Precision work routinely requires magnification to increase the visual size of objects. As the eye comes closer to an object, the apparent size and detail is increased. The ability of the eye to focus, however, diminishes at distances closer than ten inches. Use of a magnifier provides increased focusing power, resulting in the visual effect of a clearer, larger image of the object.
Four Characteristics of Magnifiers:
1. Power of Magnification
The ability of the lens to increase the visual size of an object. The symbol X (times) is used to identify this factor. For example, a 3X magnifier triples the size of an image.
*Light rays are bent by the curved lens surface resulting in an image that looks larger than the actual object.
2. Working Distance (Focal Length)
In addition to power, a magnifier will also be classified for working distance. For example, a 2X at a working distance of 5” means that the magnifier will provide double magnification and be in clear focus when held at a distance of 5” from the object.
*The amount of space between the object and the magnifier: the shorter the working distance the higher the power.
3. Field of View
The size of the area that is seen though the magnifier is the field of view. This decreases as the power increases.
4. Depth of Field
The distance that a magnifier can be moved from an object and still have the object in focus is the depth of field. The higher the power, the shorter the depth of field.
Certain magnifiers has lens systems which correct optical distortion or aberration that might otherwise be present as follows:
Distortion: Uncorrected lens- “straight lines” curve towards the center
Chromatic and Spherical Aberration: Uncorrected lens-size and shape differences occur in a pattern consisting of one size dot.
Achromatic- Lens that eliminates chromatic aberration.
Aplanatic- Lens that corrects for spherical aberration and coma.
Chromatic Aberration- Inability of a lens to focus light of different colors to a point
Coma- Blurry appearance around an object viewed through uncorrected lens.
Corrected- Lens system that corrects for distortions and aberrations to produce a sharp image.
Spherical aberration- Lens defect that produces an image that lacks contrast/