Eyepiece

An eyepiece is the lens assembly through which an observer views an image formed by an optical instrument’s objective. It magnifies that image and directs light toward the eye or a sensor.

Objective lens: An optical element or assembly that collects light from an object and forms an image. The eyepiece magnifies the image this component forms.
Real image: An image formed where light rays from an optical system actually converge. Many eyepieces are positioned to view the real image formed by an objective.
Telescope: An optical instrument that collects light from distant objects to make them appear larger or brighter. A telescope eyepiece sets the magnification and apparent field used for visual observing.
Barlow lens: A negative lens assembly that increases a telescope’s effective focal length when placed before an eyepiece. Unlike an eyepiece, it sits ahead of the viewing assembly and modifies the converging light.
Angular magnification: The ratio between an image’s apparent angular size through an optical instrument and its size viewed unaided. It describes how strongly an eyepiece enlarges the apparent image.
Virtual image: An image that appears to originate from a point where light rays do not actually converge. The observer commonly perceives the eyepiece’s output as a virtual image.
Microscope: An instrument that uses lenses to produce enlarged images of small objects. A microscope eyepiece magnifies the intermediate image produced by the objective.
Relay lens: A lens system that transfers an image from one plane to another within an optical instrument. A relay lens transports an image internally, whereas an eyepiece presents it for viewing.
Exit pupil: The image of an optical system’s aperture stop formed by its final optical elements. The eyepiece forms the exit pupil, which determines where the eye receives the light.
Focal length: A distance that characterizes how an optical system converges or diverges light. Eyepiece focal length helps determine instrument magnification when paired with an objective.

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