Monday, May 22, 2006

25

Near Point of the Eye

The near point of the eye is found when the uncorrected refractive error of the eye is added to the accommodative ability of the eye. If the amplitude of accommodation is 10D, the near point is 10cm in front of the eye (specifically, 10cm in front of the vertex of the cornea which is used as a convenient reference point)

Question: What is the effect on accommodation, for a given near viewing distance, for a spectacle versus contact lens corrected myopia? For a hyperope?

Answer: When a myope is fit with contact lenses, they may experience asthenopia, or “focusing” difficulties when doing close work. The symptoms generally subside as the individual adapts to the greater accommodative stimulus. However, this can be a more of a problem for the myope who is approaching presbyopia. This is because the myope, when viewing a near object, will accommodate more when his ametropia is corrected with contact lenses than when it is corrected with spectacle lenses.

For the hyperope who is approaching presbyopia, they will experience less difficulty when reading with a full contact lens correction than with the equivalent spectacle lens correction. This is because the hyperope, under the same conditions, will accommodate less with contact lenses than with spectacle lenses.

The effect is greatest with high refractive errors. For example, a spectacle-corrected myope may be able to read without bifocal glasses, but require reading glasses with contact lenses. Conversely, a hyperope may be able to forego reading glasses with contact lenses, but need bifocal glasses when wearing spectacles.

The change in lens position from the spectacle plane to the corneal plane is primarily responsible for the change in the stimulus to accommodation.

Question: In general, when using the direct ophthalmoscope, which patient provides the larger image of the retina, the hyperope or myope?

Answer: Myopes will have a larger image, hyperopes smaller. This is related to the total power of the eye.

Question: Where is the secondary focal point for an uncorrected myope found?

Answer: The secondary focal point for a myopic eye is anterior to the retina.

Question: Where is the secondary focal point for an uncorrected hyperope found?

Answer: The secondary focal point for a hyperopic eye is behind the retina.

Question: Where is the far point for an uncorrected myopic eye found?

Answer: The far point is between the cornea and infinity.

Question: Where is the far point for an uncorrected hyperopic eye found?

Answer: The far point is beyond infinity or behind the eye.



0 Comments:

Post a Comment

<< Home