Definitions of Magnifier-Related Concepts

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 then 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 calculating Magnification Power is:

MP = D (Diopter) + 1
               4

Example: 20 D + 1 = 6X
               4

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.

Source: Howard Electronic Instruments