I am sorry if this a stupid question but I have to ask in order to learn.
Is possible such a combination?: A Canon crop camera with a (I think not existing)speedbooster and any EF lens in order to gather the "extra" light the lens is providing and the sensor, due to its smaller surface, is not able to use. Would it improve APS-C sensor ISO performance?
Thanks in advance and sorry if it is a completly dumb question.
Speedbooster is basically a focal reducer. IMHO, it's had nothing to do with sensor size as long as the original lens produce an image circle that can cover the sensor, but focal reducer will usually reduce the distance where the image is focus on, so it's difficult (if possible) to maintain flange focal distance of any lens and mount of the same type. That's why speedbooster only appear for SLR lens to mirrorless mount, so speedbooster for EF lens to EF mount might not be possible, but speedbooster for EF lens to EF-M mount is possible.
Have a nice day.
Actually, it's a question of a physical entity (set of lenses) being interposed between the rearmost lens element and the camera mount. It might not be impossible with the Canon telephoto lenses that can take a teleconverter (and consequently have free space behind the rearmost lens element). The Speedbooster is essentially the exact opposite of a teleconverter.
To answer a question the OP had asked, yes- it does increase the amount of light. The Metabones one increases the equivalent angle of view by .71x and the light by 1 stop IIRC. And I am probably generalizing- these numbers might be specific to FF>APS-C. Too late for me to think it out