Tele-Tessar 25cm f/4.5 (SKa 4)

The middle lens of the SKa 4’s three, mounted through the camera’s bayonet rather than folded inside it as the 15cm was. Hasselblad Historical gives its aperture as f/4.5 and its weight, alone, as 1.7kg.

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.

A Tele-Tessar is Zeiss’s telephoto arrangement — the Tessar’s front cell followed by a negative group — which shortens the barrel relative to the focal length. On a camera bolted into an aircraft belly that matters less than it does on a hand-held one, but the SKa 4 already weighed nearly fifteen kilograms with its shortest lens fitted and its mounting bracket alone weighed 6.2kg, so nothing about the installation encouraged extra length.

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. The source’s specification table gives the aperture range as f/4.5 to an unrecorded minimum, one of several entries where the author marks the figure as information he is still seeking.