I'm always confused when you only use the f/ratio to explain things...
It's strange that nobody here do the difference between the F stop wich is the focal ratio, and the T stop wich is the "speed" = light quantity ratio...
Of course those 2 ratio are linked by the aperture of the lens, but the number of transparent elements and the matter used is influencing the ratio of light transmitted to the sensor.
For example : saying that FF @f/2.8 become MFT @f/5.6 is wrong for me, because it's about light quantity. Focal ratio doesn't change, the focal distance is the same, and the blur generated is same. But the light quantity, the T stop, is divided by 2, wich is impacting to the iso/exposure/aperture choice. => [email protected]
/2 become a [email protected]
/4, i prefer to say that.
It's the same with the speed booster. It's about "speed" / light quantity. You concentrate the light, like a magnifying glass, you gain "speed" / light ratio.
By concentrating the lightning, you also reduce the image projected on your sensor. That mean a wider focal, a smaller image which suits better on a smaller sensor, the focal ratio is the same, but the speed ratio is boosted.