Every now and then I come across some photographic accessory that I've never come across before. This Sigma Telemac Vario 2x Auto Converter is in this category. Sigma Telemac Basically, it seems to be a teleconverter, in this instance in M42 mount, that enables variable conversion ratios from 2x to 3x. It has a base unit with several elements, the rear element moving in a helicoid between stages marked 2x, 2.5x and 3x. Two extension rings are supplied, one marked 2.5x and the other 3x. In practice, if one requires 2x multiplication of focal length you mount the lens on one end of the converter and attach the other end to the camera, with the helicoid set to 2x. If you want 2.5x, you remove the converter from the camera, add the ring with the 2.5x marking and reattach, lining up the 2.5 index mark. It's the same procedure for 3x, first removing the 2.5x ring and replacing it with the 3x. Here's a pic of the thing attached to a camera with the 3x ring fitted, with the 3x mark on the teleconverter unit revolved to match the index mark on the ring. Mounted I've not come across a teleconverter of this design before; it seems to be rather rare, though I'm sure there must be other similar ones out there. I tested it on a Sony full-frame camera using a 35mm Super-Takumar f/2.8 lens, fully expecting a dire loss of image quality, but was quite pleasantly surprised. There was a very slight loss of sharpness, but none of the dreadful CA's small teleconverters usually produce. Here are two images, the first one full-frame with the 3x ring fitted, and the second a crop from the centre of the image. Full Frame Crop If anyone out there is acquainted with this or other similar teleconverters, please add your comments.