Ed Lyons resolves a more unique version of double vision
One of the great things about my role as a SportsVision Optometrist is that it gives me the opportunity to meet many different people, all of whom have a common love for shooting. Some clients have no visual issues but want to come to see me as part of their pre-season checklist, and the psychological boost knowing their eyesight and eye health is 100 per cent can be a valuable commodity to a seasoned competitor and the total beginner. Others have pretty simple issues and straightforward solutions, for example, changing someone out of monthly reusable contact lenses and prescribing high quality daily disposables for shooting in competitions.
Contact lenses can get dirty over time, as lipids and proteins can be laid down on the lens surface even with scrupulous cleaning. The lens can also get damaged by handling, especially with rough hands or long nails. This can contribute to a reduction in the optical clarity the wearer perceives.
While not all prescriptions can be catered for in daily disposable lenses, the parameters are growing year on year so more people than ever can wear them successfully. Not only will a nice clean fresh lens give better vision, they tend to be thinner, allow more oxygen through and be more comfortable.
All that being said, while simple cases make for a relatively easy clinical session, it’s always nice to get a challenge.
A relatively new shooter from Norfolk emailed me last year about her rather unique visual situation. When shooting teal, high crossers or driven, she saw two clays. In normal eyes, seeing two clays is often a symptom of looking at the barrel. If visual fixation is entirely on the target, one may perceive two barrels. This is entirely normal and nothing to be concerned about, and despite what some may purport, it does not indicate an eye dominance problem.
To follow this requires understanding Physiological Diplopia. Also known as Phys Dip, it is “a normal phenomenon in which objects not within the area of fixation are seen as double.” Our eyes can only point accurately at one target, other objects, either in front or behind, will appear double – though in everyday life we ignore this doubled image.
You can try this yourself at home by pointing at a clock on a wall or a light switch. Look at your finger, and the distant image should split apart, look back at the light switch and you should be aware of two fingers. Interestingly, and this is important for all two eyed shooters, the image on the left is the one that the eye on the right sees, and vice versa. However, with this case, the symptoms only happened when she was looking upwards.
It turned out she had ocular surgery on a weak eye muscle many years ago, which fixed her everyday double vision, but when her head was on the stock and she was looking for a high bird, the one eye couldn’t work in tandem with the other, and hence two images were seen. This also happened in non-shooting situations such as looking for a book on a high shelf.
It can be pretty easy to fix double vision that is consistent by using prisms – as we touched on in last months article. Prisms are usually made within the lens, and as such come into effect throughout. The issue we have is that if we use a corrective prism in her glasses to sort the double vision on up gaze, it could then induce double vision in every other point of gaze.
Fortunately, there are prisms available that can be cut and stuck on to a lens to work where we want them to, so I ordered a series of Fresnels and booked the client to come back to see me when they and her new prescription Randolph Edge glasses had arrived. I always ask my clients to bring their gun, mainly so we can calculate the best centres for their lenses, and here it was especially important for the fitting of the Fresnel lens.
It has only been five weeks since the lens was fitted and she reported back that she had hit her personal best at her local shooting ground’s competition.
Vexed by varifocals
Inside each eye, we have a lens that helps us to focus, much like the zoom function on a camera. As we get older, this lens begins to lose its elasticity and efficiency. We first tend to notice this when we have difficulty with small print, finding ourselves having to hold things a bit further away or moving reading material into the light.
It is now common place to wear varifocal glasses to combat this in everyday life. A set of varifocal spectacles has a distance prescription at the top and in the straight ahead position, then an intermediate and near zone further down towards the bottom of the lens. As such, the visual effect a varifocal gives the user depends upon where they look within it and also their head posture. Most varifocals suffer from some distortion towards the edge so as you can imagine, putting ones’ cheek to a stock and looking for a target that is slightly off-centre can lead to a blurry picture.
So if we wish to see both the targets in the distance and the scorecard but not suffer the distortion of a varifocal, what can we try? Bifocal is a simpler solution than a varifocal as there are only two power zones within the lens thus no peripheral distortion.
At the back end of last year, Chris Willett, a Sporting shooter from Kent, messaged me regarding this issue. He was seeing the top rim of his Oakleys when shooting high birds and was finding the lack of near focus frustrating so he decided to make the journey up to Wolverhampton so we could address the issue.
It is important to get the position of the bifocal correct: too low and it will be hard to find, too high and it will be in the line of sight. The pictures show the measuring process with Chris’s gun mounted, and looking straight ahead. Happy with the measurements and the frame choice, Chris made his way back and we started creating his lenses.
This was an occasion where we didn’t get it right first time, as the bifocal segment came out too high. I emailed Chris the picture shown and the lens was remade with a lower segment. He was so chuffed we made another set in 26ED in the lovely Pilla 580 Carbon design.
This article originally appeared in the April 2017 issue of Clay Shooting magazine. For more great content like this, subscribe today at our secure online store www.myfavouritemagazines.co.uk
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