Someone was asking about this adaptor the other day. Here is the official brochure for it.

I have to admit that unlike Thierry I did not notice any light loss when using this adaptor.

http://www.sonybiz.net/images/product/X/XDCAM-HDgc(brch).pdf

The one I saw was optically clear so I wouldn't expect much light loss. The only guaranteed loss is the angle of view.

And here's the Canon version, the LO-32BMT. It states there will be no change in f/stop when using it. The download link is http://www.usa.canon.com/html/industrial_bctv/pdf/LO-32BMT_Page.pdf