There are many patients who complain to me regarding variation in prescription lenses from one clinic to another.  It is disturbing to a person for many reasons:

  1. Was I using the wrong prescription?
  2. Has my eyesight deteriorated by wearing a wrong prescription?
  3. Is there something wrong with my eyes?
  4. Is there a medico-legal repercussion of this issue?

We shall start by understanding the fact that purpose of corrective lenses is to give a person clear and comfortable vision. The visual requirement of a person varies with the profession - airlines pilots, IT professionals, welders, microprocessor assembly workers, drivers, teachers, farmers, students etc.  A farmer may never realise the need of corrective lenses for distance as his visual requirement is gross!

The first step in determining the power of prescription lenses is to carry out "Cycloplegic Refraction".  Eye drops are put in the eyes to temporarily defeat power to focus.  This gives an idea of "Objective" baseline refractive status of the eyes.  Effect of these medicines is completely reversible.  A patient feels increased glare of lights and is unable to focus at near distance when cycloplegia takes place.  Effects last for a few hours in case of Tropicamide and Cyclopentolate and a few weeks when Atropine is used.  Use of Atropine is mandated in children below 10 years of age especially when there is a coexisting Squint problem.  A post cycloplegia test (PCT or PMT) is carried out after the cycloplegic effect is completely reversed.  This is also referred to as "Subjective" acceptance and prescription are finalised.  Subjective Acceptance is guided by Objective Refraction and the two may not  always be the same.

In the case of Hypermetropia (plus power for distance), a practitioner will generally prescribe the highest power with which a person gets comfortable 20/20 vision. In the case of Myopia (minus power for distance) generally, lowest power is prescribed with which a person gets 20/20 vision.  Students and IT professionals may be under corrected slightly to prevent eye strain.  In the case of astigmatism, a practitioner will perform a red/green, Worth's 4 dot test to arrive at an appropriate solution.

The point to be appreciated is that a person can have 20/20 vision with several combinations and a practitioner has to choose the most appropriate one! The accuracy of determining the power is important as it guides a practitioner but selection of appropriate power has several other determinants as swell.

Presbyopia is an age-related phenomenon that occurs due to inability to change focus for near due to age-related changes in the lens.  Reading glasses or glasses for near work are required generally around 42 to 45 years of age.  The addition of power depends on upon the profession, working distance, ambient light conditions, the height of a person etc. and hence the variability.   

Automated Refraction devices are available to practitioners these days that determine power with accuracy, but it is equally important to maintain their optics and calibrate them regularly. Any error in alignment of prisms, dust deposits, fungus growth, weakness of Infra-Red light source can induce inaccuracy.  There is a fogging device in many instruments that defeat the power to focus, any problem with that can induce an error. 

Generally, Myopia is over corrected and Hypermetropia is under corrected when "Cycloplegic Refraction" is avoided.

In conclusion, we can say that "Cycloplegic Refraction" is important for the accuracy of prescription and also gives a chance to the practitioner to examine the eye completely.  Appropriateness of a prescription should be discussed with a practitioner. "Proper centring" of lenses must be ensured for comfortable and clear visual experience.