Small Focal Ratio and Aberrations

Most binoculars have objectives that operate at around f/4 or f/5, although there are some specialist astronomical binoculars, intended for use at relatively high magnification, that have greater focal ratios.

Most optical aberrations are exacerbated with "fast" (i.e., low focal ratio, thus photographically "fast") objectives. For a normal achromatic doublet that does not use exotic glasses, the rule of thumb is that the focal ratio must be no less than three times the diameter of the aperture, measured in inches (1 inch = 25.4mm), for axial (longitudinal) color correction to be acceptable. This is equivalent to stating that a 50-mm objective must work at f/6 and a 100 mm at f/12, or that the limit for f/5 is 42 mm. This latter equivalent is a reason for the good reputation for optical quality of many 42 mm binoculars. If optical quality is to be maintained at greater apertures without a concomitant increase in focal ratio, either expensive exotic glasses or extra len elements or both must be employed. Several modern astronomical binoculars have slower focal ratios, such as f/7.5 or f/8. Lower focal ratios have light cones that are more obtuse; and obtuse light cones are more demanding of eyepiece quality than those that are more acute. This means that for image quality not to be compromised, better eyepieces are needed and thus greater expense is required.

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