Why and How to Transpose a Bifocal Prescription Into a Single Vision

Why and How to Transpose a Bifocal Prescription Into a Single Vision

07/09/2021 Off By admin

Bifocals of today are as thin as any other lens, and they come in all forms – round, blended, D or FT28, 35 segments. The job of an Optician is to determine what type of bifocal best meets any individual patron’s needs.

Bifocals are needed and useful whenever a person is prescribed two fields of vision for an ocular condition called presbyopia. The patient needs distance vision and near vision for reading things up close. The type of bifocal is usually determined by the Optician and patient after a series of questions such as:

  • Does the patient want a visible bifocal segment?
  • Does the patient spend a large amount of time reading?
  • What type of career/lifestyle?

What if, however, a person really doesn’t want nor feel that they need a bifocal? Depending on their lifestyle and cares, many times a that bifocal can be transposed into single vision lenses – one for distance vision and/or another for reading only.

An example of this would be if a patient’s bifocal prescription is as follows:

OD: -0.50 sphere

OS: -0.25 -0.25 x 172

Add: 1.75

Clearly, the distance prescription (OD & OS) is 20/30 or better when it comes to visual acuity, therefore, there is no need to even use glasses for distance vision as there is little correction to obtain optimal vision. However, as far as the add or bifocal reading power, there is a need for correction in the near vision.

In this example, the patient would prefer to not use bifocals as the doctor has explained that she doesn’t need them yet while driving and things of that nature, but most of her visual problem comes from reading. On top of that, she doesn’t want to spend the extra money if she doesn’t need to even have distance vision inside the bifocal.

Surprise! The patient can save money and only use her eyewear for when she needs them most…reading. In order to change the prescription to a single vision from a bifocal prescription, simply use mathematics – algebra.

OD: -0.50 sphere

OS: -0.25 -0.25 x 172

Add: 1.75

Simply add the bifocal power (+1.75) to the sphere powers. The result is as follows:

-0.50 + 1.75 = OD: + 1.25

-0.25 +1.75 = OS: +1.50 -0.25 x 172 (The cylinder power and the axis always remain the same!)

Now the patient can save money while feeling better about her overall wear when it comes to eyeglasses.

Again, this is only one reason as to why an Optician may feel it best to change a prescription to single vision. Other reasons may exist, and it is up to that eyewear professional what is needed for that particular patient. If, however, there is an issue or questions with any changes, Opticians should always contact the prescribing physician.