The Dallmeyer Octac was one of a number of Oscilloscope lenses made by the company. Others that I know of include the Oscillograph Anastigmats: a 51mm f1.5, 75mm f2.8 and 75mm f3.5, and an Oscilac 51mm f1.0. All these lenses are designed for recording traces on Oscilloscope CRT tubes, and thus are optimized for close distances. In addition they all tend to be quite fast, in order to capture the faint traces at decently-high shutter speeds. As far as I know all of them are far from flat field, being designed with negative field curvature in order to keep all parts of the curved tube surface in focus.
This particular lens arrived to me incomplete–only the front element group was present. I was about to give up on it, when I hit upon an idea: I had sitting in storage an old Ilex Oscillo-Paragon 56mm f1.2 lens. It was something that I had picked up cheap for experimentation. It had proved to be too difficult to use, because it did not achieve focus past a few centimeters. However the design was roughly the same as that of the Octac: two relatively symmetrical element groups with a shutter between them. I wondered what would happen if I combined the front Octac group with the rear group of the Oscillo-Paragon. What would I get?
The answer can be seen below. It has, interestingly enough, remained the same focal length and aperture as the Oscillo-Paragon: ~56mm and f1.2. But not only does it now focus to several meters (and could even focus to infinity if I wanted it to), it has a very unique character unlike any lens I have seen.
It is not easy to use–very low contrast and a lot of highlight blooming, but under the right circumstances I find the rendering both unique and pleasing. See what you think 🙂