Craig Jones sent me a few questions about manual focus for macro ports. The airlines lost his gear (what a bummer!), so he's building his underwater camera system up from scratch. He's used both Canon and Nikon systems, but he really likes the Nikon 70-180mm zoom macro, which is why he's leaning to Nikon (though he seems to like Canon AF better).
His question was about macro ports...specifically, if I have an opinion about how the Inon MRS and Nexus macro port systems compare. Craig seems to have done a lot of homework about the pros/ cons of both.
Some basic background if you're not familiar with the systems: The Nexus macro system uses gears you attach to the lens and manipulate via knobs on the housing or port, depending upon the lens combination. The Inon system uses a magnetic ring outside the port, with magnetic collars you put on the lens, so you rotate the magnetic ring just like you would rotate the focus ring on a lens on land. These considerations only matter if you're using manual focus.
I've used both systems, and both work well. There are, of course, differences.
Of all the MF macro systems I've tried, the Nexus system has the best feel. It's difficult to quantify, but there's something about the Nexus manual focus gears that makes it possible literally to feel the adjustments you're making, almost as well as you could if you were holding the actual lens. The man behind the Nexus housings is a former race car driver and is a stickler for detail, so that may contribute to the quality of the gears.
The drawback is that with precision gears, you need to line everything up perfectly before you go into the water. If the gears are slightly off, they may not work. If you go for super-macro configurations (greater than 1:1), it can sometimes be difficult to get everything to line up properly until you build up some experience setting up the system.
Once you know what to do though, the gears are smooth as silk.
The Inon system takes an entirely different approach. You put magnetic collars around your lens, directly on the focus ring. When you place your camera in the housing/ port, you use a magnetic ring outside the port to control the magnets on your lens inside the port.
There are a few advantages to this system. First, you have fewer holes in your housing (no knob or gear penetration) and for anyone who uses manual focus on land, it's a familiar concept...rotate the collar to focus the lens. Also, the magnets are fairly strong, so even if you're a bit off in placement, the system will work. Finally, if you're trying super-macro, it's much easier to line everything up and you don't have to build custom gears for different configurations.
The drawback is that the magnets in the external magnetic collar are built to fail. I'm sure they weren't designed with this intent, but from day one out of the factory, the external collar is in a gradual process of deterioration that will cause the magnets embedded in the collar to break free of their enclosure and lock up the system. It's only a matter of time. I wrote about this problem earlier when it happened to me.
Of course, this decay doesn't happen overnight, and you can prolong the life of your magnetic focus ring by removing it from the port as soon as you get the system. If the ring is stored on the port (i.e., the way it's shipped), the built-in obsolence accelerates.
I use this system now because it fits the housings I have now. Despite the obsolence issue, the system works well. It doesn't "feel" as smooth or fine as the Nexus system, but it gets the job done. There's a bit of "crunchiness" or granularity to the rotation of the ring, such that you might find it difficult to make fine adjustments if you're accustomed to a gear system like Nexus. This is probably due to the use of intermittently spaced magnets (instead of fine-tooth gears).
Both systems work though, so the bottom line is that if you need or want to use MF and are debating these two systems, you should pick whichever system goes with your preferred housing and camera/ lens system, and whichever system you personally have an easier time handling (try them on land first if you can). Neither system is perfect, but then again, few things in life are.