I contacted Soligor about their 2x tele-converters as I noticed that many firms have released new converters and that some (Soligor included) have special converters for the digital Canon EOS series. They answered (quickly, I may add 
) that the 2x convertors are only coated from one side but that because the sensors in digital cameras reflect light, they require double coating. There are two new issues to me in this:
- The sensors reflect light? Does this disturb the picture with any lens when using a slow shutter speed?
- What kind of coating is used for converters (or lenses in general) and what is it used for? (obviously it has to do with reflection, but how/what?)
Get it here