"Fit" isn't a single number
Two lenses can share the exact same power and feel completely different on your eye. That's because comfort doesn't come from the prescription — it comes from how the lens sits on your cornea. And how it sits is decided by two numbers working together: the base curve (BC) and the diameter (DIA).
Most clear lenses are sold in a single base curve and diameter — one shape for everyone. If your cornea is flatter or steeper than that "average", the lens sits too tight or too loose, and you feel it all day. The number that captures this is called sag.
Base curve (BC) × Diameter (DIA) → Sag
Same prescription — so why do dryness and sharpness change so much when you switch lenses?
Sag = how deep the lens dips from its edge to its centre.
No single spec sets it — base curve and diameter decide it together.
That depth is what makes a lens sit loose or tight on your eye.
They offset: a flatter curve can be balanced out by a larger diameter.
Illustration exaggerated for clarity. Real differences are ~0.1–0.2mm — but enough to change how a lens fits.
- Screens dry the film → edge catches your lid → grit feel
- Decentred / off pupil → blur & low contrast in dim light
- Edge conforms → no grit feel, even on screens
- Centred on your pupil → sharp even in dim light
Flatter cornea suits a flatter base curve; a steeper cornea is the opposite. Confirm your corneal parameters before you choose.
So it was never "you"
That dry brand, the scratchy one, the one that blurred at night — each was just a sag that didn't match your cornea. Too deep and the lens sits tight: the edge digs into your lid (that foreign-body, scratchy feeling), tears pool under the centre, and the moment a screen dries your tear film it gets worse. Off-centre, and your vision goes soft and low-contrast in dim light.
What to do about it
Two practical steps. First, find your base curve and diameter — they're on the box of any lens that did feel okay, or your eye-care professional can tell you. Second, match them: if your cornea runs flat and lenses always feel tight, dry or blurry in dim light, you likely want a flatter base curve.
Across moodyClear, the S-Series sits a touch flatter (8.7 base curve) while A and M run at 8.6 — so there's room to match how your eye is shaped, not just your power.