I strongly recommend you check out this article before reading to understand better how a speedbooster works : https://zebrazone.tv/what-is-a-speedbooster/
EF mount lenses roughly fall into two categories : Full Frame lenses and APS-C lenses. Full Frame lenses are designed for full frame sensors such as the 5D or 1DX. On Canon series you can spot these lenses with the red ring. On the contrary, APS-C lenses produce a smaller image circle designed for crop sensors such as the 7D or the T2i, very close to the super 35 format found into many professional cameras such as the URSA Mini.
Since Full Frame lenses produce a bigger image circle than APS-C lenses you will want to use :
- Speedbooster XL 0.64x for Full Frame lenses.
- Speedbooster Ultra 0.71x for APS-C lenses.
I’d recommend going for the Speedbooster Ultra since you can use both types of lenses and don’t have to worry about compatibility. Be aware that both cannot be used with the cheap EF-S canon mount.