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Principles of Magnification

Magnifiers enhances one's ability to perform a task or enjoy a hobby. This information is to aid in understanding how magnification works and how one can get the most most out of the magnifier he or she is using. The principles of magnification apply to all magnifiers employing simple lenses or combination of lens.


Focal Length: The distance from the center of the lens (or lens system) to the viewed object. Working distance is the distance from the bottom of the lens, or the lens housing to the object when the object appears at maximum magnification without distortion. It is important to remember that as magnification increases, the focal length decreases.

Diopter (d): A term used to identify the refractive (light bending) capacity of a lens. In magnifiers, there is a direct correlation between focal length and diopter. To find the diopter of a magnification lens, follow these steps. With the eyes 10" above the lens, move the object to be viewed to the point the greatest distance below the lens where it remains in sharp focus. Measure this distance and divide into 1 meter (39.37"). The result is the diopter of the lens - e.g., if the object is at a 13" distance than it is a 3-diopter lens (39.37/13 = 3d). Each diopter increases the size of the viewed object by 1/4 (25%) when the object is at its full focal length from the lens.

Field of View: The distance across the lens surface to which the viewer brings both his eyes (note: eyes should be 10" above the lens). It is important to note that as magnification increases, meaning the lenses used are stronger, viewing areas and focal length decrease.

Magnification: The degree to which the viewed object is enlarged. Magnification is usually expressed by a number followed by an "x", the symbol used to express power or the size of the object in relationship to its actual size. The formula for calculation Magnification Power is: 

 diopter/4 + 1 = X

Common diopter/power relationships
Diopter Magnification % Bigger than object Focal Length
3 1.75X 75% 13"
4 2.00X 100% 10"
5 2.25X 125% 8"
7 2.75X 175%


8 3.00X 200% 5"
9 3.25X 225% 4.5"
11 3.75X 275% 3.75"
13 4.25X 325% 3"
16 5.00X 400% 2.5"
18 5.50X 450% 2.25"
20 6.00X 500% 2"

Magnification Example If a dime was this small unmagnified ...

3 diopter

5 diopter

10 diopter

15 diopter