This is an elementary question that I should really know the answer to but I do not completely. I have the Nauticam 4.33 dome which is used with the Sony S6XXX series (and others) with the Sony 16mm pancake and with both the wide angle and the fisheye snap on converters. But I also have a Rokinon 10mm fisheye lens. It produces cleaner images for surface photography but it is manual focus and manual focus aperture. I would preset probably f11. But I would either need to have a gear printed for focus or preset focus. Domes produce a virtual image, can I preset focus to that virtual image and then not really need to worry about focus? If distant or near dome objects are not clearly focused it really does not matter perhaps for CFWA and fisheye shots? 

 

I once had an Ikelite bubble housing with a Nikon FM camera. I recall presetting focus and aperture but for the life of me I have no idea where the slides are from some of those shots nor do I recall if it was okay or not. I do not remeber.

Yes you could preset when focusing very close and it works quite OK on land, but as you focus very close with the lens it becomes more sensitive and you can't use hyperfocal distance like you can on land.  In CFWA you want to focus on one of the closest objects as more of your depth of field is behind the focal point.  If you don't you might find parts of the nearest object are out of focus.

 

The other thing to consider is with CFWA you may need to change distance a little due to obstructions or the size of your subject, which could be annoying.  Long story short if you are going to use a MF fisheye lens get a focus gear.  You'll probably need focus peaking to help you focus.

I have rigged a 7Artisans 7.5mm manual fisheye with a focus gear and tried using it underwater, with a SeaFrogs 4-inch dome port, but found it more trouble than it's worth. As Chris says, behind a dome, it is very sensitive to focusing, so fixed-focus is not feasible, and even with a focusing gear and focus peaking, it's still way too much of a hassle to manage.