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Everything you need to know about Flat Lens and Dome Port Lens Elements for surf photography and underwater housings

What is a dome port and why do you use it for surf photography and under water camera housings? 

A dome port is a concentric lens that adds an optical element to the camera lens. When the curvature of the dome's inside radius center is situated as close to the nodal point of the lens as possible, the dome port greatly decreases refraction, radial distortion, axial and chromatic aberrations.

When using a dome port, all light rays pass through unrefracted, allowing the "in-air" lens to maintain its angle of view. A "virtual image" is formed optically just inches in front of the lens. When using a dome port to photograph a subject underwater, the lens must be focused on the "virtual picture," not the subject itself.

Because of the dome port, the underwater focus markers on the lens are completely wrong. As a result, underwater lens calibration is necessary. Above water, the dome port serves as a clear glass or acrylic, with no extra optics.

What is a Flat port and why do you use it for surf photography and under water camera housings?

Flat ports also known as macro ports are ideal for macro lenses, greater than 35mm. Flat ports will magnify the image more than a dome port, allowing a lens to reach its true magnification potential. A flat port will magnify an image approximately 28%. 

When using a flat port in you're under water housing, you might notice a small amount of chromatic aberration at the edges of your content, this does get more noticeable when using wider lenses. Fortunately the  chromatic aberration can be easily fixed in post production.

While some people believe that you shouldn't use a macro lens with a dome port as you would have to get too close to the subject to get the shot and could potentially damage your dome. However, many under water photographers have got amazing results. One thing to watch out for is vignetting when using wide-angle port in a macro port / flat port.

 

What are the advantages of using a dome port?

Advantages 

  • The image is not magnified due to the refraction of water, which means you retain the "wideness" of the lens
  • Colour and sharpness is better with a wide angle lens, since you can get closer to your subject due to advantage #1
  • Water particles are pushed further away from the camera lens allowing for better focus, clarity and sharpness in images
  • Less chromatic aberration (colour fringing)

 

What are the disadvantages of using a dome port?

 

Can you shoot macro content behind a dome port?

Macro images and video can be shot through a dome port, but be careful not to scratch the port as you get closer to your target subject. Because there is less chromatic aberration and you must come closer for the same shot, there will be less water between you and the subject, image quality will actually be greater than behind a flat port.

Magnification can be up to 35% less when using a dome port, when compared to a flat lens. This is useful when shooting medium-sized fish or larger macro subjects. Sometimes I shoot with my Sony 60mm macro lens from behind the GDome XL dome port with great results.

 

What are the different port extensions available for surf housings?

Different ports and extensions can and should be used with different lenses, luckily if you purchase a GDome XL it comes standard with a flat and dome port extension. You can also buy the 80mm extension which extends the port for the longer lens. 

Different length extensions are essential with the dome ports, as different lenses need to fit inside the port, also wide angle lenses must have proper placement inside the dome housing for optimal image quality.

Flat VS Dome Lens Ports

The lens port is one of the most crucial and misunderstood features of an underwater camera housing. From the inception of underwater photography in 1893 until 1931, when hemispherical dome ports were first used to account for the refractive qualities of water, flat ports were all that were available. Both flat and dome ports have a role in underwater photography, and understanding the theory and practice of each is essential. The dome port is preferable for underwater photography with wide lenses, while the flat port should be utilized if the shot starts or ends above water, or if you need to capture a close-up with a long lens.

 

Flat Port – The flat port is unable to correct for the distortion produced by the differences between the indexes of light refraction in air and water. Using a flat port introduces a number of aberrations when used underwater. They are:

  • The bending of light waves as they move through different media of optical density is known as refraction (the air inside the camera housing and the water outside the lens port). The lens is magnified by 25%, giving it the same magnification as if you were looking through a face mask. In addition, the focal length of your lens rises by about 25%.
  • Flat ports feature progressive radial distortion, which becomes more noticeable as wider lenses are employed, because they do not affect light rays equally. The result is a progressive blur that gets worse as the aperture on wide lenses gets bigger. Because the direction of travel of light rays traveling through the port's centre is at right angles to the water-air interface, they are unaffected.
  • Chromatic Aberration - When white light is refracted, the colour spectrum is split. White light's component colours do not travel at the same speed, so light beams going through water, glass, and air will bend unevenly. When light is broken down into its constituent hues, the colours gradually overlap, resulting in a loss of sharpness and colour saturation, which is more noticeable with larger lenses.

 

Dome Port – The dome port is a concentric lens that acts as an additional optical element to the camera lens.

 

When the curvature of the dome's inside radius center is situated as close to the nodal point of the lens as possible, the dome port greatly decreases refraction, radial distortion, axial and chromatic aberrations. 

When using a dome port, all light rays pass through unrefracted, allowing the "in-air" lens to maintain its angle of view. A "virtual image" is formed optically just inches in front of the lens. 

When using a dome port to photograph a subject underwater, the lens must be focused on the "virtual picture," not the subject itself. 

Because of the dome port, the footage markers on the lens are completely useless for underwater focus. As a result, underwater lens calibration is necessary.

 

Zoom gears

Some underwater housings allow you to change the zoom with zoom gears. A zoom lens will need a fully customised zoom gear to allow you to zoom your lens underwater. The price of these can vary from $80 to $180, so shop around for the best deal. 

 

Focus gears

When taking 1:1 or larger macro photography, some photographers choose to manually focus a macro lens. Each lens will require its own set of bespoke focus gear. Others choose to utilize the "lock and rock" method (lock focus and rock back and forth), change focus points, or use continuous focus and then press the shutter release when they have attained the desired focus. 

Many people don't require gear focus equipment as the GDome XL allows you to touch the rear screen of the camera for touch focus.


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