This is a classic Hasselblad "Chimney" viewfinder. It is designed to slide onto the top of the camera body in place of the standard folding waist-level finder. It provides a bright, shielded view of the entire ground-glass focusing screen with high magnification (usually around 2.5x). The large rubber eyecup blocks out stray light, making it much easier to achieve critical focus in bright outdoor conditions. Unlike a prism finder, this chimney style does not correct the left-to-right reversed image, so the view remains mirrored.