Corrective and Prescription Diving Lenses:
In general: Corrective and Prescription lenses are to correct the vision or focusing problems due to the eye’s curvature. The three basic components of a prescription are the spherical, cylinder and axis values. The adjustment needed to correct the spherical and/or the cylindrical curvature is measured or expressed in diopters. Units of lens power are called diopters.
Fortunately, the human eye has the ability to change shape to focus on objects at different distances. Because the eye has this flexibility, it make it possible to make lenses with standardized curves in incremental steps. The correction/prescription indicates which of these standardized lens curves will improve the individual’s sight when combined with eye’s natural focusing characteristic. More specifically, the eye is fitted with the closest standard diopter curve that will help the eye focus an image. For diving mask lenses, the lenses curvatures are usually fabricated in .5 diopter graduations or radii, e.g., +1.0 diopter, + 1.5 diopter, +2.0 diopter, etc. If the correction/prescription value falls between the standard manufactured diopter curves that are available, the closest manufactured curve is used to fabricate the lens.
The spherical value of a lens corrects for farsightedness or nearsightedness. Plus spherical lenses correct for farsightedness (hyperopia). A minus lenses corrects for nearsightedness. If there is only one point toward which all light rays converge or from which diverge. Many prescriptions include only a spherical correction.
A spherical correction is made to help the to eye focus if the eye lens is essentially symmetrical but lacks a proper curvature. The prescription usually indicates this as a single number for each eye and indicates whether it is a plus (+) or negative (-) correction. These values are express for each eye and will be for distance. The value will be expressed for the “OD” or right eye and “OS” for the left eye.
Usually, the plus and minus values are less than 8 diopters for negative values and less than 4.5 diopters for plus values. If the correction is of a higher value than there may not be a lens available or a high index optical glass may be required to make the corrective lens (see the section on lens material for more information). Please request a quotation for higher diopter lenses.
Also, on low volume masks, the higher diopter prescription may not be recommended. The lens may be too thick and it may come to close to the user’s eye or forehead. Masks with smaller lens sizes or larger volume may be more desirable in this situation.
Strong plus (+) diopter lenses cause “Tunnelvision” under water and may not be suitable for satisfactory correction in every instance. The degree of “Tunnelvision” varies with the strength of the plus(+) diopter value.
Side window masks increases the user’s peripheral view. When tested out of water, the view is clear and the symmetrical. When sub-merged, the effects of refraction may result in the side view and the frontal view not matching. The refraction difference may create a size difference when an object is view through the side window and the front window or the edge alignment of an object will be different between the two windows.
Use of clear lenses is recommended when the maximum amount of light energy is needed for a visual activity. Colored glass absorbs certain light waves and reduces the amount of light reaching the eye.
The cylinder values of the prescription corrects for astigmatism. In general, the eye is not symmetrical but has two curvatures across the eye’s natural lens shape, e.g., the eye is shape more like a football than a softball. The prescription expresses the second curvature with two values. The first value indicates the second radius of curvature and is noted as the cylinder correction while the second value is expressed as an axis correction. The axis correction is an angular measure starting from a particular circular reference point and indicates the location of cylinder radius. These values will be expressed for the “OD” or right eye and “OS” for the left eye.
On occasion, if the individual’s cylindrical adjustment is small ( 1 diopter or less), it may be possible to adjust the spherical correction to provide some compensation for the cylindrical values. This adjustment may only be suitable for snorkeling activities and when it is hard for the individual justifying the investment in fully corrected lenses. Obviously, the correction will not be prefect and will have some distortion.
Literally, bifocal refers to a lens that has two focal points. The terms is specifically applied to lenses made up of two parts, each having a different refracting power, and therefore a different principal focal point; one part, as a rule, being intended for distant vision and the other for reading and close work.
If a bifocal is require for reading, it will usually appear on the prescription as an “Add” value for a given eye, i.e., there will be an “Add” value expressed for the “OD” or right eye and/or “OS” for the left eye.
In general, if the individual is only going to do snorkeling, the individual will only need to see distance for this activity. However, if the individual is using the mask for diving then the bifocals may be necessary to read the gauges and computer screens.
Lenses are made impact resistant by a glass tempering/strengthening process. Lenses comply with the impact resistance required by ANSI Z86.22, CEN, USFDA and Federal regulations.
However, impact resistant lenses can break or shatter if subjected to undue force or impact.