Mamiya's RB67 soft-focus portrait lens — the leaf-shutter Sekor C 150mm f/4 with effect disks.
The Mamiya Sekor C 150mm f/4 Soft Focus is a specialist portrait lens for the Mamiya RB67 medium-format system, from the C-series line of the late 1970s. Unlike the standard telephotos it is designed to deliver a controlled soft-focus effect, prized for flattering portraiture and pictorial work on 6x7. It occupies a distinctive niche in the RB67 catalogue.
This is a manual-focus lens native to the Mamiya RB67 bayonet mount, with a focal length of 150mm and a maximum aperture of f/4. It carries a built-in leaf shutter, with the aperture set on the barrel and focusing by the camera bellows. The lens uses removable perforated disks to adjust the degree of soft-focus effect, allowing the amount of spherical-aberration glow to be dialled in.
At 150mm on 6x7 the perspective suits portraiture, and the soft-focus design adds a controllable halation around highlights that softens skin and lends a romantic quality. The effect is strongest wide open and reduces as the lens is stopped down or when the supplied disks are changed. This makes it a deliberate creative tool rather than a general-purpose telephoto.
The 150mm f/4 Soft Focus is uncommon and appeals to portrait and fine-art photographers, so clean copies can command a premium. Confirm the soft-focus disks are present, as they are easily lost and hard to replace. Test the leaf shutter at all speeds, inspect the glass for haze and fungus, confirm the aperture is dry, and verify the bayonet and shutter coupling before buying.