FLAT VS DOME PORTS
Underwater, light refraction bends light rays as they pass from water into the air inside the housing port. This makes objects appear approximately 25–33% closer and larger, while reducing the lens’s angle of view (FOV) by up to 28%.
Flat ports do not correct this refraction. They are simpler, lighter, and more affordable—well-suited for macro photography, where magnification and a narrower FOV are beneficial. They can also be used for surface or surf photography with zoom or prime lenses, although they are not ideal for over-under (50/50) shots due to difficulty in maintaining a clean waterline and image sharpness across both elements.
However, flat ports are not suitable for wide-angle use underwater, as they further reduce FOV and can introduce distortion, chromatic aberration, and edge softness.
Dome ports, by contrast, introduce a curved water-to-air interface that preserves the lens’s original FOV and focusing behavior. This makes them essential for underwater wide-angle and fisheye photography, including reef scenes, large subjects, and over-under compositions. Dome ports are also required for ultra-wide or fisheye lenses in surf photography.
While dome ports provide superior image quality in these scenarios, they are larger, more expensive, and can still introduce some optical challenges—such as chromatic aberration or edge softness.