Understanding Vision Underwater

You're not seeing the whole picture with a flat mask

Wearing a flat mask is like looking through weak binoculars. Everthing gets bigger and closer, but you lose your peripheral vision.

The Nix mask also has a wider FOV simply due to lenses being closer to the eyes

Below is a comparison of what it is like to dive with a normal flat mask compared to a Nix dive mask. I've also provided the HydoOptix as an additional point of comparison

Magnification and Peripheral Vision

In air, your eyes can move horizontally to achieve a field of view (FOV) of approximately 180°. With a flat mask, the FOV is reduced to about 90°. The Nix dive mask increases the FOV to 120°, while the HydroOptix mask provides an even wider FOV of 150°—though this comes at the cost of a significantly higher internal volume.

In the images below, the masks are at the bottom, and the rays extend from the eyes to show the field of view. The index of refraction of the glass lenses and water is correct

If you want to explore the raytracing models for yourself, you can follow these links:
Nix Mask, Flat mask, HydroOptixs

Distortion and Corrective Lenses

Due to refraction, light from objects bends as it passes through materials with different refractive indices. The green arrow dots represent the true object suspended in the water, while the red dots indicate where the object appears to the viewer wearing the mask. You'll notice that the flat mask causes distortion near the edges, but the Nix mask minimizes distortion and places the object at the true distance.

If you want to explore these raytracing models for yourself, you can follow these links:
Nix Mask, Flat Mask, HydroOptix