Mamiya's RB67 standard lens — the leaf-shutter Sekor C 90mm f/3.8 for 6x7 medium format.
The Mamiya Sekor C 90mm f/3.8 is the standard normal lens for the Mamiya RB67 medium-format system, from the mid-1970s C-series. It was the lens most commonly supplied with RB67 bodies and served as the default optic for studio and general 6x7 work. On the format it gives a natural, normal field of view.
This is a manual-focus lens native to the Mamiya RB67 bayonet mount, with a focal length of 90mm and a maximum aperture of f/3.8. It carries a built-in leaf shutter like all RB67 lenses, with the aperture set on the barrel and focusing performed by the camera bellows. The optical design covers the full 6x7 image circle.
At 90mm on 6x7 the perspective corresponds to roughly a 44mm lens on 35mm, close to a true normal view that works for portraits, general shooting and landscape. The leaf shutter allows flash sync at all speeds, a key advantage for studio use. The lens renders with good contrast and even sharpness across the large frame when stopped down.
As the RB67's standard lens, the 90mm f/3.8 is plentiful and affordable on the used market. Because it carries a leaf shutter, fire it at all speeds to confirm clean, accurate operation, and check for stickiness or sluggish blades. Inspect the glass for haze and fungus, confirm the aperture is dry, and verify the bayonet and shutter-cocking coupling engage properly.