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By Stephen F Tonkin

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In addition, the cost of providing matching eyepiece pairs, thus doubling the number of eyepieces required when compared to a conventional telescope, is not one that can be ignored, especially where good quality eyepieces are used. However, this is ameliorated to some extent by the development of binoviewers that incorporate the facility of multiple magnifications without changing eyepieces. An advantage that is not common to conventional binoculars is the ability to use high magnifications without the need to collimate two optical tubes.

By comparison, the field of view of the unaided eyes is approximately 45 degrees (excluding peripheral vision). Some manufacturers tend to be optimistic in their stated fields of view. In practice, 65 degrees appears to be the upper limit for an apparent field; all binoculars I have used with wider apparent fields have suffered from severe deterioration of quality and easily noticeable vignetting in the outer part of the field, and those of ultrawide angle have also appeared to have a poorer image quality even in the center of the field when compared to standard field binoculars of a similar price.

Some affect the quality of the image, others affect its position. They are: • • • • • • Chromatic aberration: error of quality Spherical aberration: error of quality Coma: error of quality Astigmatism: error of quality Field curvature: error of position Distortion: error of position Chromatic aberration is an error of refractive systems and is therefore of consideration for all binoculars. Because any light that does not impinge normally on a refractive surface will be dispersed, single converging lenses will bring different wavelengths (colors) of light to different foci, with the red end of the optical spectrum being most distant from the lens.

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