I have loved using Back Button Focus (BBF) for photographing wildlife on land and in the water I shoot with a Canon 5D markIV and nauticam housing. It seems like BBF would have many of the same advantages for underwater use, including that if I'm shooting during the day and then it becomes evening past sunset, my camera doesn't have enough light to focus and will refuse to take images (I don't want to carry a focus light). With BBF I would never have that issue, I could set the focus manually with light from a strobe in video mode and never worry about it again. However, when I setup my 5D for BFF and put it in my housing, trying to reach the AF-on Button and the shutter at the same time really strains my hands and there's no way I would want to do that for 5+ hours straight.

I could modify the lever on the outside of my housing to be more ergonomic and reach close to my shutter, see about reassinging it to another button, change the AF settings underwater, or just go back to having the shutter half press be the focus. How many folks are using BBF and does anyone have any suggested work arounds for the placement of the shutter and the AF-on button? Rassinging the AF to the * button would be even more challanging

Here's a pic of me trying to hit the shutter with my pointer finger and AF-on button with my thumb at the same time. I basically can't grip the camera and can barely reach both:

The standard approach to deal with this as I recall is adjusting the handle in and out to get to a comfortable setting. I don't have this particular housing to judge but the way you are holding it seems odd, probably just the way it has been photographed, typically I wrap by 3 non-index fingers around the handle, extend the index finger for the shutter lever and the BBF lever falls under my thumb.

It looks like your palm is well back from the handle, mine is in tight contact- when holding for shooting. My hands aren't as supple as they once were so I vary my grip and periodically hold with my left hand while stretching my right and adopt a looser hold when travelling as opposed to shooting.

To be it looks like if you gripped tighter with the palm tight to the handle you could reach more easily. Combine that a looser hold and resting stretching your right hand when not shooting would be my initial thoughts. Experiment with moving the handle in and out as well if our housing accommodates this.

27 minutes ago, Chris Ross said:

typically I wrap by 3 non-index fingers around the handle, extend the index finger for the shutter lever and the BBF lever falls under my thumb.

I have the 7dmkii in a nauticam housing and this above is pretty much how I hold it, too. I have bad hands, size XS gloves, and find I can hold steady, reach the buttons and shoot without issue doing this. Also as Chris says, if I'm not actively shooting, I hold the housing a variety of ways depending what's comfy in the moment.

good luck!

Wait wait wait. Maybe I'm wrong

Did you set the BBF to act as one single button press?

I'm under the impression that you need to keep rhe BBF pressed to maintain focus. Hence your problem.

I'm asking because on my Panasonic camera there's an option for this: focus lock works only until the button is pressed. On my previous camera it was the default and I changed it immediately.