The simplest of optical devices intended to enhance our visual capability is the magnifier. This is a simple positive lens with a focal length of 100 mm or less. (Biocular, Sherlock Holmes-type magnifiers, which may have longer focal lengths, will not be covered.) When a simple positive lens is placed close to the eye, it permits the object being viewed to be moved inside the near point of the eye, thus increasing the apparent size of that object along with the final image size on the retina.
The most common lens forms used for magnifiers are the biconvex element, the achromatic doublet, and the Hastings triplet. While all of these are commercially available at reasonable prices, we will consider the optical design of each in order to gain some insight into their performance characteristics.
Several more complex, multicomponent lens forms are used as magnifiers. The design and performance of one such magnifier will also be covered. In every case the final visual image quality, in terms of on-axis and off-axis resolution, will be determined and compared.
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