I have done a few blackwater dives and really enjoy them.  I will be spending a couple weeks in the Philippines early next year and am looking forward to doing several more.  

Most recently I have been using the Nikkor Z105 on a Z8, with the MFO-3.   I recently re-watched an interview with Mike Bartick from a year ago, in which he enthused over the AF-S 60mm on a Z8 for blackwater.   I have been tempted to give that a try, since I already have the lens, but it means taking not only the lens but an FTZ and I will be taking the 105 and MFO-3 either way.  Not a big deal, but I am always dealing with weight and size limits on the airlines.

I am guessing no one has really done any comparison between the 60 and the 105/MFO-3 on a mirrorless body, but if so, I would be interested in hearing about it.  My guess is that there is no benefit to the 60mm in this situation.  If I can meet weight and size limits,  I might just take both lenses and experiment, but as I said, I would enjoy hearing about others' experience.

I have penned a hands on review of a MFO-3 using a Nikon Z8 and Z105 which included some blackwater shooting - https://www.underwaterjournal.com/nauticam-mfo-3/

Currently, I no longer see the point of shooting with a 60mm Macro anymore.

I have a different take. The 60mm setup is still compact, but adding the MFO-3 to the 105mm brings the total length to around 200mm. That's probably too long and heavy for some divers.

One of the claimed advantages of the MFO3 is that is corrects for the distortion and abberations caused by the flat port, these abberations are more of a concern for shorter focal lengths/wider fields like you get with a bare 60mm lens. So it should be sharper than a bare 60mm. The MFO3 is fairly hefty however the in-water weight is quite low and it balances quite well in use.

34 minutes ago, Chris Ross said:

One of the claimed advantages of the MFO3 is that is corrects for the distortion and abberations caused by the flat port, these abberations are more of a concern for shorter focal lengths/wider fields like you get with a bare 60mm lens. So it should be sharper than a bare 60mm. The MFO3 is fairly hefty however the in-water weight is quite low and it balances quite well in use.

Chris, you’re using it on an M43 system with the Zuiko 60mm, right?

Thank you for the article, Walt. Nicely done, by the way. Pretty much confirms my expectations. Jyuny1, the 105 and MFO-3 is long, but I have not found that to be a problem, and I use it extensively on daytime macro dives to have that "2 lenses in 1" benefit. Interestingly, Alex Mustard has expressed a preference for using a 90 or 100 on blackwater, but I found that focal length difficult to use in that setting and pretty much left the MFO-3 in place on previous blackwater dives.

If I lived near dive sites and could dive more often I would experiment more, but with the cost and time constraints of being a long way from dive locations, I have to rely a lot on others' experiments and experience when I get curious.