Underwater Cinematography – 8K and Beyond
by Pawel Achtel ACS, Director, Achtel Pty Limited
Compared to filming on land, filming underwater always poses challenges. But it is not just about keeping the camera dry. The most challenging aspect of filming underwater is the optics. It is the ultimate factor limiting image quality when filming below the waves. Much of the underwater content that is claimed to be 4K or 8K is limited to approximately high-definition quality or less by its optics, even though it may be recorded on an 8K-capable camera. Fortunately, 8K-capable underwater solutions are now available – solutions used in Avatar: The Way of Water.
For decades the state-of-the-art was to place traditional lenses designed for filming on land behind flat or dome ports. While this appears to be a reasonable solution, it became apparent that aberrations, image plane curvature, and distortions caused by those ports severely limit the image quality achieved when using housed lenses. The answer is a submersible lens.
What is Submersible Lens?
A submersible lens is a lens that has been specifically designed to take sharp pictures underwater. It doesn’t produce sharp images on land. Its optical design incorporates a water medium as part of its optical formulation.
Submersible lenses were pioneered by Nikon about half a century ago. They were branded Nikonos lenses to offer a high-quality solution for range-finder and DSLR underwater still picture cameras. With the arrival of digital cameras, Nikon abandoned its underwater offering. Nikonos lenses, to this day, offer the highest image quality (by far) when it comes to underwater photography.
In the test images below, we look at 1:1 frame crops from a 5K image captured with a flat port, dome port, and Nikon Nikonos 15mm submersible lens.
Dome ports, used for wide-angle lenses, produce image plane curvature combined with astigmatism when placed underwater, causing extensive blurring of images. Dome ports preserve the angle of view (no image magnification) nor significant geometric distortions. However, they suffer from a loss of sharpness due to image plane curvature (the image is not focused on a flat image plane, but rather on a curved surface) and relatively poor optical performance from the taking lens that needs to be focused on a very close virtual image. Most wide-angle land lenses are not optimized for close-focus performance and usually deliver compromised image quality. In ideal conditions, such systems are limited to approximately 2K equivalent sharpness (about 1000 line pairs per picture width). This limitation falls far short of today’s high-resolution camera standards.
Flat ports perform even worse. While having negligible effect out of the water, a flat port produces severe chromatic aberrations (rainbow-colored edges), pin-cushion distortions, astigmatism, loss of sharpness away from the center of the frame, and (often undesirable) image magnification. An example of a flat port can be a dive mask. For wide-angle imaging, a flat port limits image sharpness to approximately 1K (standard definition) quality.
Submersible lenses, such as Nikonos 15mm, are specifically designed to take sharp images underwater and do not suffer from any of the abovementioned problems. Typically submersible lenses can reproduce one or two orders of magnitude more detail and sharpness than housed land lenses. Their small size also allows side-by-side and beam-splitter configurations for stereoscopic 3D.
The optical performance of Figure 2 was measured using Spatial Frequency Response (SFR) process and MTF function in Imatest software (Figure 3).
The MTF (Modulation Transfer Function) graph shows the sharpness (contrast) of a “cherry-picked” modified Nikonos 15mm f/2.8 submersible lens on a 9.5K digital sensor.
The MTF graph shows contrast (Y axis) versus spatial frequency (detail). It shows that this lens exhibits exceptionally high contrast up to the resolving limit of the sensor (Nyquist limit at 0.5 cycles per pixel) and even beyond, suggesting that the combined MTF is probably limited by the (9.5K) sensor and not by the optics at the maximum aperture. While stopping down, the lens’s optical performance (contrast) is expected to improve even further.
Such outstanding optical performance is impossible with a housed lens behind a dome or flat optical port.
UNDERWATER STEREOSCOPIC 3D
Some content creators, like James Cameron, wish to capture images in a stereoscopic 3D format.
Like on land, there are two main camera configurations for filming stereoscopic 3D underwater. The Side-by-Side camera configuration (Figure 4) has the benefits of simplicity, small size, and no light loss. The drawback is that this configuration has fixed interaxial distance (stereo base), limiting its use to filming bigger subjects from some distance away to avoid excessive horizontal parallax.
With the Beam-splitter configuration, one camera films through a half-mirror, and the second camera reflects from the half-mirror (Figure 5). This configuration benefits from an adjustable interaxial distance allowing closeups and working with subjects of any size. The drawback is the device’s size, complexity, and, until recently, beam-splitter configurations have been only available for housed (flat port) designs, dramatically reducing image quality, limiting the angle of view, and introducing undesirable vertical disparities resulting in 3D that creates discomfort and eye strain when watching on the screen.
ACHTEL DeepX 3D Submersible Beam-splitter
DeepX 3D is the world’s first patented camera system designed to capture stereoscopic 3D images better underwater using submersible lenses. Unlike other underwater 3D beam-splitter systems that house a beam-splitter behind a flat port, DeepX 3D is wholly submerged in water. Such extreme design allows for a wide angle of view, no geometric distortions, and no chromatic aberrations associated with traditional approaches. It improves sharpness from corner to corner. Jim Cameron’s team in Avatar: The Way of Water used this patented system for the first time.
DeepX 3D allows underwater immersive, high-resolution stereoscopic 3D images with more detail than possible with traditional housed systems.
Figure 6 shows the modified and “cherry-picked” Nikonos 15mm lenses used on Avatar 2: The Way of Water (serial numbers #216940 and #202669). These lenses are the actual ones used on the DeepX 3D Beam Splitter system when filming underwater sequences by James Cameron’s underwater team. The lenses were carefully selected among over 100 to ensure consistency and sharpness across the frame.
[Note from 8K Monitor, we’re genuinely excited to see 8K finally coming into its own in another domain and are eager to see the new Cameron movie. For more information about underwater 3D capture, check out this podcast with Pawel Achtel.]
I had no idea Nikonos 15mm lenses were used in rhe Avatar. A great build is timeless!