The process of getting used to a pair of progressive lenses is called “adaptation”. Thankfully, today’s progressive lens wearers live in the era of computer assisted lens design. Thanks to computers, we can design your progressive lenses specifically to minimize or even eliminate your progressive lens adaptation experience altogether. Providing my patients with the best progressive lens adaptation experience requires understanding the variables that contribute to each patient’s unique adaptation experience, and then customizing their progressive lenses specifically for their eyes. In fact, the desire to give you the very best possible progressive lens experience was one of my prime motivations for opening my Calgary Optometry Clinic, Doig Optometry.
The longer I practice optometry, and the more I study progressive lens design, the more I’m convinced that magnification effects determine the patient’s adaptation experience. Specifically, adaptation problems are due to a difference in magnification in the upper (distance) viewing zone of the progressive lens, and the magnification in the lower (reading) zone.
Spectacle lens magnification is a product of two variables: the power of the spectacle lens, and the shape of the lens.
Spectacle Lens = Shape X Power
Magnification Factor Factor
The shape factor is the magnification that results from the shape of the front of the lens. We call it F1 in the following formula:
Shape = 1
Factor 1 - (t/n) F1
There are two other things that can increase or decrease the magnification that comes from the shape factor. The thickness of the lens (t in the formula above), and the index of refraction of the lens material (n in the formula above). Index of refraction is a measure of how well the lens material bends light. If the material bends light really well, it allows for greater control over thickness and the shape of the front of the lens.
The power factor is the magnification that results from the power of a prescription lens. The power of a spectacle lens can be measured at the front surface of the lens, or at the back surface of the lens (F’v in the formula below). The power of a prescription lens can also be measured at a certain distance from the lens. These three measurements will all be different. The important point here is that the magnification due to the prescription power of a spectacle lens will vary at different distances from the lens. In other words, as your progressive lenses move closer or further from your eyes, the size of the image you see will change. The change will be larger or smaller depending on your prescription. That’s why we measure the distance between your eyes and your glasses at my Calgary Eye Clinic, Doig Optometry. This distance is called “the vertex distance” (v in the formula below).
Power = 1
Factor 1 - v F’v
The take-home message from all this math can be summarized this way:
The front of a progressive lens creates magnification due to its shape and thickness. The back of a progressive lens creates magnification due to its power and its distance from the eyes.
Prior to 2008, when every progressive lens was manufactured in a similar, conventional way, the reading prescription was always on the front and the patient’s distance prescription was on the back of the lens. This had a huge impact on the magnification profile of these conventional progressive lenses because:
- The front of a progressive lens creates magnification due to its shape, and the front of a conventional progressive lens is shaped differently at the bottom compared to the top.
- The front of a progressive lens creates magnification due to its thickness, and the bottom of a conventional progressive lens is thicker than the top.
- The back of a progressive lens creates magnification due to its power, and the bottom of a conventional progressive lens has a different power than the top.
- The back of a progressive lens creates magnification due to its distance from the eyes, and the bottom of a progressive lens is further from the eyes than the top.
Thankfully, a critical breakthrough in spectacle lens manufacturing occurred in 2008 and 2009 that allowed us to move the reading prescription from the front surface of the progressive lens to the back. This single change in the manufacturing process very nearly eliminates all the magnification differences that occur at the front surface of the progressive lens because the shape of the front surface is the same from top to bottom. Also, moving the reading prescription to the back of the progressive lens decreases the magnification differences at the bottom of the progressive lens simply because it’s closer to the eyes. This means that it’s now possible for your eyes to have a more similar magnification experience when they move from the top to the bottom of your progressive lenses. This is why today’s free-form progressive lenses (when properly designed and manufactured) are much easier to adapt to than yesterday’s older, conventional progressive lenses.
If you need progressive lenses that are easy to use, the eye doctors and staff at Calgary’s Doig Optometry would love to create a customized progressive lens design, specifically for your eyes. If you’d like us to, we’d love to minimize, or even eliminate any frustrating adaptation period in your new progressive lenses. So please call our Calgary Eye Care Centre, Doig Optometry to book your next eye appointment. See you soon!
Note: This blog was is for informational purposes only. It was written with the goal of making the complex issue of progressive lens adaptation more understandable for progressive lens wearers. In order to achieve this goal, certain optical and mathematical concepts were simplified in order to make them more understandable. If you were able to identify them, congratulations! You are on your way to becoming a masterful expert of optics. If you have any questions or insights about the management of magnification in progressive lens design, I welcome your comments. You can reach me via the contact page on this website, or by email at firstname.lastname@example.org.
- Dr. Doig