Okulära

Okulära are the eyepieces of optical instruments such as telescopes, microscopes, and binoculars. They are situated closest to the eye and work with the objective lenses to produce the final magnified image that the observer sees. The eyepiece influences not only magnification but also the apparent field of view and eye comfort.

Magnification is usually the objective’s focal length divided by the eyepiece’s focal length. For example, a telescope with a 1000 mm objective and a 10 mm eyepiece provides about 100x magnification. Eyepieces are commonly built for specific barrel standards (such as 1.25-inch or 2-inch in astronomy) and may offer different apparent fields of view. Important practical characteristics include eye relief (the distance from the last optical surface to the eye) and, in some designs, diopter correction for users with vision differences.

Eyepiece designs range from historical to modern. Early types include Huygens and Ramsden; later common designs include Kellner and Plössl. Modern eyepieces also include orthoscopic and wide-field variants, often optimized for brightness and edge-to-edge sharpness. In microscopy, eyepieces are labeled by magnification (for example 10x, 20x, 40x) and are typically designed to be parfocal with related objectives so that focusing is maintained when switching eyepieces. Lens coatings reduce reflections and improve light transmission.

The term okulära is the Swedish plural for eyepieces and is used in Swedish-language technical literature to refer to this component across various optical instruments.