I'd like to take a moment to discuss a few important characteristics of soft contact lenses and how they are fit, which makes several posts I have made on this thread a bit easier to understand, and it will give the interested diver more information with which to take to their next visit with their eye care practitioner.
Soft contact lenses have 3 major characteristics that determine how they fit; their diameter and base curve. Thickness and material are also lens characteristics of importance but are not relevant for this particular discussion because they don't affect the movement of the lens all that much.
The base curve of the lens is the radius of curvature of the lens, and for most soft contact lenses ranges between about 8.3 and 8.9, although there are some contact lenses that fall outside these parameters. The lower the number, the tighter the radius of curvature and it naturally follows, that the fit of the lens will be tighter.
The diameter of most soft contact lenses ranges between 13.5mm and 15.0 mm, and again there are lenses that fall outside these numbers but that covers about 98% of contact lenses.
The base curve has a greater effect on how tight a lens fits than the diameter, however a larger diameter lens will be less likely to be washed out of the eye because there is more lens material under the upper and lower lids. Older divers who wear contact lenses may have looser eyelids and therefore may have more of a problem with contact lens loss than a younger diver, all other factors being equal.
For purposes of Scuba Diving, patients may want their eyecare practitioner to be aware of the fact that they are interested in a lens that will be less likely to fall out of their eyes due to unexpected exposure to water and to work in the direction of a tighter fit. A contact lens that is too tight will limit exchange of tears and oxygen to the cornea and may be difficult to remove which can cause irritation or even damage to the cornea.
Patients with contact lenses that are too tight often complain of discomfort or pain only after removing the contact lens at the end of the day.