I have been wanting something like this for ages (wide angle converter that's behind the lens)

Russell, if you can believe that a 1.4x rear tele converter will lose you a stop of light and there is no one arguing that point. you then also have to believe a 0.71x rear wide angle converter will gain you a stop of light. a 35mm F2.0 on FF will give you x amount of light over a given area, that same 35mm F2.0 on APS-C will also give you that same x amount of light over a smaller area. the density of light hasn't changed. however when you take x amount of light on a FF sensor and compress it into a smaller area such as an APS-C sensor you're now all of a sudden increasing the density of that light by a factor of 1.5x (lets say 1.4x as thats what the adapter claims) essentially you're adding a whole stop of light over a given area. and that 35mm F2.0 becomes a 24mm F1.4 on an APS-C with a speed booster.

--
Mike from Canada
"I am not a great photographer! God is a great creator! All I do is capture His creation with the tools He has provided me."
'I like to think so far outside the box that it would require a telephoto lens just to see the box!' ~ 'My Quote :)'
http://www.michaelfastphotography.com/galleries/VP-BDI_3a.jpg
http://www.airliners.net/search/photo.search?sort_order=views%20DESC&first_this_page=0&page_limit=180&&emailsearch=mighty_mike88%40hotmail.com&thumbnails=