To jump straight to the transposition activity; CLICK HERE.

Understanding Optical Prescriptions: Transposing to Alternative Sphere-Cyl Form

When it comes to understanding optical prescriptions, one of the key skills for both optometry professionals and students is the ability to transpose a prescription from its original sphere-cylinder (sph-cyl) form to its alternative form. This skill is not only crucial for accurate lens creation but also for effective communication within the optometry field.

Why Transpose an Optical Prescription?

Transposing an optical prescription is necessary because there are two different notations used for prescribing astigmatic corrections: the plus cylinder form and the minus cylinder form. The choice between these forms often comes down to personal preference or equipment requirements, but regardless of the reason, the ability to convert between these two is essential.

Back in the 1800s, when eye care professionals started making glasses, they used to put the part of the lens that corrects for astigmatism (called the cylinder) on the front side of the lens. This was the norm, and so they noted it down as a plus (+) cylinder in the prescriptions.

However, a century later, two eye care professionals, Glancy and Tillyer, figured out that this wasn’t the best way to make lenses. They showed that putting the cylinder on the back side of the lens, closer to the eye, and marking it as a minus (-) cylinder was a better choice. This helped keep the important parts of the lens aligned for better vision.

Even though they proved this was a better method, it took a long time for everyone to switch over. Some eye doctors (ophthalmologists) still used the old plus cylinder method for a while, even though the industry was moving towards the minus cylinder.

Therefore, it’s not really about which one is better overall; it’s more about what’s more convenient for the specific situation. The good news is that eye care professionals can switch between the two notations easily, so they can use whatever works best for the task at hand.

Situations that Require Transposition:

There are several situations that may require you to transpose an optical prescription to its alternative sphere / cylinder form. These include:

Inter-Professional Communication: Eye care professionals may need to share prescriptions with each other, with differences in the convention as to which their specialty records them (such as an optometrist, using minus cyl form, referring their patient to an ophthalmologist, who uses plus cyl form or an optometrist requesting a prescription be made up for lenses at an optical lab, that requires their prescriptions to be in plus cyl form).

Patient Records: When reviewing patient records, the prescription may be noted down in the opposite way to the way that you would do it. Therefore being able to transpose between the two forms yourself is important to interpret what the record is telling you.

Prescription Verification: Ensuring that the prescription is correct and can be cross-verified in different formats. Often the lenses sent from the optical labs to the optical practice will have their prescription in plus cyl form, thus if using minus cyl form, you would need to transpose to verify that they have sent you the correct lens prescription.

Step-by-Step Guide to Transposing Prescriptions

Here’s how you can transpose an optical prescription into its alternative sphere-cyl form:

STEP 1: Assess the spherical power, the cylinder power and the axis. Knowing the components of each is key. The diagram below demonstrates which part of the prescription is the sphere, cyl and axis respectively

STEP 2: Calculate the New Sphere Value: Add the sphere value to the cylinder value to get the new sphere value.

New Sphere = Original Sphere + Original Cylinder

STEP 3: Change the Sign of the Cylinder: If the original cylinder is positive, change it to negative, and vice versa.

New Cylinder = − (Original Cylinder)

STEP 4: Adjust the Axis: If the original axis is 90 degrees or less, add 90. If it’s more than 90 degrees, subtract 90. This is because the principle meridians of a toric lens are at 90 degrees to one another and run from a scale from 1 to 180 degrees.

New Axis = Original Axis ± 90

Example:

Let’s transpose the following prescription:

• Original Prescription: +2.50 -1.50 x 20
• New Sphere: ( +2.50 + (-1.50) = +1.00 )
• New Cylinder: ( -(-1.50) = +1.50 )
• New Axis: ( 20 + 90 = 110 )

The transposed prescription would be +1.00 +1.50 x 110.

Conclusion

Transposing an optical prescription ensures that regardless of the format, the optical power remains the same, providing the patient with the correct visual correction. Whether you’re a seasoned optometrist or a student, mastering this process is a valuable part of your toolkit.

The Eye Care Advocate has created a set of flashcards to help test your ability to transpose into the alternate sphere/cylinder form. Try them below and see how you get on!

Test Your Skill

---

If you found this Skill Centre activity on transposing optical prescriptions into its alternative sphere / cylinder form, please consider sharing with your peers. If you wish to have the latest updates and features to The Eye Care Advocate’s website, subscribe below!