There are three types of lens available for dive masks. These are Single-Vision, Bifocal and Reading Segments. Single-Vision is used primarily for correcting distance vision and the lenses will fill as much of the lens aperture as possible, within the limits of the lens blanks we have to use, the style of the mask and the individual prescription.
Bifocals use most of the lens area for correcting distance vision, with an area at the base of the lens for correcting close vision, for reading gauges, camera settings etc. Read segments are placed in the lower portion of the lens aperture and are used to correct near vision, where no correction is required for distance. For cost purposes, they are classed as single-vision lenses.
There are no varifocal lenses available for dive masks. This is simply because there are no flat-fronted, lens blanks made that would enable us to do this.However, bifocals probably account for some 50% of all the orders we receive. These look like one single piece of glass, with a small, virtually indiscernible, 28mm reading segment at the bottom. This segment is well out of line of sight when you are swimming ahead, but positioned to be exactly in line of sight when you want to read your computer or gauges, check camera settings or look at anything up close. We carefully set the height of the top of the segment depending on the make and model of mask.
For specific applications, such as photography, we can provide a much larger read area if required.However, it is only possible to supply larger read segments in those instances where a read segment is all that is required. With no correction for distance vision, the read segment is, in effect, a single-vision lens, and can be as large as we care to make it, within the confines of a 65mm diameter lens. However, this will only be a viable solution if your distance vision is good without any correction. Modern bifocals are made using 'fused lenses', whereby the read segment is set into the overall lens. This gives it fixed dimensions of approximately 28mm wide X 15mm high. We can adjust the height at which the segment is positioned, but we cannot change its size.
The only way we can give you both distance correction and a LARGER read segment, is by resorting to older style 'Franklin Split' lenses. This entails having two separate pieces of glass in each aperture, one for distance and one for reading, butting together with a horizontal join near the centre of the lens aperture. Not the most attractive solution, but nevertheless effective.