Coin Magnifiers & Loupes
What Is a Coin Magnifier?A coin magnifier is a lens that increases the apparent size of objects seen through it. It may be a single lens, thicker at the center than at its edge, or it may be a compound lens made of several lenses mounted or cemented together. By moving closer to an object we are able to see it in more detail. But the focusing power of our eyes is limited and we are able to see clearly only down to about 10 inches. A magnifier, in effect, adds focusing power to the eye, enabling us to move closer than 10 inches to the object and to see more detail. We see the effect as an increase in the image size. Depending on its power, a magnifier makes it possible to see an object clearly as close as one-half inch from the eye.
Why So Many Different Magnifiers?Basically, the purpose of a magnifier is to enlarge the image of a coin so that its details may be seen more clearly. This is a function of the power of the magnifier. However, three other factors affect the performance of a magnifier and its suitability for certain jobs: field of view, depth of field and working distance (focal length). The four factors are interdependent; if the power is increased, the other three become smaller, and so forth. In selecting a magnifier, you should consider all four factors.
Power of MagnificationThe power of magnification refers to the capacity in the lens to increase the image size. X, the symbol used with a number in denoting the power of a magnifier, is quite simply the multiplication sign, "times." Thus, a 2X magnifier creates an image size twice as large as that which the unaided eye sees at 10 inches. A 3X magnifier triples the image size, and so on.
Focal LengthFocal length is the distance at which a magnifier must be held away from an object to achieve clear focus and maximum magnification. In a 2X magnifier the focal length is approximately 5 inches (the lens thickness is a factor); in a 5X magnifier it is 2 inches; and in a 20X magnifier it is 1/2 inch.
Field of ViewField of view is the size of the area that can be seen at any one time. In a magnifier, a number of things influence the field of view: the diameter of the lens for instance. However, the power of magnification primarily determines the size of the field of view-the higher the power the smaller the field of view.
Depth of FieldDepth of field is the distance that you can move a magnifier toward or away from an object and still have the object in focus. It also refers to the depth of the area in front of or behind the viewed object that can be seen clearly. Like the field of view, the depth of field has an inverse relationship with the power of magnification-the higher the power the shorter the depth of field.
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Glossaryaberration-The failure of a lens to bring all the rays of light to exact focus, causing a blue-red image.
achromatic-A lens which corrects for chromatic aberration; transmits light that forms images practically free from prismatic colors.
aplanatic-A lens which corrects for spherical aberration and coma.
astigmatism-A defect in which the lens fails to unite rays of light from an external point at a single image point, thus giving an imperfect image or vision.
chromatic aberration-The inability of a lens to focus light of different colors at a simple point.
Coddington-A corrected lens, named after its British inventor, Henry Coddington.
coma-The blurred appearance or hazy border surrounding an object viewed through a lens which is not free from spherical aberration. concave-Describes a lens surface which is hollowed; interior of a curved surface.
convex-Describes a lens surface that curves or is rounded outward.
corrected-A lens or lens system which corrects for aberrations; remedies deviations of light rays from object to eye to produce a clear, sharp image.
crown glass-Optical alkali-lime glass having a low dispersion and usually a low index of refraction.
curvature of field-When a plane field is not imaged as a plane, or the outer part of the field is not imaged in the same plane as the center and therefore appears out of focus; as opposed to flatness of field. curvature of lens-The amount of sharpness of curve in a lens surface. diopter-The amount of power in a lens needed to focus parallel light at one meter.
dispersion-The separation of light into its component colors, as in passing through a prism.
distortion-That defect of a lens whereby the images of straight lines appear curved.
double lens magnifier-A magnifier composed of two single lenses. flatness of field-Appearance of the image to be flat; a plane in the object will be imaged as a plane as opposed to curvature of field. flint glass-A heavy, brilliant glass containing lead and having a high dispersion and usually a high index of refraction.
focus-The point at which light rays through a lens intersect to form an image.
Hastings Triplet-A highly corrected magnifier composed of three simple lens elements cemented together to form a single lens. highly corrected-A magnifier or lens in which virtually all aberration is eliminated.
image-The likeness or picture formed by a lens; the optical counterpart of an object.
meniscus-A crescent-shaped lens-one which is concave on one surface, convex on the other. It may be converging or diverging.
plano-Pertaining to flat; a plano lens surface has no curve.
plano-concave-A lens with one surface flat, the other curved inward. (See concave)
plano-convex-A lens with one surface flat, the other curved outward. (See convex)
refractive index-The ration of speed of light in a vacuum, or in a given medium to its speed in a different medium.
semi-corrected-A magnifier or lens in which only part of the aberration is eliminated.
spherical aberration-A defect in a lens which causes marginal and central rays to focus at different distances from the lens, producing an image which lacks contrast.