Underwater Optics by Pawel Achtel, July 2002
- General Characteristics of Underwater Cinematography
- Filtration of Red Spectrum
Water is a very effective colour filter of about 0.12 red. This means that you lose 1 stop of red for every 2.5 m. Consequently, if not corrected, images underwater look blue or green.
To correct the colour balance colour filters and artificial light must be used. When ambient light level is low it is easier to bring full colour spectrum using underwater light. Conversely, powerfull lighting must be used to override ambient light in clear water on a sunny day.
Even if powerful artificial lighting is used, still colour andjustment is necessary. If the lighting is mounted on the camera you lose 1 full stop of red with only 1.25 m from the subject. This is because light travels from camera to the subject and back.
- Water Clarity
Sea water contains many different particles. These are small organisms, silt, air bubbles and salt solutions of different densities. All of them can greatly affect water clarity and impact the image quality.
The best way to overcome poor water clarity is to decrease the distance between the lens and the subject. For this reason telephoto lenes are of little use underwater. Instead, it is better to get closer to the subject and either use wide angle or macro lens.
Another way to increase image clarity is to use artificial light directed at the subject, but avoid highlighting the space between the lens and the subject. This avoids back scatter and makes the subject appear clearer and more contrasty.
- Lens Coating
Every glass to air surface can reflect between 4-10% of light entered resulting with light loss, ghosting and reflections. It is therefore recommended that the inner side of a port be coated with anti-reflection coating for maximum performance.
- Flat Port
- When to use flat port underwater?
Flat port should be used when angle of view does not exceed 60°. Using flat port with wider angles results in two types of distortion: chromatic aberration and image shape distortion. All those distortions are inevitable and get worse as the angle of view increases. Also, image in a flat port will be magnified.
- Chromatic Aberration
Chromatic aberration occures when different wavelengs of the visible colour spactrum can not be focused in one plane. This type of chromatic aberration results in blur, flare and poor colur reproduction. It can be also noticed as multi coloured edges around the subjects. This effect increases with the angle from the centre. Beyond 60° this chromatic aberration becomes clearly noticable and is inevitable. This is because when light goes from water to air different colour wavelengths are refracted at slightly different angles. The best way to reduce chromatic aberrations is to decrease the angle of view.
- Shape Distortion
With an increase of angle of view the image will become distorted. Flat port causes pin-cussion distortion and the effect becomes stronger with an increase of the angle of view. Because straight lines do not apper often underwater, shape distortion are not easily detectable on static images. However, if during panning and rotating of the camera against the background, shape distortion becomes apparent. The effect can make the viewer dizzy and is not desirable. To avoid this distortion it is necesary to use dome shaped port for any wide angle work.
- Image Magnification
Water has different optical density than air. The effect of different refractive index of air and water causes that objects appear larger by a factor of approximately 4/3. Consequently, the angle of view is reduced.
This effect may be desirable for macro and closeup shots, but is unwanted for wide angle work. Consequently, a dome port must be used for wide angle shots. Correctly aligned spherical dome port does not cause any change of image size.
- Camera setting When Using Flat Port
Chromatic aberrations and shape distortion increase with the angle of view. To reduce their effect it is therefore important to reduce angle of view as much as possible. This can be achieved by increasing object to camera distance and zooming into the subject. However, increased object distance would create problems of decreased water clarity and filtraton of red spectrum. A better solution is to decrease f-stop when shooting with flat port. I suggest aperture of f/2 when using a flat port. When iris is fully opened, the light comes into the port at a lower angle and therefore the chromatic aberrations will be reduced.
Further, when shooting through a flat port, the distance between the lens and the port should be as large as possible. This has two advantages: it reduces the angle of view and subsequently reduces chromatic aberrations and shape distortion as well as provides longer path for light through the air and shorter through the water, which improves the image clarity.
- Dome Port
- Dome port should be used when angle of view is greater than 60°. Correctly aligned dome port does not introduce significant image distortion or chromatic aberrations. Also, it does not affect the image size. However, at short image to lens distances an effect known as "curvature of field" becomes a problem.
- Nodal Point of a Lens
Nodal point is where incoming rays converge inside the lens. It is also a point, which when used as a centre of rotation of the lens would not result in any image shift. It is very important that this point is aligned with the geometric centre of the dome port. If those two points are apart, the resulting image will be subject to geometric distortions, focusing problems and chromatic aberrations. Also, the image size will be affected.
The difficulty here is that the nodal point changes with the zoom. In a typical lens the nodal point is located closer to the front in a wide angle position and moves backward when zooming in. It is therefore impossible to zoom through a dome port without creating distortions of the image. In addition to that, for larger focal lengths the position of the nodal point is located far back demanding a large diametre of the dome. It may also prove physically impossible to position the lens far forward for larger focal lengths.
There are two main difficulties when focusing through a dome port. The first one is the the image plane is curved, not flat. This causes either the centre or the edges of the image to be in focus. The second difficulty is that the image is projected on an imaginary surface in front of the dome at a smaller distance. This requres the lens to focus on this image that is much closer than the actual subject. This requires MOD (minimum object distance) to be as short as possible, but, even then, the lens may not be able to focus on the image. There are number of methods to overcome this problem. To focus on a closer image a positive diopter of a suitable strength may be installed in front of the lens. This however degrades the optical performance of the lens. It may also not be a practical solution for a wide angle lens and cause vignetting. A beter method is to adjust the back focus or use macro for close focusing. To do so one has to increase back flange distance.
- Curvature of Field
When shooting in air, the focus plane is a flat surface. Filming a perpendicular wall and focusing at the centre would make the whole wall appear in focus. However, when shooting through a dome underwater, the surface of focus becomes convex when looking from the lens. Therefore, the same flat wall would be out of focus at the corners when shooting underwater. At small distances the focusing surface approximates to a sphere. Consequently, the subjects that are in focus in the corners would be closer to the camera than if they were in air. To reduce this phenomenon it is important to increase depth of field by stopping down. By increasing depth of field the effect becomes less apparent. To increase depth of field it is best to set the aperture to f/16. This requres good lighting conditions. Shooting through a dome port with large apertures (f/2 or f/4) will cause difficulties in focusing close subjects on the entire area of the image.
- Camera Setting When Using Dome Port
I recommend not to use ND filter with a dome port underwater. Instead, if the lighting conditions are good, set the aperture to f/16 or smaller. This will increase depth of field and allow flat subjects to stay in focus on the entire area of the image.
The focusing range of a typical lens may be sufficient to focus on a subject that is a few meters away. But, this may prove difficult to focus on a subject that is 0.6m away from the lens. Using back focus or macro may be necessary to focus on close subjects as their virtual images are projected at a considerably smaller distance.