Overview:

Domes make it better underwater

Domes help correct for aberrations that occur in water photography because light changes speed as it passes through water rather than air. Domes provide better results than flat ports, especially for split (half up, half down) lenses, and make it easier to obtain separation, as well as more "spectacular" separation. While dome ports don't take anything away from your above-water work, their impact or difference on the surface is irrelevant. Domes are ideal for staggered layers and underwater work.

Size matters

In short, bigger is optically better for a number of reasons. But it is heavier, bulkier and more expensive than a smaller dome. If image integrity and results are a major concern, choose a larger dome (180 mm dome diameter). If portability and ease of use are paramount, the smaller dome (120 mm dome diameter) will give you great "bang for your buck".

Glass is optically superior

Star Optic’s domes are made of optical glass as glass domes are optically superior to plastic or acrylic and resilient to scratch and harsh environment.