The Definition Of A Diopter As Related To Optics
For optical devices that bend light, the unit of measure is referred to as a diopter. By definition, a diopter is the reciprocal of the focal length measured in meters. This means that a lens of 1 diopter will bring light rays into focus one meter away. Three diopters will bring light rays in to focus 1/3 of a meter away.
The higher the diopter power of a lens, the closer an image will be brought in to focus. How does this relate to everyday things like eyeglasses, magnifying glasses, and mirrors?
For starters, an eyeglass prescription is written in diopters of either a plus or minus form. Plus powers are for farsighted eyes and the eyeglass prescription will enlarge the image. Minus powered lenses are for nearsighted eyes, and these lenses tend to minimize an image.
When someone reaches the age of 40, their eyes become presbyopic and usually require the assistance of reading glasses to see up close. Readers are in plus powers, they enlarge an image. Usually over the counter glasses are available in powers ranging from +1.00 to +3.00 diopters.
Visiting the eye doctor to purchase a pair of cheater glasses with a power of +2.00 diopters will essentially be the same as enlarging the image as if it were ½ of a meter closer. In reality, this enlarges the image enough so your eye can relax and see the details. In order for this to work, the lens must be within a ½ meter of the object being viewed.
A magnifying glass works much the same way. Magnifying glasses and plus powered eyeglasses actually create what is called a virtual image. The larger the power of the lens, the larger the viewed image. Conversely, the higher the power, the smaller the lens must be. This is because all magnification and diopter power comes from the curvature of the lens. And the higher power lenses must have thick centers. A larger lens will create a thicker lens, adding more weight, and more distortion.
Magnifying mirrors work in much the same way as magnifying glasses, only they are reflecting light back at the subject rather than letting it pass through. The object being viewed through the mirror must be within the diopter powers range. For example, if the mirror is 4 diopter strength (mirrors annotated as 4x) then you must be within ¼ of a meter to view yourself. When you get beyond that point, the mirror portrays and inverted and blurred image.
Lenses used for magnification are convex, while lenses used for minification (eyeglasses for myopia) have concave surfaces. They are opposite than plus lenses as they have thin centers and thick edges.
This is why it so important for an optician to analyze an eyeglass prescription before helping you select a a frame for eyeglasses. Most prescriptions will require different style frames or lens type in order to satisfy a person’s eyeglass prescription.
Another way to look at an eyeglass prescription for a nearsighted person is this. Say your prescription is -5.00. This means that for you to see an object clearly, that object must be within 1/5 of a meter, which is 20 cm, which is about 7.9 inches. In short, a -5.00 myope’s world is blurred beyond 7.9 inches, unless they are wearing eyeglasses or contact lenses.
Thank goodness for diopters and the wonderful world of optics. Otherwise the huge majority of our population that is nearsighted would be in a huge blur.
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