While trial lens kits are staple tools for eye care professionals, auxiliary lenses often fly under the radar—yet they’re critical for accurate refraction, vision testing, and diagnosing eye conditions. Let’s demystify these unsung heroes and break down exactly what each auxiliary lens does (and why it matters).
During monocular visual acuity tests (e.g., reading an eye chart with one eye at a time), the BL lens completely covers the non-test eye to prevent peeking. This ensures results reflect the tested eye’s true vision—no cheating allowed! It’s the most reliable way to isolate vision for single-eye assessments.
Unlike the fully opaque BL lens, the FL lens is semi-transparent (frosted), which reduces anxiety in pediatric patients who dislike total darkness. It’s also ideal for outdoor testing, where harsh light might make a black occluder uncomfortable. The FL still blocks enough light to prevent peeking—without overwhelming young or sensitive eyes.
The PH lens has a tiny pinhole that focuses incoming light directly on the retina, cutting out blurriness from uncorrected refractive errors (e.g., myopia, hyperopia). Here’s how it works:
- If a patient’s vision improves with the PH lens, their blurriness likely stems from a refractive error (easily fixed with glasses/contacts).
- If vision doesn’t improve, it may indicate an underlying eye condition (e.g., cataracts, macular degeneration).
This makes the PH lens a quick, powerful diagnostic tool for separating “correctable” blur from serious issues.
PL lenses have zero refractive power—meaning they don’t change vision at all. They’re used as placeholders in trial frames to:
- Maintain balance during testing (so the frame doesn’t shift).
- Confirm that vision changes are due to other lenses (not the frame itself).
Think of the PL lens as your “control” for accurate comparisons.
Astigmatism causes blurriness because the eye’s cornea is irregularly shaped (like a football instead of a basketball). The SS lens has a narrow, vertical slit that isolates light to one meridian (direction)—your first step to finding astigmatism. Here’s how to use it:
- Fog the Vision: After correcting spherical power to best-corrected visual acuity (BCVA), add a weak positive lens to slightly blur the patient’s vision.
- Rotate the Slit: Spin the SS lens 360 degrees. Ask the patient if the chart gets clearer or blurrier at any angle—changes mean astigmatism is present.
- Find Axis & Initial Power: Stop at the angle where vision is clearest (this is the astigmatism axis). Add small spherical lenses to sharpen vision in that meridian. The difference between the clearest and blurriest meridians gives your initial astigmatism power.
The SS lens is your starting point for astigmatism—before fine-tuning with a cross cylinder.
Binocular fusion is the brain’s ability to merge images from both eyes into one clear picture. The GF lens tints vision green; the RF lens tints it red. When used together (e.g., with a red-green test chart), they force each eye to see a different color. If the patient can fuse the two colors into one image, their binocular vision is healthy. If not, it may signal convergence issues or eye teaming problems (common causes of eye strain or double vision).
Latent strabismus (heterophoria) is an eye turn that’s “hidden” until binocular vision is disrupted. The MR lens has parallel cylindrical rods that refract light into a line (instead of a point). Here’s how to test it:
- Have the patient look at a penlight with one eye through the MR lens.
- The MR eye sees a line of light; the other eye sees a dot.
- If the line and dot are misaligned, heterophoria is present.
This helps diagnose subtle eye alignment issues that might cause headaches or blurred vision—even if the patient doesn’t notice them.
IPD (pupil distance) is the distance between the centers of both pupils—and it’s critical for glasses. If lenses aren’t aligned with the pupils, vision will be blurry or uncomfortable. The CL lens has a crosshair target; pair it with a penlight to:
- Align the crosshair with the patient’s right pupil.
- Repeat for the left pupil.
- The distance between the two points is their IPD.
The CL lens is faster and more precise than manual measuring—perfect for patients of all ages.
Once the SS lens identifies astigmatism, the 0.25X cross cylinder lens is your “fine-tuning” tool. It has two axes: one that increases astigmatism and one that decreases it. Here’s how to use it:
- Flip the lens back and forth while the patient looks at a chart.
- Ask which position is clearer (“1” or “2”).
- Adjust the astigmatism power/axis based on their answer.
This step ensures astigmatism correction is exact—the difference between “good” vision and “great” vision.
