While the speedboosters have been out for quite a while now, I still regularly see people expressing confusion as to how they work in making the lens wider. Maybe this could be a useful reference point... Today I was enjoying reading the Google Translate version of an article by Marco Cavina entitled (in English): "A Tribute to the Legendary Immortal Kubrick and the Planar 50mm f/0.7." It's an excellent article, as all of Cavina's are. But this image struck me as particularly useful in helping to convey how the Speedbooster works. The famous Planar 50mm f/.7, also used a "speedbooster" in its design, though the technical term is converging lens group. So here it is: How a Speedbooster/converging group functions: It's clear here that the converging group has the opposite function as a teleconverter. Beyond that, the article itself in its google translate form is still quite engaging, if you have the time.