As eyecare teams, we deal with patients’ prescriptions all day – but how do different prescriptions correct sight? In this month’s OA Corner, we look at different prescriptions, and how spectacle lenses can maximise a patient’s visual potential.
The eye has a refractive power of approximately +60.00D and the length of the eye should be such that a distant object should be clearly focused on the retina when the eye is unaccommodated. If this is the case, the eye is said to be emmetropic, which means no correction is needed. However, if the power of the eye is too strong, or too weak, too long, or too short, light will not focus on the retina and the retinal image will be blurred.
Myopia
Consider the prescription RE: -2.00DS. This is a prescription for a short-sighted (myopic) patient. In this case, the power of the eye will either be too strong, or the length of the eye will be too long, and so the image from a distant object will fall in front of the retina, meaning the retinal image will be blurred and the patient will not be able to see distant objects clearly.
As the image falls in front of the retina, a concave, or minus lens, is needed to diverge the light, and so push the image further back onto the retina, thus providing the patient with clear distance vision. This is how a minus lens corrects vision for a myopic patient.
Hypermetropia
Now consider the prescription +2.00DS. This is a prescription for a long-sighted (hypermetropic) patient, and in this case the power of the eye will either be too weak or the length of the eye will be too short and so the image would fall behind the retina. This means a convex, or plus lens, is needed to converge the light to bring the image forward onto the retina. This is how a plus lens corrects vision for a hypermetropic patient.
Astigmatism
A prescription may also contain a correction for astigmatism, and this can also be in conjunction with myopia or hypermetropia. Astigmatism means the eye needs a different correction in one meridian, compared to the other. Imagine the shape of a rugby ball, compared to a football. The football has the same curvature all round, whereas the rugby ball has different curves at 90˚ to each other.
In terms of the retinal image, an astigmatic eye will have a different curvature in both meridians and so two images will be formed. This means that a lens with two different powers will be needed to bring both images to a focus on the retina.
Both meridians might be myopic (both images in front of the retina), both might be hypermetropic (both images behind the retina), or one might be myopic and the other hypermetropic (one image in front and one behind the retina). It is also possible that one meridian will focus on the retina and the other meridian might be myopic (in front) or hypermetropic (behind).
Presbyopia
A prescription may also contain a reading addition. This is required when the patient finds it difficult to see clearly for near vision and usually begins after age 40 and is called presbyopia.
The crystalline lens in the eye is usually flexible, and the eye is able to adjust its power from distance to near. However, as the lens ages, this becomes more difficult and so some additional plus power is needed in the form of reading spectacles. This additional plus power replaces the reduction in plus power when the eye is no longer able to add this power once presbyopia begins.
Hopefully this article has provided some understanding for optical assistants about how lenses correct vision. Next month, we will look at a selection of lens types available and the different ways they correct vision.
Sue Deal FBDO R is a practising dispensing optician, ABDO College examiner, senior tutor and supervisor for dispensing opticians. She is also a practice visitor and external moderator for ABDO. She was recently awarded the ABDO Medal of Excellence for her outstanding services to the profession.