Hi Nick. I’d be a little wary of the G7X if I were you, it does not behave like most other compacts so needs some consideration. If you are thinking of going wide angle, the G7X is particular in that its lens is shorter when at its widest. The result of this is that if you screw a wide angle lens to the front of the standard port, you are left with a large air gap between the camera lens and the port glass with a result that the corners of the image become very soft and out of focus with heavy vignetting around the borders of the image. The alternative is to zoom in which then means you lose much of the benefit of the wide angle lens.
Nauticam have overcome this issue by offering interchangeable ports for this camera, a shorter port for wide angle and a longer port for macro. When you have the short port installed, you cannot zoom in beyond about 40mm as the lens extends and hits the glass. This does mean that you lose flexibility that you would normally expect with a compact camera but you do end up with good image quality for both wide angle and macro. As far as I’m aware, other manufacturers do not offer a similar solution.
We have sent a setup to a French magazine, Chercheurs d’Eau, who are currently running tests with the Nauticam, Isotta, Recsea and Ikelite housings for G7X with a variety of wide angle lenses so this will be worth watching.
If you are looking for flexibility to change from wide to macro underwater, I’d still personally think the Sony RX100 Mk2 is the winning choice for underwater. This camera though does not perform well underwater for white balancing and the flash recycle time tends to be longer than the Canon.
I’ve just come back from an Alex Mustard workshop in Raja Ampat where I was predominately using a 140mm minidome on the D750. I was using it with the Sigma 15mm and sometimes with the 1.4x Kenko teleconvertor. I have to say that it has changed the way I think about FX underwater as I love the practicality of minidomes but did not want to compromise on quality. I am very pleased with the corner sharpness of the 140mm even up to F7.1 with the Sigma 15mm. Here are some images taken with the minidome.
1. D750, Sigma 15mm, F9, 1/60, ISO 500, 140mm minidome
2. D750, Sigma 15mm, F11, 1/60, ISO 500, 140mm minidome
3. On peut s’approcher très proche au subjet avec le minidome
D750, Sigma 15mm, F11, 1/125, ISO 320, 140mm minidome
With the 7D, I’d turn off the half-shutter press off so you are focusing on the rear button, find a suitable subject, no current or swell, and a place where you can put a sand spike to rest the housing on. Use the rear lever to focus to the magnification you want approx then release. Using the sand spike, gradually ease the housing forwards and backwards until you see the subject in focus and then pull the shutter. Take lots of shots as the DoF is very minimal especially as you increase to 1:1 on the lens. Think about the out of focus areas and how they will appear in the image. Breathe and work within depths that you are very comfortable with. Spend 30 minutes with a subject.