Filters and dome ports for water

Why underwater footage goes blue

Water absorbs light unevenly, taking the long wavelengths first. Red disappears fastest — most of it is gone within the first few meters — then orange, then yellow. What is left at fifteen feet is predominantly blue-green light, and that is all your sensor has to work with.

Your eyes compensate automatically, which is why the dive looked vivid and the footage does not. The camera records what actually arrived.

A filter is a corrective: it cuts the blue-green that is over-represented, rebalancing the light before it reaches the sensor. That is the crucial difference from fixing it afterwards.

Why you cannot fix it in the edit

Pushing red in an editor does not recover red that was never recorded. The red channel at depth has received almost no light, so amplifying it amplifies noise — you get a muddy, grainy, slightly pink image rather than a corrected one.

An in-camera white balance mode helps a little, because it is at least applied before compression, and it costs nothing. It is not a substitute for a filter.

Which filter, at what depth

Two things to take from that table.

Depth ranges are specific, and using the wrong one is worse than none. A filter cut for 15-75ft used at three feet overcorrects: there is still plenty of red at that depth, and the filter adds more, so everything turns magenta.

Blue water and green water need different filters. A red filter in green water pushes the image further toward a color cast rather than away from it. Lakes, quarries and temperate coastal water are usually green; tropical reef is usually blue.

Dome ports: what they are actually for

A dome port is a large transparent hemisphere that fits over the lens, and it does one thing well: the over-under split shot, with half the frame above water and half below.

It works because the dome pushes the water line away from the lens, so both halves can be in focus at once. Without one, a split shot is a blurred band where the surface crosses the lens.

The costs are real. A dome is large and awkward to swim with, it is fragile, it collects droplets on the upper half that need constant wiping, and it does nothing at all for ordinary underwater footage.

Buy one if split shots are specifically what you want. Do not buy one because an underwater video looked good — that was probably a filter.

The housing requirement

Both filter sets here state that they fit the GoPro Protective Housing only. That is not a preference: the frames key to the housing's lens surround, and there is nothing to attach to on a bare camera.

So the practical decision is a chain. If you want filters, you need the housing. If you have the housing, you need anti-fog inserts, because a sealed housing fogs. And you accept muffled audio.

For surface snorkeling with a camera rated past your depth, that chain is worth thinking about — you may be better off with no housing and slightly bluer footage than with the whole stack.

Practical handling

Small parts, in water, on a moving boat. Three habits.

Tether the frame or keep filters in a case clipped to something. A dropped filter at depth is gone.

Swap at the surface, holding onto the boat. Not while treading water with a camera in one hand.

Rinse filters with the rest of the kit. Salt in a frame mechanism seizes it exactly like a mount thread — the salt routine.