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150. Glasseye (Heteropriacanthus cruentatus) (iridescence)


A Glasseye (Heteropriacanthus cruentatus) moves in and out of the darkness of the nook it calls home. By doing so, the scales of the fish bounce back the light of my underwater video lights. This phenomenon is called “iridescence”. Iridescence is characterised by a change in colour or luminosity with the direction of observation. In other words, if the direction of the object or subject changes, the colour or luminosity changes as well.

Iridescence is produced by tiny crystals of guanine that grow on the scales of fish. These guanine crystals are very thin plates stacked on top of each other. When combined with cytoplasm, a gelatinous liquid that fills the inside of cells, it creates the typical metallic lustre characteristic of fish.

The morphology of the crystals, their size, and orientation are controlled by the fish in a strange way; the fish expend energy to actively interfere with the natural crystallisation process to form these thin crystalline plates. Scientists believe that inhibitory factors within the skin cells interact with these tiny crystals while they’re growing.


Although scientists are still studying this phenomenon, underwater videographers know for sure that the iridescence of a fish’s scales acts as a mirror, reflecting the light emitted by underwater video lights. Because of this reflection of light, it is not recommended to set your underwater video lights at full power. Good quality underwater video lights have the option to regulate the strength or intensity of the light beam. One way to avoid this strong reflection is by not illuminating your subject directly or by lowering your Lumen (light strength) output.

Another way to counter this excess of light is by switching on your ND filter. Some cameras have a built-in Neutral Density filter that can be turned on by simply flicking a switch. Better quality cameras often have a built-in ND filter, which makes filming underwater a lot easier.

If you have a “zebra” option on your camera, it is wise to turn this feature on so that it can guide you in choosing the right settings to avoid "burning" parts of your image. Burning refers to overexposed areas in your image, usually completely white with no visible details due to the absence of colours.

To reduce the amount of light that enters your camera's sensor, you can decrease your camera’s ISO setting or change your aperture. Decreasing the exposure time of the frames (shutter speed) is another option, but it is not recommended to play with this setting as it should be double of your frame rate (for example, frame rate 24fps = exposure time or shutter speed 1/48 sec and frame rate 30fps = exposure time or shutter speed 1/60 sec, etc….) to retain the natural smoothness of motion in your footage.

Usually, in underwater videography, it is better to underexpose your images than to overexpose them. In underexposed images, all colours are present and can be enhanced in most editing programs, whereas in overexposed images, some colours are burned away and unretrievable in post-production.

More on this topic:

For another post about the Glasseye and a more in-depth description about its ability to see in dark circomstances please go to our vlog post 81 or click on this link: https://www.beyondscuba.com/post/the-eye-of-the-glasseye-heteropriacanthus-cruentatus

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