Prismatic glasses use prism to improve reading for people with macular degeneration and other vision limiting conditions. They are useful for patients whose two eyes are approximately equal in vision and require relatively lower magnification. They are often beneficial for reading the newspaper and for writing checks.
|Dr. Randolph Kinkade and Eileen Zitnik with |
Prismatic Glasses for Macular Degeneration
Patients with macular degeneration and other vision limiting conditions need more magnification in order to read. Regular glasses have a limit on how much magnification they can provide. With mild to moderate vision loss, prismatic magnifying glasses can offer reading improvement.
Prisms are used in higher power reading glasses for patients with equal vision in both eyes. High magnification lenses require both eyes to turn in far enough to see a single image and to be able to read comfortably.
There are three significant challenges when trying to gain extra reading magnification when reading with both eyes is one of the goals. The first challenge is the lens power's relationship relative to the viewing distance and depth of field. The second challenge is the limits of binocularity (fusion of the two eyes) due to our eye anatomy. The third challenge is our reading habits of where we hold our reading material.
|Dr. Kinkade's Patient|
+8-Diopter Full-Frame Prismatic
Lens Power and Viewing Distance
As a lens becomes more curved and thicker it becomes more powerful. As the power increases, the focal length or viewing distance (i.e., where you hold the reading material from your glasses) becomes shorter.
There is no way to get around the laws of optics. For gaining the most magnification and clarity, and assuming there is no distance refractive error, the reading material must be held at the focal length of the lens. Stronger lenses have shorter focal lengths, and therefore, stronger lenses require you to position the reading material closer to the lens. That is the law.
Over-the-counter reading glasses are labeled in diopters. They come under powers of +4.00 diopters. Diopters are a measure of focusing power. The higher the diopter value, the quicker the lens focuses images. For the average reader, with no distance refractive error, a pair of +2.50 diopter lenses focuses reading material best when held at 16" (i.e., a normal reading distance).
Patients with macular degeneration and other vision limiting conditions need more magnification in order to read. We can gain higher magnification by holding reading material closer than the normal 16". The closer the material is held to the eyes, the more magnification is gained. As the reading material is held closer, higher dioptric power lenses are needed to focus what is being read at the close viewing distances.
|+6-Diopter One-Piece Prismatic|
Designs For Vision
For someone without a distance refractive error (i.e., no farsightedness or nearsightedness) the following table is educational:
Focal Distance in Inches Based on Dioptric Lens Power
Wearing a +4.00 diopter lens requires the reading material must be held at 10".
Wearing a +5.00 diopter lens requires the reading material must be held at 8".
Wearing a +6.00 diopter lens requires the reading material must be held at 7".
Wearing a +8.00 diopter lens requires the reading material must be held at 5".
Wearing a +10.00 diopter lens requires the reading material must be held at 4".
Wearing a +12.00 diopter lens requires the reading material must be held at 3".
Prismatic reading glasses come in different dioptric powers: +4.00, +5.00, +6.00, +8.00, +10.00, and +12.00.
Another challenge with stronger lenses is their shorter depth of field. Although a lens can precisely focus at only one distance, the depth of field is the distance on either side of this point where the reading material still appears sharp enough to read. The shorter the depth of field the more critical it is that the reading material be placed and held at the precise focal distance.
Functional Reading Depth of Field Based on Dioptric Lens Power With Small Print
+2.50 diopter lens 13.00" - 20.00"
+4.00 diopter lens 9.50" - 10.50"
+8.00 diopter lens 4.75" - 5.25"
+12.00 diopter lens 2.82" - 3.12"
+12.00 diopter lens 2.82" - 3.12"
As you can see from the above, when viewing through a +2.50 diopter lens the reading material can be moved 3-4 inches closer or further away from the sharpest focus point of 16" before it blurs. With a +12.00 lens you only have to move the reading material an eighth of an inch away from its focal distance of 3"and it will be blurred.
Eye Convergence and Anatomy
When we look into the distance, our eyes are looking straight ahead and parallel with each other. The closer something is to the eyes the more the eyes have to converge (turn in). The eyes sit in the orbits in the skull and each eye has six muscles used for positioning the eyes. At the normal 16" viewing distance the eyes can easily and comfortably converge. For viewing distance less than 12" it becomes more difficult for us to converge enough to remain binocular (i.e., use both eyes).
A prism bends but does not focus light. A prism creates rainbows by bending each color wavelength differently. In glasses, prisms are used for several different things and not for making rainbows. For prismatic reading glasses, prisms help reduce the convergence needed in order to see binocularly.
|Prism Reduces |
Base-in prisms (the think part of the prism is placed towards the nose), in various amounts, are used to reduce the amount of convergence needed to remain binocular depending on the power of the prescription lenses.
The closer you hold things the more prism is needed to remain binocular.
Rule of Thumb
Prism Required Based on the Convergence Demand
4 lens diopters with 6 units of prism
6 lens diopters with 8 units of prism
8 lens diopters with 10 units of prism
10 lens diopters with 12 units of prism
12 lens diopters with 14 units of prism
The longer someone has read at 16" the more adaption time it takes to learn to read at closer distances. You have to learn to bend your elbows and allow the print to get closer to the eyes. The muscles in your arms (proprioceptors) will feel funny at first. The more magnification needed requires more time learning and more practice.
Start reading with large print so the power and depth of field are not so critical.
The Take Home Messages1. The more magnification required, the stronger the lens has to be.
2. The stronger the lens has to be, the closer you need to have the reading material to your eyes and
at the focal length.
3. To read binocularly you need relatively equal vision in both eyes.
4. To read binocularly at close distances you need prism to help with the convergence of the eyes.
6. It takes practice and motivation to learn new ways of seeing.