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.