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Descriptions, Definitions, Recommendations
Types of Lenses
Referenced below is some helpful technical information including lens types and terminology definitions. Use these guides in addition to the magnifiers recommendations below to help you choose the correct magnifier.
Positive Lens - A single piece of optical glass or acrylic with
two convex surfaces (curved outward). Designed for low magnification.
Simple Negative Lens - A single piece of optical glass or acrylic with two concave surfaces (curved inward). Used in conjunction with positive lenses.
Achromatic Lens - A positive simple lens cemented to a negative simple lens. Achromatic lenses are corrected for two colors and produce flatter fields of view at higher powers.
Double Lens - Two simple lenses used as a system but not cemented together. This system produces a far superior image than a simple lens.
Power: The number of times a lens or lenses multiplies an image with respect to the original size. Low power is recommended for scanning large surfaces. Higher powers are generally used for inspecting small areas.
Field of View: The maximum area seen through a magnifier. The entire field of view may not be in focus. See Flatness of Field.
Flatness of Field: Due to the physical laws of optics, the outer part of the image formed by a lens may be out of focus. The greater the power and curvature of the lens, the more pronounced this problem becomes. Magnifiers with multiple lenses can reduce this problem. The visible area which appears in focus is the flat field.
Working Distance: The distance from the magnifier lens to the object being viewed. As magnification power increases, the working distance decreases.
Aberration: This is a distortion caused by the lens. Spherical aberration is distortion of the image, as explained in Flatness of Field. Chromatic aberration is distortion of colors being viewed, as explained in Color Distortion.
Color Distortion: Lenses produce a prism effect which causes the image to develop false color fringes. This is due to the fact that different colors focus at different points. Achromatic lenses correct this problem by focusing many colors at the same point.
Parallax: Parallax is the apparent misalignment of two different items when viewed from an angle. It is the cause of improper registration when stripping multiple color jobs. Parallax is eliminated when viewing straight down on registration marks instead of from a slight angle.
Coated Lens: A coated lens helps to minimize or eliminate color distortion caused by different colors focusing at different locations. Optimum color viewing is obtained from coated achromatic lenses.
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