The Small Numbers on the Side of the Box: Is 58% Water Content Good or Bad?
Turn the lens box over.
A small table in tiny print.
Water content 58%.
Dk/t 25.
Material: etafilcon A.
Modulus 0.3 MPa.
Few people know what these are.
Yet when you switch brands, this is what you should be reading instead of "moist" and "breathable".
This article is how to read that table.
Four numbers are enough.
Water Content: Does Higher Mean Moister?
The proportion of the lens's weight that is water.
From 38% up to 78%.
It is tempting to think "more water, more moisture".
Often it is the opposite.
A high-water lens is a sponge full of water.
When a sponge dries, it pulls water from its surroundings.
The lens's surroundings are your tears.
So a high-water lens in a dry environment can actually draw more tears and feel drier.
Water content is not a "moistness" number. It tells you the character of the material.
High: soft and comfortable at first, may dry by afternoon. Low: firmer feel, loses less water.
Material: Why Every Name Ends in "filcon"
etafilcon A. senofilcon A. comfilcon A. nelfilcon A.
They all end in "-filcon".
That is the international naming rule for soft lens materials.
The front is the maker's chosen name; "filcon" means a water-absorbing polymer.
Same name, same material, regardless of brand.
Two families.
Hydrogel. The older type. Water carries the oxygen, so more water means more oxygen. Soft and inexpensive.
Silicone hydrogel. From the 2000s. The silicone itself passes oxygen, so even at low water content far more oxygen gets through. Slightly firmer, and oils stick more easily.
If the box says "silicone hydrogel", it is the second.
Dk/t: How Much Your Cornea Breathes
The cornea has no blood vessels.
It takes oxygen straight from the air.
A lens is a membrane in the way.
Dk/t is how much oxygen passes through that membrane.
Dk is the material's oxygen permeability; t is the lens thickness.
Thicker passes less, so you divide.
Bigger number, more oxygen.
Hydrogels run roughly 20–40; silicone hydrogels 80–170.
A commonly cited minimum for daytime wear is 24 or above.
Overnight wear needs far more.
If you wear lenses twelve hours a day or longer, this number may matter more than initial comfort.
Modulus: How Stiff the Lens Is
How much the lens resists when stretched; in other words, stiffness.
Measured in MPa.
Around 0.3 is soft; above 1.0 is on the firm side.
A soft lens is less noticeable at first but easier to insert inside out and harder to handle.
A firm lens is easy to handle and holds its shape, but some people feel it against the inner eyelid.
Early silicone hydrogels earned their "stiff" reputation from this number.
Recent products have come down a lot.
The Four Numbers in One Line
| Number | Meaning | Higher | Lower |
|---|---|---|---|
| Water content | Share of water | Soft, may dry by afternoon | Loses less water |
| Material | Hydrogel / silicone hydrogel | — | — |
| Dk/t | Oxygen through the lens | Better for long wear | Fine for short wear |
| Modulus | Stiffness | Easier to handle | Softer first impression |
Same water content, different material: different lens.
Same Dk/t, different modulus: different feel.
Read the four together, not one at a time.
What Is Not on the Box: Surface Treatment
One thing rarely appears as a number.
Surface treatment, or wetting agents.
Silicone hydrogels attract oil, so each maker treats the surface differently.
Some build wetting agents into the lens; some coat the surface thinly.
This difference shapes "how it feels at 4 p.m." a great deal, yet the spec sheet only shows it as a brand name.
So even two lenses with similar numbers leave something you can only learn by wearing them.
The Order When Switching Brands
- Write down the four numbers of your current lens from the box.
- Look at the four numbers of the candidate.
- Check whether the material family is the same. Hydrogel to silicone hydrogel is a noticeable change.
- If you wear lenses long hours, do not let Dk/t drop below what you have now.
- If you dry out in the afternoon, avoid very high water content.
- BC, DIA and power must match the prescription, of course. (How to read it)
- Buy one box and wear it for a week.
Three Common Misconceptions
"Higher water content is a better lens." No. It is a different character.
"Expensive lenses pass more oxygen." Price and Dk/t are unrelated. Read the box.
"Lenses from the same company are similar." One company sells both hydrogels and silicone hydrogels. Read the material name.
Advertising Uses Adjectives, the Box Uses Numbers
"Moist", "breathable", "natural".
Adjectives cannot be compared.
58%, Dk/t 147, senofilcon A, 0.7 MPa.
Numbers can.
The small table on the side of the box is information the maker is legally required to print.
It is more honest than the advertising.
Read it before switching brands, and "why is this one so different" happens far less often.
References
- US Food and Drug Administration, Types of Contact Lenses
- American Optometric Association, Contact Lenses
- International Organization for Standardization, ISO 18369-1: Ophthalmic optics — Contact lenses — Vocabulary
- American Academy of Ophthalmology, Contact Lens Types
This article provides general information. Specification values come from manufacturers' official data and vary by product. Discuss lens type and brand changes with an eye-care professional.