All About Base Curve 

Base curve refers to the front curvature of a lens and is measured in diopters. You can determine total dioptric power of a lens by adding the base curve (front surface) and the ocular curve (back surface).

The base curve chart illustrated below contains very general information regarding base curve selection. How base curves are chosen and optimized may differ with lens design technology or laboratories. More detailed charts are available from your laboratory or lens manufacturer. 

Lens Power and Base Curve:

• • Note that powers which fall around plano up to about +/- 2.00 diopters are designed on a +6.00 D curve on the chart below, but modern ophthalmic frames are widely made to ideally accommodate +4.00 D base curves. This has evolved to be the new standard curve for lower power lenses to suit wider frame compatibility.
  • As lens powers increase in the plus range, the curves become steeper.
  • For minus lenses, the base curves flatten as the power increases.

Base curve charts are designed to be guides, and the recommended base curve for any given prescription can vary from one laboratory or manufacturer to another, especially with “borderline” prescriptions.

Optical base curve theories have changed over time, and it can be difficult to choose an appropriate method for selecting the optimal base curve, if a chart or recommended base curve is not provided from the lab or manufacturer. Below are three optical theories that can assist in determining an appropriate base curve.

Corrected Curve Theory

Opticians and laboratories currently use the corrected base curve theory, which uses the average eye rotation as 13.00 mm. Most spectacle lenses made today are shaped so that aberrations or distortions are minimized. Namely, curvature of field, which is a lens aberration caused by the spherical power differences from the center to the peripheral edge of a lens. Corrected curve lenses are designed so that the ocular surface (back surface), is as close to -6.00 D as can be reasonably expected given the power called for. As the eye rotates in various directions, a -6.00 D back curve allows the distance from the eye to the back of the lens to remain relatively consistent.

Vogel’s Rule/Formula

This rule of thumb can be used to determine the appropriate base curve based on the lens’s spherical equivalent power.

For Plus Powers

• BC = spherical equivalent + 6.00 D

For Minus Powers

• BC = 1/2 Spherical equivalent + 6.00

OTI’s Base Curve Rule of Thumb

Below is a base curve rule of thumb that can be considered more of an optical theory that has much variation in actual practice but can be helpful in determining an appropriate base curve if needed.

1. Determine the true power(s) of the prescription:
   • If the prescription is spherical: use the sphere value. This is the only true power
   • If the prescription is toric: break the prescription down to find both of the true powers
2. After you have determined the true power(s):
   • If the biggest (or only) true power is a plus, add +4.00 to that number
   • If the biggest (or only) true power is minus, then:
     • If the biggest (or only) true power is up to a -8.00, combine the true power with a +8.00, and the result is the best base curve
     • If the biggest (or only) true power is between -8.12 and -15.00, use a plano base curve
     • If the biggest (or only) true power is over -15.00, a specialty lens such as a concave front or myodisc may be needed

FAQs for opticians commonly appearing in exams:

• • What is a base curve? The base curve is the primary surface curvature of a lens that determines its optical characteristics.
  • How to measure base curve of glasses? Use a lens measure or lens clock to accurately determine the base curve of a lens. 

Practical Application

For opticians, understanding and applying knowledge about the base curve is crucial. Whether you’re adjusting existing glasses or creating a new prescription, knowing how to interpret and use base curve charts can make a significant difference in patient satisfaction. Ensure that you can confidently measure, specify, and explain the base curve to your clients, enhancing their understanding and comfort with their eyewear.

In summary, mastering the base curve basics is an integral part of your preparation for the ABO/NCLE exam and your practice as an optician. The knowledge not only helps in passing the exam but also in delivering optimal visual solutions to your patients.