Product Spotlight: Size Lens Kit

Size lenses are a specialized tool that, while used rarely, are invaluable for doctors who want a diverse toolkit to help patients with challenging binocular vision issues. Size lenses offer afocal magnification, meaning they are Plano (zero power) lenses that alter the perceived size of an image.

When Are Size Lenses Appropriate?

Size lenses are typically not needed when the image size difference (Aniseikonia) can be calculated directly from a large difference in the patient’s prescription (Anisometropia). In those cases, the imbalance can often be corrected using standard optical calculations (like Shaw calculations) during manufacturing.

Spectacle Magnification/Aniseikonia Calculator

Size lenses become necessary when a size imbalance shows up outside of the normal refraction. The most common scenario is a patient who:

• Complains of symptoms similar to double vision or struggles with binocular discomfort and difficulty fusing images.
• Has a history of retinal imbalance, such as a retinal detachment or epiretinal membrane surgery.

In these cases, the surgery or retinal abnormality has created two vastly different image sizes that no amount of standard prism or refraction changes can fully balance.

Using the Size Lens Kit

The size lens kit is a simple, subjective tool that helps you determine the magnification needed to restore functional vision.

The Kit and Assessment

• The kit contains five afocal lenses: 1%, 2%, 4%, 5%, and 7% magnification.
• Magnify vs. Minify: A single lens can achieve both:
  • Holding the convex side away from the eye magnifies the image (e.g., +7%).
  • Holding the concave side away from the eye minifies the image (e.g., -7%).
• Stacking: Because the lenses are afocal, you can stack them to achieve higher magnification (e.g., a +7% and a +5% stacked equals +12%).

Subjective Testing is Best

While highly precise Aniseikonia tests exist, in a functional setting, a subjective patient response often works best. You can test different percentages until the patient confirms, “Wow, this is so much better.” This functional assessment helps the patient achieve maximum comfort in the real world.

Manufacturing for Magnification

When incorporating magnification into a final prescription, there are important optical considerations to keep in mind:

The Golden Rule: Always Magnify the Smaller Eye

We almost always seek to magnify the smaller eye rather than minify the larger eye. The reason is that minifying requires using curves that interfere with typical manufacturing parameters, which severely limits the patient’s field of view.

Prescribing the Change

On the prescription, you simply note the required percentage of magnification (e.g., “Left Eye: -1.00 + 7% magnification”).

The Optical Trade-Offs

Magnification is physically accomplished by manipulating three parameters during lens manufacturing:

1. Center Thickness
2. Base Curve (the front curve of the lens)
3. Vertex Distance (the distance from the eye)

Achieving large magnification amounts (e.g., +12% for a Plano lens) requires large changes in base curve and center thickness. A Plano lens with 12% magnification might be up to 25mm thick (about one inch). Patient comfort and aesthetics must be considered.

Alternative Solutions

If the required magnification creates an unacceptable aesthetic problem (too thick), it may be possible to combine contact lenses with an offsetting change in the glasses prescription to achieve the necessary magnification without excessive thickness.

The size lens kit is a vital assessment tool that can help you diagnose and treat patients who have run out of conventional options.

Learn More about the Size Lens Kit
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