Olympus E-M1 with 8mm fisheye lens

[CGreathe 0]

Ive been using my Olympus e-m1 with the Olympus f1.8 8mm fisheye lens in a F.I.T glass dome port without a 20mm extension ring behind it, but recently chatted to someone who recommended putting the 20mm extension ring on (nauticam housing). Any other thoughts on this from anyone?? Is there a benefit?