In other words … optical help for divers having trouble reading their computers, gauges, and digital screens.
by Quentin Bennett, image Richard Robinson
Even divers get middle aged, and one of the biggest nuisances from this stage of life on is the difficulty most of us suffer trying to read and see things that are close up. This includes reading our dive computer and gauges. The phenomenon of reduced ability to focus the eyes is called presbyopia, and I argue that it is a sign of maturity.
One extra factor underwater is that the reduced levels of light induce the pupils to dilate, further exacerbating the difficulties. The first time that I remember noticing early presbyopia was once when I was diving in deep dark conditions and the realization that I had great problems seeing my computer and gauge.
There are a number of ways that we can get over this difficulty and these all depend on our personal requirements. The existence of a distance refractive error is another factor in our approach to the problem. I will endeavour to discuss the various options in the correction of presbyopia for diving. Generally contact lenses will only correct distance vision and dont help with the correction of presbyopia. However there are bifocal contact lenses that will help with the correction of both distance and near vision. There may be difficulties with larger pupils in the darker underwater environment causing the lenses not to function as well as on land.
Many recreational divers only need to see their depth and tank contents gauges and this only requires a little corrective lens at the bottom edge of their divemask glass on the side of their gauge. Sometimes a little lens made from aliphatic polyurethane, a plastic possessing a negative electrostatic charge, will be more than adequate. Because of the charge it will adhere to the inside of the mask glass of its own accord. These do have to be looked after because they can be lost relatively easily.The segment is like a capital D on its back. I normally would place it so that the top of it is level with the lower eyelid when looking straight ahead.
Digital photographers wish to see their camera screen well and this will mean that they need rather good vision at near. I would expect most presbyopic photographers to prefer near correction for both eyes, and for it to be very good and sharp. For really sharp near vision properly ground bonded glass lenses are the best answer, and they are also far more rugged and serviceable. These can be made as just small D-shaped near lenses. For divers requiring a distance correction lenses can be made as a one piece bifocal which is bonded to the inside of the mask glass.
These days ordinary bifocals are becoming quite unusual in everyday life, having been replaced by progressive lenses. However it is impossible to make progressive lenses so that they can be bonded to the inside of a divers facemask. We have managed to mimic many of the characteristics of a progressive lens by utilizing a technique known as simultaneous vision or monovision, for near vision only. We correct the near vision in the dominant eye and correct intermediate vision in the non-dominant eye. Most people appear to adapt to this readily.
What I used to do before I retired was to discuss with the diver his or her requirements, their normal prescription and make the dominant eye the closest focus. We all have a dominant eye, and this is the one that we use if we aim or sight something like a rifle. For example my own right eye is dominant and I have made its near focus at just about a foot, which is perfect for my wrist computer or digital camera screen. The left near lens is focused at about twice this and is pretty good for finding smallish creatures and is adequate for reading my pressure gauge which comes under my left arm.
So using both eyes together my mask is set up for me to focus on near objects from a bit under a foot to almost a metre. That works well for the type of diving that I generally do. Because I have a small distance error, I have that corrected in the top part of my mask and the close vision separately at the bottom. That means that each eyes lens is made up as a one piece glass fused bifocal which is bonded to the inside of my mask glass
The difference in power between the distance part of ones correction and the near is termed the addition. Again, as an example, in my own divemask I have a 3-dioptre addition before my right eye and a 1.50-dioptre addition before the left.
My dive mask correction is quite different from my normal everyday progressive spectacle lenses which are very conventional with +2.25 dioptre additions in either eye. Younger people have lower prescriptions and my figures wont be relevant for them, but the overall principle of differing additions in either eye is.
I know that this sounds like a lot of gobbledegock, but if you take this article with you to your optometrist or dispensing optician they will understand it easily.
To translate a little I will explain the dioptre, which is the power measurement of a lens. The dioptric power of a lens is the reciprocal of the focal length in metres; ie 1/metres. For example a lens with a focal length of 50cm or half of a metre has a power of 2 dioptres. A lens with a focal length of 33cm or one third of a metre is of 3 dioptres power.
So we could look at my earlier example of my own divemask where I have an addition of 3 dioptres before the right eye and an addition of +1.50 dioptres before the left eye. So my right eye bifocal segment focus is approximately 33cm and the left approximately 66cm. The top distance part of the lens is focused to infinity so if I am on the surface I can see the boat clearly.
Do remember that the eyes and visual system are immensely complicated and what I have written is very generalistic. Everyone is different, but a practitioner can work this out pretty well.
To sum it up for mature divers.
For simple recreational diving where one only has to look at a pressure gauge and computer or compass a small lens bonded in the bottom of the mask on the side of ones pressure gauge is adequate. This may be an aliphatic polyurethane stick-on Optyx 20/20 lens or a glass plano/concave lens bonded to the inside of the facemask glass below the normal line of sight.
For a digital photographer or scientist two lenses may be needed, either with similar additions or else utilising monovision or simultaneous vision for near vision only.
For those requiring a distance correction in addition, lenses can be made up in a glass fused bifocal form with a plano anterior surface to correct both distance and near.
Any glass lenses will need to be ground with a flat front surface that can be bonded to the inside of the divemask glass plate. This may have to be done overseas, so give your practitioner lots of time.