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Lens Diameter If you could hold an object right against your eyeball and still have it in focus, you would make very tiny images appear very large because they would seem to be as wide as your entire field of view. Individual letters of newsprint would appear as wide as your desk!

Magnifying lenses change the angle that light travels as it approaches the eye so as to make objects appear larger. This increases the apparent size of the object and makes it appear to be closer. The lens can further increase the apparent size by allowing one to focus on objects held very close to the eye. This combination is summarized by a number which specifies the total magnifying power of the lens.

Higher powers produced by using a single lens require a smaller radius of curvature for the lens. This limits lens size because lens diameter can not be any larger than twice its radius. For this reason, as power increases, lens diameter and field of view decrease.

At 5x power the diameter of a single lens is seldom more than 2" and the field of view is 0.4" -- 1/5 of the diameter. At 10x power the lens usually does not exceed 1.0 in. in diameter, so the field of view of such a lens is 0.1". Higher magnification lenses are correspondingly smaller and have even smaller fields of view. Magnifying newsprint by 20 times results in lower case letters which appear almost one inch tall and completely fill the field of view of the lens, so text has to be read one letter at a time.

Combining high powers and larger diameters can be accomplished by stacking lenses on top of one another. This permits utilization of a combination of lower power, wide lenses to achieve higher magnification. However, use of multiple lenses increases cost and produces more distortion unless further (and more expensive) measures are taken to undistort the images. As a result, it is best to use low power for scanning larger surfaces and high power for scanning small areas.

Since optical magnification is subject to the above limitations, technological alternatives are often utilized to achieve the combination of high power and large diameter. These include video and projection based magnifiers which typically cost many hundreds or even thousands of dollars.

Fact: Rare meteorite found near Lake Huron
One of the world's rarest meteorites has been discovered by an amateur rock hound strolling along a boulder-strewn shore of Lake Huron.
Carl Young, a 78-year-old retired pipefitter from London, Ont., stumbled across the unusual boulder last April while looking for driftwood on a beach near Southampton, a small port about 230 kilometres northwest of Toronto.
An avid rock hound for 25 years, Mr. Young immediately suspected his find was something unique when he stooped to lift it and found it surprisingly heavy for its size - slightly smaller than a basketball.
He lugged the 2.5-kilogram rock home to his carport, where it sat for three weeks until he tested it and discovered it was not only magnetic, but when cleaned, gleamed with bright green crystals the size of fingertips surrounded by highly reflective metal.
Mr. Young hauled his find to the University of Western Ontario in London, Ont., where it was positively identified as a rare class of meteorite found only three times in Canada.
"I had a suspicion what it was," Mr. Young said. "It's a once in a lifetime opportunity."
Researchers at UWO suspect that, like most other meteorites, Mr. Young's discovery originated in the asteroid belt, a region filled with thousands of rocks that revolve around the sun between the orbits of Mars and Jupiter.
What has them excited is not its origin but its composition. The rock is a pallasite, made up of a rare combination of metallic and silicate minerals that has been found only 52 times throughout the world.