Tessar 15cm f/4.5 (SKa 4)

The standard lens of the SKa 4, and the one with the strangest mounting arrangement in the whole Ross line. On the SKa 4 the 15cm lens was fixed inside the camera and folded away when one of the longer lenses was fitted to the bayonet; using it meant attaching a clear plate in the bayonet opening. The SKa 4a dispensed with the folding arrangement altogether, which is what made that variant smaller and lighter.

Ross Aktiebolag of Göteborg was the company Victor Hasselblad ran through the war, building aerial cameras for the Swedish air force, and it is where the firm that later made the 1600F learned its trade. Ross built the bodies and, unusually, the shutters; it made no glass at all. The lenses came from Germany — Zeiss, Schneider and Hugo Meyer — and Hasselblad Historical notes that no documentation has been found describing how they were imported into a neutral country that badly needed optics and rather more badly needed to be left alone.

The SKa 4 was a different proposition from the hand-held HK 7: a 12×12cm camera mounted permanently in an aircraft’s belly with the lens pointing straight down, running special 13cm double-perforated roll film and stamping the date, hour and minute of exposure on the film edge. The whole camera weighs 14.9kg with this lens fitted.

Nothing Ross fitted was anti-reflection coated; coating reached Swedish military optics after the war, and these lenses date from before it.

Hasselblad Historical’s account of the Ross cameras is the only substantial source on them, and its author marks several specifications as information requested — an open call to readers for detail he does not have. Maximum aperture, weight, element count and production quantities for this lens are among the figures that no published source gives, so what is recorded here is deliberately short. Hasselblad Historical also lists a 15.2cm Cooke Aviar among the SKa 4’s lenses, so there appear to have been two lenses of about this focal length available for the camera.