloplegic refraction is +5.50D.
Absolute, manifest, faculatative + latent hyeprmetropia
- Absolute = +3.00D
- Manifest = +4.50D
- Facultative = +1.50D
- Latent = +1.00D
What is the name of the type of image formed in an astigmatic eye?
Sturm’s conoid
What is regular astigmatism?
when the principal meridians are at 90 degrees to each other and lie at or near 90 degrees and 180 degrees
What is oblique astigmatism?
the principal meridians are at 90 degrees to each toerh but do not lie at or near 90 and 180 degrees
What is irregular astigmatism?
the principal meridians are not at 90 degrees to each other
Which type of astigmatism cannot be corrected by spectacles?
irregular astigmatism
What is compound hypermetropic astigmatism / compound myopic astigmatism?
when rays in all meridians come to a focus behind the retina (hypermetropic) or in front of the retina (myopic)
What is simple hypermetropic astigmatism / simple myopic astigmatism?
rays in one meridian focus on the retina, the other focus lies behind (hypermetropic) or in front of (myopic) the retina
What is mixed astigmatism?
one line focus lies in front of the retina, the other behind the retina
What is anisometropia?
when the refraction of the two eyes is different
What degree of anisometropia between eyes in hypermetropic patients is large enough to cause amblyopia?
> 1D can cause amblyopia in the more hypermetropic eye
What degree of anisometropia between eyes in myopic patients is large enough to cause amblyopia?
>2 D or higher
Why are myopic patients less likely to develop amblyopia in anisometropia?
in hypermetropia, the individual eyes cannot accommodate by different amounts as accommodation is a binocular function, therefore the more hypermetropia eye remains out of focus
however, in myopia both eyes have clear near vision
What can the pinhole test tell you about reduced visual acuity?
if due to refractive error, the pin hold test will be much better than unaided acuity
if due to ocular pathology or neurology cause, pinhole acuity will show no improvement (in macular disease it may be even worse than unaided acuity)
What is the range of refractive errors beyond which the pinhole will not correct vision to 6/6?
+4 Dto -4D
What is a stenopaeic slit and what is it used for?
elongated pinhole, only allows light in the axis of the slit to enter the eye
can be used to determine refraction and principal axes in astigmatism
How is the stenopaeic slit used?
- Slit is first rotated to a position in which the clearest vision is obtained.
- Spherical lenses are added to give further improvement in acuity.
- The slit is then rotated through 90° and the spherical lens power adjusted to give best subjective acuity.
How do you calculate cylindrical correction from the stenopaeic slit?
it is the algebraic difference between the two spherical corrections used, and its axis is that of the original direction of the slit
What is the far point of an eye?
position of an object such that its image falls on the retina of the relaxed eye i.e. in absence of accommodation
What is important about the far point of the eye when prescribing a lens?
the far point of the eye must coincide with the focal point of the lens
the focal length, f of the correcting lens must be approx. equal to the distance, r of the far point from the principal plane