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.
So if Speedbooster (SB) is a focal reducer then
where f is the focal length, and D is the diameter of the entrance pupil (effective aperture). It is customary to write f-numbers preceded by f/, which forms a mathematical expression of the entrance pupil diameter in terms of f and N. For example, if a lens's focal length is 10 mm and its entrance pupil diameter is 5 mm, the f-number is 2 and the aperture diameter is f/2
Does really work like that?