Diopter and Magnification

What is a Diopter, anyway?

A simple explanation, or, as simple as it gets..

Check the dimes at left. If the smallest is unmagnified...the others are 3, 5, 10 and 15 diopter.
Read below for the explanation.


  • 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.
Common diopter/Power/Focal Length
Diopter Power % Bigger than object Focal Length
3 1.75X 75% 13"
4 2.00X 100% 10"
5 2.25X 125% 8"
7 2.75X 175% 5.5"
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"

Selecting The Right Magnifier

  1. Determine the desired magnification for your needs. Remember, as you increase magnification, you decrease both the focal length and the viewing area.
  2. Check to find out the correct diopter you need to achieve that magnification.
  3. Note the focal length and lens diameter that correspond to the magnification and lens diopter you have chosen, and make sure they are suitable for your task.
  4. As a general rule, because the working distance will be less than 8" above 5 diopter, stereo microscopes are recommended for rework purposes only.

Tips on Proper use of a Magnifier

To take best advantage of the comforts built into illuminated magnifiers, please keep these points in mind:

  1. Use both eyes. Magnifiers are designed as "working tools". They can be used as comfortably as a pair of glasses.
  2. Position the lens so that it is a proper distance from the work area, yet close enough for your eyes (8" to 10") so that you have the maximum magnification without distortion. Do not lean back away from the lens to increase magnification.
  3. Chair height and work surface should be positioned so the operator can maintain good posture while working.

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