If you have a pair of glasses and do not know what the prescription in the lenses is, there are a few simple ways to determine whether the glasses are for farsightedness, nearsightedness, or astigmatism. While these methods will not determine the exact prescription in the glasses, it can give you an idea as to the power in glasses.
Hand Neutralization or “Shaking” Glasses
One of the simplest methods of determining the power of glasses is through hand neutralization.
This method does not require any special equipment or tools and can be done in any setting.
To perform this technique, you only need the glasses which you are trying to determine and a distantly viewed target which is best if it is a large, straight line such as a door frame or edge of a wall.
Begin the process by holding the glasses at arm’s length away from you, then find a target like a door frame to view that is several feet away.
Look at the door frame through the glasses at the target, slowly move the glasses to the side and see if the image of the door within the glasses moves in the same direction or the opposite direction of the direction the glasses themselves were moved.
The image inside the glasses may be blurry or appear slightly distorted. This is not a problem or concern, and the technique can be completed even if there is some blurriness.
Glasses for Nearsightedness
Glasses for nearsightedness will have a minus power and the lens will have a minification effect on the world.
When using the hand neutralization technique, these type of glasses will always cause the target to move in the same direction as the glasses.
If the glasses are extremely high powered, the amount of the movement will be much more than if there is only a low powered lens in the glasses.
Hand Neutralization for Glasses for Farsightedness
Instead of the image moving in the same direction as with glasses for nearsightedness, glasses which are designed for farsightedness will possess the opposite attributes.
Glasses for farsightedness will magnify the world viewed through them and when using hand neutralization will cause the image to move in the opposite direction of the glasses.
Glasses for Astigmatism
Astigmatism differs from nearsightedness and farsightedness as it causes the eye to view things as stretched or distorted.
Glasses which correct for astigmatism can also be for nearsightedness or farsightedness.
When performing hand neutralization, if the door frame or other straight line target does not appear to line up correctly and is tilted to either direction, there is some amount of astigmatism correction in the glasses.
Why Hand Neutralization Would be Needed
In the majority of cases, you will either already know the prescription power of the glasses or will be able to use equipment to determine the power.
However, if you find a pair of glasses or if a pair of glasses is donated for a charitable cause without optometric equipment, hand neutralization can be a great technique to assist with determining the type of glasses.
Our eye doctor at Local Eyes Optometry in New Braunfels, TX excels in prescription of glasses, contact lenses and the diagnosis of a variety of eye diseases. Call our optometrist at (830) 627-9272 or schedule an eye exam appointment online if you would like to learn more about hand neutralization. Our eye doctor, Dr. Marcus Page, provides the highest quality optometry services and eye exams in New Braunfels, Texas.