Just to add more info on the original #3 comment \" A thick diffuser will make the flash duration longer and this makes the risk of glare higher.\"
Some shots below using styrofoam diffuser of increasing thickness. You will just see as the diffusion increases by adding more layers from none intially, the contrast decreases and bright spots spread giving a more even light. The light loss is approx 1 stop per 3 layers, so the flash duration in the last shot was approx 4 X longer than the first shot.
