I assume you are talking about the lever which push the AEL/AFL key on the back of the E-M1 which is at the top rear of the right hand side of the housing. The behavior of this key can be customized. Press menu, scroll down to the gear symbol, select Gear A menu, and scroll again to AEL/AFL. Now you have up to 4 modes to determine what this key can do. In Modes 1 and 2, it locks the auto exposure. In mode 3, it activates S-AF if your AF is set to S-AF or MF. In mode 3 or 4, it starts the C-AF when your AF is in this mode.
When changing this setting, you have similar explanation displayed on the screen.
Suggest you check the user manual as there are some relationship with the trigger/pre-trigger behavior (check on page 98).
I confirm what DivingMeCrazy advise. I just made the test, where the underwater mode is assigned to either Fn key in one of MySet preset. The flash fires every time you trigger. When switching to another MySet where the underwater mode is not assigned, the flash doesn't fire anymore. The procedure to configure the E-M1 is provided in the Aquatica AE-M1 user guide (page 6).
Another (simpler ?) way to do so is to access the "Direct Control" menu by pressing "OK". Scroll vertically to reach the flash settings and you can choose between always on, red eye reduction, always off, 2nd curtain and manual value (info key + rear dial to change the value by 1/3 step from full to 1/64).
I didn't have the chance to make a side by side comparison. I have the Aquatica housing since a few weeks and had a chance do dive twice with it so far.
The housing is very well built. The commands are mostly very ergonomical. From your right hand, the trigger, the front dial, the video recording trigger and the AEL/AFL commands are very easily accessible ! The various keys on the rear are well located and well sized. The two other top dials (mode, rear dial), are a bit less comfortable to use ; you may decide to customize your E-M1 to put the function (speed, F) on the front top dial to prioritize its ease of access.
From you left hand, you mainly have direct access to the zoom/focus knob.
There are other access such as port release / lens release which do not require to open the housing if you just need to change the lens between two dives.
The FN3 and FN4 keys are not present on the Aquatica housing. I think the Nauticam one has on of the two, and the picture of the Subal makes me think it has both. Note that the Aquatica housing is built the same way as the olympus one, where the storbe has to remain in its low position. The Olympus procedure is also recommended by Aquatica ; it requires to assign the underwater function to any FN key which then becomes relatively useless. So you would ultimately loose only one FN key.
Aquatica comes in standard with moisture detection and low pressure monitoring inside the housing. You may decide to purchase the valve option, which allow to seal the housing before going to dive. The system works pretty well (I'm still paying attention to the battery life). Note that you need to use the pump both for making vacuum and removing it. The valve system is pretty tiny and doesn't affect the housing handling.
The housing handles are the classic Aquatica ones, so they are very ergonomical.
The port chart is already pretty extensive. I have the dome port for the Pana 8mm fisheye, which is very good. I also have the flat port for the Oly 60mm macro which can also work with the Oly 12-50 kit zoom. By the way the 12-50 zoom ring is less expensive than the Nauticam one, however, there is no way to switch to motor zoom nor macro mode. The flat port is equiped with an M67 thread allowing to add close-up wet lens. I wanted to have a flip holder ; Aquatica doesn't have any for this port ; so I took one from sagadive. It is find but add strong vigneting when using with the 12-50 zoom (I can't go wider than 18mm or so)
Aquatica has also a single dome port for wide angle non fish-eye lens (SW8) with different extension rings, in order to fit the mostly used wide angle lens (oly 9-18, pana 7-14 ...). Shame that this approach was not the same for macro ports (one port for the pana 45mm, another one for the oly 60mm)
From a weight point of view, the system is not so light and require floats if you want it to be neutral. I currently have 4 jumbo stix floats, which is not enough in salt water. From a size point of view, the system is more compact than a DSLR (even APSC/DX) system, but not that much. I would personnally evaluate it at about 15% (I didn't do the math)
For the strobe sync, the housing can get S&S optical connectors directly. The housing is shipped with two adaptators for direct fiber adaptations.
For the arms, you have various options :
- 2 1/4"-20 threads on top of each handle
- 1 1/4"-20 thread on top of the housing (above the strobe). May be convenient position for the focus light.
- 3 1/4"-20 threads on the bottom of the housing. Convenient for a 1 ball mount or to install a tripod. I wanted to put a T-plate base here but the distance between the wholes do not match the ones of the T-plates
From a pricing point of view, my expected system was pretty much the same price with Nauticam as with Aquatica.