Focus on Fundamentals
The thickness of lenses is quite possibly the second-most-important factor in a patient’s satisfaction with their eyewear, following right on the heels of how well they see through their glasses. In some cases, it can take precedence: Before the patient even puts the glasses on, they are seeing the thickness of the lenses.
Knowing the factors of where and why lens thickness presents itself and how to minimize or prepare your patient for the inevitability of lens thickness by aligning expectations and the reality of their prescription or frame selection will lead to improved patient satisfaction and a better experience for the dispensing optician.
“Why Are My Lenses So Thick?!”
Lens thickness comes down to four core factors—prescription power, lens material, frame size, and decentration. Some of these factors can be controlled and others are simply an obstacle to overcome.
Prescription Power
The stronger or higher the prescription power, the thicker the lens.
→ In a minus-powered lens used to correct nearsightedness (myopia), the lens thickness will be most noticeable at the edges or outer rim of the lens. This thickness location is due to the concave nature of the lens that is created by the curvature needed to correct the diverging light entering the eye.
→ In a plus-powered lens used to correct farsightedness (hyperopia), the lens thickness will be most prevalent in the optical center of the lens, curving outwards and tapering off at the edges of the lens. This is due to the convex nature of the lens that allows light to converge properly onto the retina.
→ In a cylinder or astigmatic lens, the lens thickness varies across different meridians due to the cylindrical power correction. The lens thickness in an astigmatic lens typically shows more prominently in certain areas depending on the axis, due to the way the cylindrical power is ground into the lens to correct the specific astigmatic meridians.
Lens Material
Lens material selection should be weighed against the patient’s prescription requirements. If a patient’s prescription is high, a higher index of lens material will reduce the thickness and weight. But if a patient’s prescription is low, a higher index of lens material may only influence the overall cost of the lenses, rather than benefit the patient with reductions in lens thickness. Using a lens thickness calculator such as opticampus.opti.vision/tools/thickness.php can help to quickly determine the difference between materials and give a tangible expectation of the patient’s lens cost versus lens thickness.
For more on lens materials, click here.
Frame Size
The larger the frame, the larger the required lens blank, and when the diameter of the lens increases, so does the thickness of the lens. For example: A +5.00 lens in a 60 mm lens has a center thickness of 3 mm, while the same Rx in a 70 mm blank will have a center thickness of 7 mm. With a minus-powered lens, a smaller frame size is less likely to capture much of the thickest portion of the lens edges, cutting out the lens shape from the thinnest center portion of the lens.
Decentration
This stands for the deviation from the lens center and the optical center requirements of the wearer. When horizontal (pupil distance) and vertical (heights or OC) decentration is minimized, the results are thinner lenses when mounted in the frame, particularly in higher prescription powers.
Even strong prescriptions can look significantly thinner if a frame is selected that aligns a patient’s horizontal and vertical optical centers with that of the frame.
How to calculate lens decentration: First calculate the frame PD (A + DBL = frame PD). Then frame PD - patient’s PD = decentration.
Tools To Measure Lens Thickness
→ Lensmeter or Lensometer: A device that measures the power and axis of a lens but can also measure the thickness of lenses. A lensmeter typically includes a gauge or dial that provides measurements of the lens thickness at various points, especially at the lens’s center and periphery. If your lensometer does not measure thickness, it can be used to mark the optical centers to identify the measuring location for plus-powered lenses.
→ Calipers: Used to precisely and accurately measure the thickness of a lens. They can be used at the center, edge, or any location on the lens to measure the thickness. This essential tool can also ensure lenses meet minimum center and edge thickness for safety and mounting into certain frame styles.
→ Digital Calipers: Offer more precise digital readouts of lens thickness at specific points across the lens.
Ways To Align Patients’ Expectations
It isn’t always easy to align a patient’s expectations of how thin a lens can be with what is technically or physically possible based on their prescription, lens material, and frame choice. Using visual aids and having an honest and frank conversation about what the finished product will look like is always better to take place prior to making the glasses, as opposed to having a difficult conversation about the finished product at dispensing.
In some cases, it is the professional duty of the optician to refuse to create a pair of glasses using a frame or lens material that is unsuitable for the patient and will lead to unnecessary thickness, weight, and visual experience. Using visual tools can help establish a patient’s expectations for what the finished product will look like and will allow the optician to guide the patient toward a better frame and lens material selection.
→ Visuals: Do the math. Numbers can easily create a more tangible expectation of what the finished product will look like. Using OptiCampus or a lens calculating app can compare the lens thickness based on index of material and frame size. Some of the apps also have visuals that go hand in hand with the calculations.
→ Lens Blanks and Mounted Demos: Having demo lenses of the same power in different blank sizes can allow a patient to see and feel the thickness and weight difference between blank sizes. Mounting the same power in two different lens indexes or blank sizes also gives patients a tangible example of what the glasses can look like. This can potentially convince the high-prescription wearer that the oversized frame they wanted isn’t going to be the finished product they desire.
