Pentax SMC Pentax 67 400mm f/4 ED [IF]

Long telephoto with extra-low-dispersion (ED) glass and internal focusing (IF). In 35mm-equivalent terms, 400mm on 6×7 corresponds to roughly 200mm full-frame — true long-tele territory, usable for distant landscape, wildlife (large mammals, birds at moderate distance), and sports.

ED glass corrects chromatic aberration that becomes increasingly visible at long focal lengths — without it, color fringing on high-contrast edges (backlit branches, water highlights, brightly-lit subjects against dark backgrounds) would mar the image at apertures wider than f/8 or so. Internal focusing means the front element doesn't rotate or extend during focus, which simplifies use of polarizing and graduated filters and improves balance on a tripod.

f/4 maximum aperture is fast for the focal length on medium format — comparable in light-gathering performance to many 35mm-system 400mm primes. Mounts on the outer bayonet (Pentax 67 lenses ≥400mm use the larger outer mount for the additional rigidity and weight-bearing capacity needed at long focal lengths). Tripod use is essentially mandatory; the lens is far too heavy for sustained handheld shooting.