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Magnification Knowledge & Facts |
Power (X):
The number of times a lens multiplies the image.
Focal Length (FL):
The working distance between the lens and the object where the image is sharply defined.
Field of View:
The maximum area seen through a magnifier.
Depth of Field:
The allowable variation in focal length in which a lens remains in clear definition.
Aberration:
Spherical aberration distorts the image. Color aberration separates the colors of the spectrum.
Corrected Lens:
Lenses which are ground with variable radii or a combination of lenses or various types of glass spaced or stacked to form a lens system.
All of the above factors are variables that must be taken into consideration in selection and use of a magnifier. Each of these factors affect the other. The power of the lens determines the focal length, the focal length limits the field and depth of view, (i.e., the shorter the focal length, the stronger the power). Aberrations are more predominate when lenses are used at improper focal lengths, especially in the stronger powered lenses.
How All magnification works:
Rays of light reflected from an object pass through the lens and are bent toward the apex of the lens (thinner part) to produce a magnified image. The smaller the radius on the surface of the lens, the more the rays are bent. Therefore, the stronger the lens the larger the image.
How to use a magnifier:
The plane of the lens in a magnifier should always be held parallel to the plane of the object being viewed. By holding the magnifier as close to the eye as comfortably possible, the fullest number of rays from the object being viewed are permitted to enter the eye and cuts down reflected light hitting the surface of the lens, thus minimizing reflection and glare.
Each Individual and the particular application of a magnifier will dictate the method of use. By experimentation the individual may find the best way to use the magnifier for his special task. It is best to employ a magnifier in the least power sufficient to serve its purpose with a comfortable focal length. Hand magnifiers are designed for use of both eyes so it is only natural to hold them further away from the eyes and closer to the object or reading material.
IRA:
Independent Reading Aid.
Diameter versus power:
Lenses are like sections cut from spheres. A section cut from a sphere the size of a golf ball would be stronger in power than a lens cut from a sphere the size of a basketball. It is apparent then that the diameter of a lens is limited in size by the radii of its surface. Hence, it is important to produce a large lens with a small radius to effect strong power.
Lens materials:
Many transparent materials may be used to manufacture lenses. The most common materials for magnifying lenses are glass and acrylic plastics. The selection of the material for the lens is based on its use. A glass lens would be harder and more scratch resistant, whereas a plastic lens is lighter in weight but would require more care to avoid abrasion.
Mountings:
The mounting used on a magnifier is most important to its utility. A lens of specific power may be mounted in a number of fashions so as to provide a convenient instrument for various uses.
Usage:
A person performing a task which requires the use of both hands obviously must find a different method of holding the same lens that a person would use to scan a surface. Lenses having strong powers and short focal lengths should be mounted when possible in fixed focus stands because a slight variation in focal length may cause image jump and distortion.
Aberrations:
Spherical lenses (especially the stronger powers) produce a pin cushion appearance with peripheral blurring. In the weaker powered lenses this effect is tolerable. When lenses of a stronger power are required, it is necessary that a system be developed by composing two or more lenses stacked or space to correct the inherent distortion; or a lens may be ground with an aspheric curvature to eliminate distortions.
As various color of the spectrum focus at different lengths, the phenomenon is also magnified and is discernible with stronger power.
