In addition to the magnification and aperture, the other numerical factor that is usually stated is the field of view. This is quoted in one of three ways:
• Degrees. This is the most useful factor for astronomers, since it gives you an indication of the amount of sky you will be able to see. The area of sky that will be visible is directly proportional to the square of the angular field.
• Metres at 1,000 Meters. This factor is more useful for terrestrial use and is currently the most commonly found alternative to degrees. The approximate conversion of this to degrees is to divide by 17.5. Thus 87m at 1,000m « (87/17.45)° = 5°. There is a conversion chart given in Appendix H.
• Feet at 1,000 Yards. This factor is more useful for terrestrial use and is currently most often found on binoculars intended for the U.S. market. The approximate conversion of this to degrees is to divide by 52. Thus 364 ft at 1,000 yd « (364/52)° = 7°. There is a conversion chart given in Appendix H.
Most, but not all, people prefer a wide field of view for astronomy. The true field of view is dependent on the magnification and the apparent field of view of the eyepiece:
True field = Apparent field + Magnification
Strictly speaking, an eyepiece can have an extremely large field of view, but this deteriorates rapidly toward the edge, so it is limited by a field stop. There is always a trade-off between field of view and edge quality. In general, a 50-degree apparent field is a "standard" field, 65 degrees and above is considered to be "wide angle," and 80 degrees and above is designated "ultrawide angle." 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.
Another problem associated with some very wide field eyepieces is the effect that is colloquially called "kidney-beaning" or "flying shadows." The colloquial names are descriptive of what you see if your eyepieces are afflicted with this problem, the correct name being spherical aberration of the exit pupil. Different zones of the exit pupil are focused at different distances from the eyepiece, so your eye is unable to focus on the entire field at once. If your eye is slightly off center, the result is these flying shadows that are the shape of kidney beans. They tend to be worse at night when your pupils are more dilated, and some people seem to be more bothered by them than others.
While wide field views are an attraction to many people (a 7-degree field shows an area of sky twice as large as a 5-degree field), magnification is an extremely important factor for binocular astronomy, and a small cluster, nebula, or galaxy that is detectable at x10 may appear to be stellar at x7.
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