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Q: What is distortion?
Distortion is an optical error (aberration) caused by the lens, that results in a difference in magnification at different points within the image.
Q: What is a focal length?
Focal length is the distance from the object to the lens surface when the object is in sharp focus.
Q: Why are magnifiers made with optical acrylic material instead of glass?
The main reasons for using acrylic instead of glass are cost and weight. Glass lenses are much more expensive to produce then acrylic lenses. Between glass lenses and acrylic lenses, acrylic lenses weigh about one-half that of glass lenses. Exceptional clarity and resistance to ultraviolet light, degradation and discoloration make the acrylic used in Ultra lenses the standard in the industry. These resins are virtually unaffected by alkalis, hydrocarbons, non-oxidizing acids, salt water, photographic or battery solutions.
Q: What is an aspheric lens?
Aspheric lenses are specially made with non-spherical surfaces, corrected to eliminate spherical aberration, and improve focusing and collimating accuracy. The tooling is quite expensive, but the results are worth it.
Q: What is a fresnel lens?
A fresnel lens replaces the curved surface of a conventional lens with a series of concentric groves molded into the surface of a thin, lightweight plastic sheet. The grooves act as individual refracting surfaces, like tiny prisms when viewed in cross sections, bending parallel rays in a very close approximation to a common focal length which if designed properly can be more efficient at gathering light then a standard lens.
This efficiency can be an advantage but there is a loss of image quality due to the visibility of the concentric rings in the lens surface. Higher grooved density can allow for higher quality images however.
Because of these factors, fresnel lenses are a compromise between efficiency and image quality.
Q: What happens if the magnifier is lifted too high over viewing image?
The magnified image will go out of focus and become distorted. Depending on the situation, the image can then become inverted and get smaller as the magnifier is moved closer to your eye.
Q: What is a bifocal lens and what are the features of a bifocal lens?
A bifocal lens is a lens that has two different powers within it. An example of this is demonstrated in the Ultra 3x-6x hand magnifier. The large portion of the lens has a magnification of 3x, and the smaller lens, which is molded as an integral part of the larger lens, has a magnification of 6x. In spectacle lenses, the upper portion of the lens has a proper prescription for distance viewing and the lower portion has the proper RX for close up viewing.
Q: What is field of view?
Field of view is the area seen through the magnifier. As the power increase, lens diameter and field of view decrease. At (5x) power the field of view is about 1 ½”. At (10x) power it is about ½”. Usually, it is best to use low power for scanning larger surfaces and high power for scanning small areas.
Q: What is depth of focus?
Depth of focus (depth of field) is the distance between the closest and the furthest points at which a magnifier in a fixed position stays in focus. The depth of field decreases as power increases.
Q: What are the advantages of lighted magnifiers?
Lighted magnifiers help an observer see more detail of an item being viewed, especially in a poorly lighted area. The light should be concentrated on the object being viewed.