When shooting underwater, the type of lens port used on your camera housing has a major impact on image quality. The two most common options — flat ports and dome ports — behave very differently once submerged due to how light refracts underwater.
Flat Lens Ports
Flat ports use a simple, flat piece of glass placed in front of the lens. While they are straightforward, durable, and often more affordable, flat ports significantly alter how light behaves underwater.
As light travels from water to glass to air, it bends, a phenomenon known as refraction. This causes images shot through a flat port to appear magnified, similar to looking through a dive mask. The result is a reduction in the lens’s field of view by roughly 25%, along with increased edge softness, distortion, and potential color fringing when using wider lenses.
This magnification can actually be beneficial for macro and close-up photography, allowing small subjects to fill the frame more easily.
Flat ports are best suited for macro, close-focus work, and scenarios where above-water framing or split-level composition isn’t a priority.
Dome Lens Ports
Dome ports feature a curved optical surface specifically designed to counteract underwater refraction. This curvature allows light rays to enter the housing more naturally, helping the lens retain its intended field of view and reducing distortion.
A key characteristic of dome ports is that they create a virtual image underwater. Instead of focusing on the subject itself, the lens focuses on a virtual image that appears closer than the actual scene. Because of this, proper lens selection, aperture choice, and focus calibration are critical for achieving sharp results.
Stopping down your aperture (e.g. f/8–f/11) helps improve corner sharpness when shooting wide-angle scenes with dome ports.
When used correctly, dome ports deliver sharper edges, wider coverage, and more natural-looking images. They are the preferred choice for wide-angle photography, cinematic underwater footage, reefscapes, wrecks, and clean over-under (split) shots.
Flat Port vs Dome Port
| Feature | Flat Port | Dome Port |
|---|---|---|
| Field of View | Reduced (~25%) | Maintains original lens FOV |
| Best For | Macro & close-ups | Wide-angle & cinematic shots |
| Edge Sharpness | Limited with wide lenses | Excellent when configured correctly |
| Over-Under Shots | Not ideal | Preferred option |
Choosing the Right Port
If your work focuses on macro subjects or fine detail, a flat port is often the better and simpler choice. For expansive underwater scenes, wide-angle compositions, and professional-level image quality, a dome port offers superior optical performance.
Choosing the correct lens port ensures you’re getting the most out of your camera system, and capturing the clearest, most natural underwater images possible.