Mamiya-Sekor SF C 150mm f/4 (RB67)

The 150mm f/4 SF is the Mamiya RB67’s soft-focus lens, a short portrait telephoto, the equivalent of 74 mm on 35 mm film by Mamiya’s reckoning, that blurs on purpose. Its instruction sheet explains the mechanism without mystery: “A special type of spherical aberration has been purposely introduced into this lens to produce photographic images that are sharp yet which have an alluring softness”, corrected in every other respect by three new types of optical glass, and the effect disappears when the lens is stopped down to f/8 or beyond, at which point it is an ordinary sharp 150. The sheet gives 5 elements in 3 groups, an angle of view of 33 degrees, f/4 to f/32, 77 mm filters, a screw-in hood, 84 mm long by 94 mm across and 840 g; Mamiya’s Interchangeable Lenses for Mamiya RB67 booklet repeats those figures, prints the cross-section on its second page, and says the lens’s own instructions travel with it. Japanese Wikipedia, citing Mamiya’s catalogue, puts closest focus at 0.856 m, which is 0.615 m from the front rim.

The softness is set in two ways. Without a disc it depends on the aperture: softest at f/4, less at f/5.6, gone at f/8. With one of the three Softness Control Discs, thin metal caps with a large central hole ringed by smaller ones, the lens is left at f/4 and the disc does the stopping: No. 1 gives about f/5, No. 2 about f/5.6, No. 3 about f/6.3. A disc goes on by unscrewing the front lens barrel counterclockwise, slipping the disc over its rear end and screwing the barrel back. Because the aberration shifts the focus at full aperture, the sheet’s routine is to set f/8, lock the depth-of-field preview lever down, focus, then open up to the working aperture and release the lever; the plain matte and checker screens are recommended over the rangefinder and microprism types. Japanese Wikipedia’s warning is the buyer’s: without all three grids the lens cannot show what it can do, so check a used example for them. There is only the one version. No K/L soft-focus lens was made, and Mamiya’s Pro SD lens page and its c. 1994 brochure both carry the same 5-in-3 lens into the Pro-SD era, at 790 g on the later page against the sheet’s 840 g. The K/L 150mm f/3.5 L of 1990 is a different, sharp lens and has its own page; the RZ67’s soft-focus lens is a 180.

No review of it was found on Emulsive, 35mmc, Kosmo Foto, analog.cafe, Shoot It With Film, Japan Camera Hunter or the KEH blog, and the Photo.net thread on user experience with it could not be read from here. What exists is a Flickr Mamiya RB67 group thread, a single forum source, whose contributors agree on the practical points: the effect needs f/4 or f/5.6 and is gone by f/8, the rendering is dreamlike with creamy out-of-focus areas, the lens is one of the sharpest in the RB line when stopped down without a disc, 400 speed film wants neutral density to hold the wide apertures, and the centre-sharp look belongs to a period of portraiture rather than to today. The RB67 Camera Guide adds the disc apertures and calls the out-of-focus highlights the point of the exercise. Mamiya’s own page called it the prime lens of many portrait and wedding photographers because the softness came without retouching. Casual Photophile, Ken Rockwell and Lomography are not consulted by this catalogue.

Buying is about the discs, then the shutter. A lens without its three Softness Control Discs is half a lens, and complete sets change hands separately at a price; count them, check the numbers 1 to 3, and look inside the front barrel for the seat they sit on. The Seiko #1 lives in the lens, is cocked by the body, and after years in a bag will often hang at T or drag at the slow speeds; before mounting, the cocking pins on the barrel must be turned all the way to the red marks, and the booklet says a lens stored for a long time should have its shutter released. Confirm the depth-of-field preview lever locks down and springs back, because the focusing routine depends on it, and that the front barrel unscrews and reseats without binding. On a Pro-SD the lens wants the 7 mm adapter ring Mamiya recommends for light sealing. No launch price in yen or dollars was found, no serial-number table exists, and no discontinuation date is published.