The story in brief with interesting insights into the origins.
ALPA introduced the Short Barrel concept back in 2007. How did this come about and what were the key aspects of this decision?
In June 2007, ALPA visited the Bibliothèque Nationale de France in Paris to help evaluate a successor to its Sinarcam 23. BNF planned to make the best possible use of the digital backs in question from Hasselblad or Sinar and to achieve the highest possible megapixel yield. In addition to multishot backs, extremely precise stitching and planarity were therefore required, and the ALPA 12 XY and ALPA/Schneider Apo-Digitare with electromechanical leaf shutters were therefore already evaluated.
Due to the working distances and the large-format originals, BNF wanted to use the ALPA / Schneider Apo-Digitar 5.6/120 mm as one of its main lenses. With a 4-fold stitch, the image circle of 110 to 120 mm was theoretically exhausted. However, the geometry caused mechanical vignetting. Since this effect did not occur with wide-angle lenses, appropriate solutions had to be worked out. This led to the development of the Short Barrel lens concept.
But what changes if a lens barrel is shortened and the complementary element is mounted on the back? The following rough sketch shows the basic problem. In the standard configuration, the sensor is very close to the camera body. The free opening is not infinite with a technical camera. If the back part is shifted considerably, the camera body shades a part of the sensor. If the sensor is now placed further back, this shadowing is substantially reduced and the usage of the image circle is optimized for the BNF. In line with ALPA's system modularity, the first lenses, where constructionally possible, were shortened by 34 mm to SB34, since macro tubes of 34 mm in length were already available.
During research, the so-called "Short-Barrel" lenses of the Mamiya RZ67 were spotted. Back in their time, Mamiya also shortened the lens barrel of some lenses. However, only space was to be created for a shift/tilt module, which could be used optionally. ALPA also referred to the concept as "Short Barrel" as a reminiscence to this RZ solution. ALPA delivered the first SB34 lenses starting in mid-2007.
At Photokina 2008, ALPA also presented tilt/swing modules using the SB concept. From now on, ALPA users could very economically use a single tilt element for any SB34 lens and only had to purchase it once. Later, ALPA extended the group of SB lenses with shorter focal lengths to SB17. Here, too, the user could continue to use and combine existing elements, such as macro tubes. Today, 17mm macro elements and tilt/shift modules can be combined and used with great flexibility. The ALPA / Rodenstock HR Alpagon 6.3 / 138 mm planned for 2019 will be offered as SB51 (34+17 mm) following this philosophy.
The Exceptions - Good to Know
The very first short barrel lenses were the SB34, as the corresponding macro tube already existed. In 2004 ALPA introduced a complete set of extension tubes for macro photography. This set initially included tubes of 6.5 mm, 16 mm, 34 mm and 52 mm in length, which were optimally matched to the existing Schneider and Rodenstock helical mounts. The old versions are distinguished by long, SWA-like locking levers.
ALPA has subsequently successfully integrated tilt/swing modules of 34 mm with an inclination of up to 12° into the SB concept. Due to the success and other customer requests, ALPA then transferred the idea to further lenses. However, not all focal lengths allow a shortening of the tubes by 34 mm. It became evident that apart from a few very short focal length lenses with a small image circle (HR Alpagon 23, 28 and 35) an SB17 version was also possible. Thus, two 17 mm elements could be economically combined for the SB34 lens tubes.
This subsequently led to the standardization of the macro tubes to 6 mm (as the shortest, finishable extension and seamless connection to the extension of the standard helical gears, the 17 mm elements (2 x 17 mm = 34 mm) and now 51 mm tubes (3 x 17 mm or 1 x 34 mm plus 1 x 17 mm).
Important & Caveat
The older 16 mm tubes are therefore NOT fully compatible with the current SB concept, as they are one millimeter too short. Of course, they can still be used as extension tubes. The missing millimeter can also be compensated by an additional extension on the screw, whereby the distance scales no longer fit.