I just received my AOI housing for an OM-1 and the 8 in. glass port.  I went with the 8-inch port because I do like over and under compositions, but now I'm wondering if it is just too big. From what I understand, I don't lose image quality by going with the 4-inch port.  Would there be any other reason other than over and under shots to choose the 8 inch port over the 4 in.? Can I focus closer with the 8 in.?  I'm coming from an Olympus housing with the Olympus glass port, which isn't nearly as large.  I have to decide whether having more drag with the large port is worth it for the options it gives me.  Yes, if I had more experience, the decision would be more clear.  I will use the setup in Indonesia and closer to home in the Pacific Northwest. Any advice or personal experience would be appreciated. 

on m43 a 100-140mm port is fine and larger gives no real advantages.  The smaller ports allow you to get closer to your subject which translates into a larger subject size.  It doesn't seem like much but a small change in subject distance makes a big difference.  You can try this out with the lens on land.

I agree with what you say Ross, but while I have no personal experience with O/U shots I am always reading that the larger the dome the easier the O/U shot is to make successful. And I think that is what the OP wants the dome for.  Could be wrong as I have no practical experience.

12 minutes ago, SFEgr said:

I agree with what you say Ross, but while I have no personal experience with O/U shots I am always reading that the larger the dome the easier the O/U shot is to make successful. And I think that is what the OP wants the dome for.  Could be wrong as I have no practical experience.

This is correct, I believe the OP is already aware that the the larger port has an advantage for over - under shots.  If you are not planning over unders, the small ports are fine for the fisheye lens.

I have zen 170mm with oly 8mm and it is very versatile ok for split shots and small focus distance