Birds of the sea: Fergus Kennedy on filming reef manta rays

First the team had to find manta rays, which took their guide some time to track down – just as it had taken Fergus time to find Komodo dragons on the nearby island of Rinca. Here, battling strong currents, they needed to cram four dives into each day to make the most of their time.

As for lenses, Fergus says: "When underwater I tend to either shoot really wide on a fisheye lens (such as the Canon EF 8-15mm f/4L Fisheye USM or the Canon EF 11-24mm f/4L USM, which are both really good underwater lenses), or I shoot macro with the Canon EF 100mm f/2.8L Macro IS USM. I have an underwater housing with a 9-inch (23cm) glass dome port on the front. On the dives where I was shooting video I had two 10,000 lumen LED video lights."

For his macro setup he switched to using a flat port (which enables more accurate focusing than a dome port) on the front of the lens. For some of the video shooting, Fergus also deployed a small underwater HDMI monitor to allow him to get a better view of what he was capturing.

Overcoming underwater filming challenges

Underwater, the main challenge was the changing light conditions at different depths. "White balance was a big issue because the colour of the light changes so drastically. As you go deeper, the light gets bluer and all the other colours get filtered out by the seawater. So I put a little bit of orange filter gel, called a magic filter, into the filter holder on the back of the lens. That gave some colour correction and effectively filtered out that really strong blue."

Fergus also recommends setting a custom white balance every time you change depth on underwater shoots, and resetting colour balance when necessary, to overcome colour casts.

On one of the last days of the shoot, the team got up close to reef manta rays in fairly shallow waters. Fergus notes: "We shot everything in Canon Log mode, which was good in an underwater environment because you do often – particularly in reasonably shallow water – have quite bright sunlight coming through the surface in the top quarter or third of the frame. The seabed is often quite a bit darker, so there's a big dynamic range, and C-Log [with its maximum dynamic range of 12 stops] definitely helps with that."

Fergus says another advantage of using the Canon EOS 5D Mark IV for shooting 4K video underwater was the crop factor. "The Canon EF 11-24mm f/4L USM lens is super wide in the first place. Once you're shooting 4K video, the 1.7x crop turns it into a useful zoom for underwater – it's kind of 17mm at the wide end, making it effectively more like a 17-40mm [lens].

"It's really versatile – you can shoot wide, scenic shots of coral reefs or the manta rays, which are big and come quite close to you, but then you can zoom in to get detailed shots of fish or corals at the tighter end of the zoom."

Fergus and the crew were living and sleeping on a boat throughout the reef manta rays shoot, so they managed to grab some editing time at the end of each shooting day. "We had mains power so we ran the laptops and did some editing," he remembers. "It was great to just be able to load the card onto the computer, grade things straightaway and check things were looking the way we hoped they would.

"There wasn't much downtime – we were either diving or editing – and you spend so much more time fiddling around with gear when you're doing underwater stuff because even something [that's relatively simple on dry land], such as changing lenses, involves more effort than normal. You have to change the port and gears in order to operate the zoom and the focus through the housing."

Despite having fulfilled a life-long ambition to film manta rays, Fergus still wants to return to shoot even more footage of one of the ocean's most graceful creatures. "I always want slightly better light or slightly calmer conditions," he says. "It's part of the photography addiction – I always want to come back and do it again."