Cousin David: Your flash photography of David (my son) inspired me to do my own experiments. I used a hallway so I have left wall, right wall, and ceiling as reflector. The experiment spreadsheet is attached and the results are posted. I used evaluative metering for camera and ETTL for flash. I did not try manual exposure and flash setting.
No diffuser, you can see how the light hits the subject but fails to light the background. It is almost like a spot light. The built in diffuser gives the most even light. The add on diffuser is an intermediate light. It probably has the most pleasing blend of shadow to give texture to the subject. At the end it is still about what is pleasing to the photographer’s eye. The exposure in each instance is not wrong. So it’s a matter of taste. Once again let me thank cousin David for the tremendous work that went into carefully changing parameters and documenting the results. Great work!!