![]() "erecting" the image) before passing to the eyepiece, so the viewer actually sees an upright target. In modern optical telescopes and telescopic sights with a dual- focal plane design, the objective image is typically already inverted upon reaching the relay lenses, and thus needed to be inverted again back into an erect image (i.e. In practice, the lens will be an achromatic doublet. If a longer distance is needed, this can be repeated. Ideally, this second image is the mirror image of the first image, so you could put an image sensor there and record the mirrored first image. If you place another lens with focal length f at the distance 2 f from that image plane and then put an image sensor at 2 f beyond that lens, that lens will relay the first image to the second image with 1:1 magnification (see thin lens formula showing that with object distance s = 2 f ). For example, in a SLR camera the zoom lens produces an image plane where the image sensor or photographic film would usually go. Relay lenses operate by producing intermediate planes of focus. They may be made of one or more conventional lenses or achromatic doublets, or a long cylindrical gradient-index of refraction lens (a GRIN lens). Relay lenses are found in refracting telescopes, endoscopes, and periscopes to optically manipulate the light path, extend the length of the whole optical system, and usually serve the purpose of inverting the image. In optics, a relay lens is a lens or a group of lenses that receives the image from the objective lens and relays it to the eyepiece. Image-erecting optical system for astronomical telescopes (erecting eyepiece) ![]()
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