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In page Twin-lens reflex camera:

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Many TLRs used front and back cut-outs in the hinged top hood to provide a quick-action finder for sports and action photography. Late model Rollei Rolleiflex TLRs introduced the widely copied additional feature of a second-mirror "sports finder". When the hinged front hood knock-out is moved to the sports finder position a secondary mirror swings down over the view screen to reflect the image to a secondary magnifier on the back of the hood, just below the direct view cutout. This permits precise focusing while using the sports finder feature. The magnified central image is reversed both top-to-bottom and left-to-right. This feature made Rolleis the leading choice for press photographers during the 1940s to 1960s.[3]