Underwater Photography Quiz

Water absorbs the long red wavelengths first, and most red light is gone within the first few meters. Orange and yellow disappear next, while blue and green travel farthest. That is why underwater photos turn blue green with depth.

Water strips out red light first, so color has to be put back. A red or magenta filter rebalances available light at shallow depths, and strobes bring full spectrum light to the subject at any depth. A blue filter would make the existing blue cast worse.

Backscatter is light from a strobe or flash bouncing off particles suspended in the water, such as sand, plankton and silt. Each lit particle shows up as a bright speck or blurry orb between the lens and the subject. It comes from the water, not from the lens or the camera.

Backscatter occurs when strobe light reflects off suspended particles between the lens and the subject. Angling the strobes outward places their illuminated cone on the subject without illuminating the water column directly in front of the lens, reducing the number of particles lit within the frame.

Water's higher refractive index bends light rays more than air does. Through a flat port, this bending makes objects appear closer and larger than they actually are, a consistent optical effect of roughly 33 percent magnification in each dimension.

A dome port creates a virtual curved surface that compensates for the flat-port refraction effect, restoring the lens's intended angle of view. For wide-angle lenses, this restoration is essential to avoid the magnification and corner distortion that a flat port introduces.

A flat port keeps the lens's full magnification underwater, and the roughly 33 percent extra magnification from refraction even helps with small subjects. Macro lenses have narrow angles of view, so the edge distortion a flat port causes on wide lenses barely matters. Wide angle, fisheye and split shots need a dome port instead.

A large dome gives more room above and below the waterline, which keeps the water surface line smooth and easy to place. The dome also creates a virtual image of the underwater part very close to the lens, so a small aperture such as f/11 or smaller is needed to keep both that near image and the distant topside scene sharp. A flat port distorts the underwater half.

O-rings keep water out of a housing, so they need regular care. Remove the ring, clean off sand, hair and grit, check it for nicks or flattening, and apply a thin film of the recommended silicone grease before refitting. Too much grease attracts dirt, and a dry or damaged ring can let the housing flood.

Water absorbs and scatters light, so with every meter of depth there is less of it, and colors drop out starting with red. Particles in the water scatter what light remains, which lowers contrast. Lens sharpness and focus do not change with depth.