I think this is a bit an issue of opinions as to what is scientifically correct and what is "practically accepted reality". Focal lenght is fixed, it doesn't change depending on the camera. So 28mm is 28mm no matter if 35mm, APS-C or M43 sensor, but the crop of the image circle that is used obviously changes, so you will change the distance to compensate for this. Theory goes on and on but.... The practical answer you want: 28mm on M43 has the field of view like a 56mm on 35mm/FF. So the Speedbooster uses whatever modification, in your case 0.72, which will give you what a 41mm lens would frame on a FF sensor. 28mm x 2 x 0.72 = 40.32mm EDIT: yeah, what dhessel said