Dave,
A lot of these short lenses are actually retrofocus designs. That's why you are seeing the longer barrel. I'm shooting with a Rodenstock 35mm HR and that is a retrofocus design as well. No center filter is required and the flange focal distance is much higher that that for the regular Rodenstock 35mm digital.
Larry
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Strictly speaking, it is a mistake to call the Rodenstock 35mm HR lens a “retrofocus design” although I can understand the confusion on this issue as some older Rodenstock sales literature makes this mistake in describing this very lens. Newer literature from Rodenstock does not make the mistake. (In my experience, German optical guys may not like to admit mistakes but they are quick to correct them!)
A retrofocus lens design, also known as a reversed telephoto design, is commonly defined by optical engineers as a lens that has a back focal length that is longer than the effective focal length. The back focal length (BFL) is the distance from the rear lens surface to the infinity focus image plane. Retrofocus designs are commonly used when a large clearance from lens to image sensor is required relative to the focal length. In other words: wide angle lenses on reflex cameras to provide necessary mirror clearance. Like the 28mm Mamiya with 63.3mm needed for bayonet mount flange to sensor and something only slightly less needed for mirror clearance.
Rodenstock literature for the 35mm HR shows that the lens housing to infinity image distance is 23.8mm which is considerably less than the 35mm effective focal length. Lens housing to image is not much different than the lens surface to image distance for this lens (strict definition of BFL).
All the Rodenstock Digital HR lenses I have seen data on are not retrofocus designs. However the Rodenstock designers have designed the lenses to reduce the angle of rays hitting the sensor at the edge of the field for the purpose of reducing sensor lens cast correction (LCC) problems and to reduce vignetting. See the Rodenstock lens diagram attached for the 35HR vs the much older design 35mm Apo-Grandagon (designed for film). The imaging ray angle at the edge of the field is reduced by optimizing the lens design to put the exit pupil of the lens as far as possible from the image plane. Rodenstock or Schneider do not provide specific locations of the exit pupils for their lenses unfortunately or it would possible to compare image ray angles at the edge of the field.
Of the Schneider Digitar lens series the only one that appears to be a true retrofocus design is the Digitar 28mm L. It has 49.3mm distance from rear of the lens cell to sensor which is much greater than the 28mm focal length.
Steve
