Even when a lens is designed identically for two different people, they can have a polarising visual experience. ALEX RIGBY discusses the visual concept likely at play.
Visual sensitivity is a concept increasingly being captured in lens design. It can be defined as the reduction in visual acuity caused by aberrations. Essentially, it’s the perception of blur, even if the script is correct.
It’s important, given patients with varying visual sensitivity can face significant challenges.
Some may report a sense of disruption or lag when moving their gaze across the lens, particularly when transitioning between different zones. This could manifest as difficulty maintaining a smooth flow when reading; for instance re-reading sentences or losing their place on the page.
In more dynamic situations, some may notice interruption in their vision when switching fixation to peripheral areas, particularly when shifting between the zones in a progressive lens. This lack of visual stability can lead to a feeling of disorientation or a sense that something’s off, even if the prescription is technically accurate.
Ultimately, the common thread is a feeling that something’s disturbing the natural flow of vision through the lens.
To achieve a truly smooth transition between near, intermediate, and distance zones, the operational fields must be adapted to match the individual’s specific visual sensitivity.
This approach goes beyond the traditional method of simply shifting aberrational patterns, often referred to as creating a “hard” or “soft” design in conventional lens manufacturing. Instead, we now look toward manipulating the underlying optical principles that generate aberrational distortions. By doing so, we can mitigate their perceptual impact and design lenses that align more closely with how a person processes visual information.
To bring this into a real-world context, we’ve likely all encountered patients at different points along the sensitivity spectrum. There are those who immediately notice even the slightest axis change in a low-cylinder prescription. Their visual system is highly tuned to even minor prescription changes.
Alternatively, we see patients who tolerate significant levels of blur with little to no complaint – sometimes even using lenses outside their intended purpose, like occupational lenses for driving.
This contrast highlights how differently people perceive and respond to blur. It’s a clear reminder visual sensitivity is highly individual, and must be factored when prescribing and designing lenses.
Differing experiences
Let’s take a moment to revisit the origin of aberrational patterns in multifocal lenses and why they occur because of changing power across the lens.
Minkwitz’s Theorem states when power increases along a central vertical line, the amount of unwanted astigmatism increases at twice the rate laterally, perpendicular to that line. For every 0.25 D increase in addition power along the corridor, there’s a corresponding 0.50 D increase in unwanted astigmatism in the periphery.
So, as patients move their gaze, they inevitably encounter zones of unwanted astigmatism. The higher the addition power, the greater the level of distortion.
Here’s where it gets interesting: some patients can pass through 0.50, 0.75, or even 1.00 D of unwanted astigmatism without perceiving any blur. For them, the areas they perceive as sharp within each zone are much wider.
But others may begin to perceive blur almost immediately at just 0.25 D of unwanted astigmatism. Those same zones feel much narrower. Even though the lens design is identical, their individual visual sensitivity causes a very different experience.
Contributing factors
1. Visual acuity – generally, patients with higher visual acuity tend to be more visually sensitive than those with lower acuity.
2. Pupil size – smaller pupils increase depth of focus, making the visual system less sensitive to aberrations, while larger pupils reduce depth of focus, causing image quality to degrade more quickly and increasing sensitivity to visual imperfections.
3. Higher order aberrations – patients who suffer with increased higher order aberrations often show lower visual sensitivity because the resulting blur reduces their ability to detect small imperfections.Those with fewer aberrations have higher sensitivity, as their clearer optical system makes them more aware of minor distortions.
For sensitive patients, the goal is to create clearer central zones in the lens, reducing peripheral distortions, minimising visual discomfort and providing smoother transitions across different areas of the lens.
For less sensitive patients, the aim is to create a more uniform aberration field across the lens, providing a wide, comfortable visual area with smooth, consistent performance.
In both cases, the end result is a wider perceived area.
Studies show only about 17% of people are currently wearing lenses truly matching their visual sensitivity, representing a major unmet need.
When discussing the benefits of lenses factoring in visual sensitivity, it can be summarised into three categories: improved visual performance, significantly reduced fixation duration in tasks like reading, and reduced mental demand offering better orientation when navigating through the lens and shifting gaze.
ABOUT THE AUTHOR: Alex Rigby is a qualified optometrist and the professional services manager for Rodenstock Australia.
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