Red filters in the Red Sea

In every marine science course I’ve ever taken, my professors have stressed two important concepts: 1) Finding Nemo is full of scientific inaccuracies, and 2) red light cannot travel deep down in the water column. While the first point upset me greatly and shattered a small yet significant building block of my childhood, the second came in handy when I became a SCUBA diver.

As a quick review, red wavelengths of light (the longer wavelengths in the visible light spectrum) are absorbed more readily by water, while blue and green wavelengths are scattered by water molecules and are able to penetrate more deeply into the ocean. (See the diagram from NOAA below). So the deeper you go down in the water column, the fewer red wavelengths of sunlight there are to reflect off of the objects around you. As a result, the things that you see and take pictures of appear more blue/green than they would at the surface when illuminated by white sunlight. Even things that would have a red color when exposed to white light (like red algae or fish) can look brown or dark green under meters of sea water. So when I cut my leg on a piece of coral 50 feet underwater this summer, it calmed me down to know that the green-black soup oozing out of my leg was just a bit of blood rather than an infected stream of unidentified bodily fluid, and when I got back to the surface, that stream of dark liquid turned bright red and looked more like normal scrape than the start of the next zombie apocalypse outbreak.

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See, the blue/green wavelengths of light get down deeper that the red ones.      Source: Kyle Carothers, NOAA-OE http://oceanexplorer.noaa.gov/explorations/04deepscope/background/deeplight/media/diagram3.html

In addition to explaining why my blood looks black under the sea, an understanding of how visible light is absorbed by water is helpful when I am trying to take pictures underwater. When I first began taking pictures underwater, the photos that I thought would end up on the cover on National Geographic ended up looking like I squirted green food coloring into the ocean before taking the shot (see below for an example).

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The green tint that my diving photos get below a certain depth.

I was able to touch a few of them up using the color balance tools on Photoshop, and that definitely helped (see below for comparison). However, it takes time to edit the images and to put in more red tones, and the pictures can easily look artificial if you overcompensate and add too much red/pink color. Which is why many people have suggested that I get myself a red filter.

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Two images from a dive in the Red Sea. The first one I color corrected on Photoshop, the second one I left as it was taken.

Up until about a year ago, I never bothered buying the red piece of plastic that fits on the front of your camera lens because. But I grew tired of showing people swampy-looking diving photos and trying to explain to them that it doesn’t actually look that green under the water. So finally I got a red filter for my GoPro, and after trying it out for the first time on a snorkeling trip, here’s what I learned:

PHYSICS IS EVERYTHING. After so many experiences with the green and blue images that I got on my SCUBA diving trips, I went out on the snorkeling boat with the mindset that a camera with a red filter would solve all of my problems. NOT the case. As the nifty diagram from NOAA shows, red light gets absorbed in water, but it does penetrate into the very top part of the ocean. The reef that I was snorkeling on was quite shallow, so a lot of red light was still present at that depth. Using a red filter ended up being overkill in very shallow water, and photos taken with the red filter only looked natural once I dove down several meters deeper.

I played around a little bit with taking pictures at different depths with the filter on and off, and I found that pictures below about 5 meters look nice with the red filter, while images taken near the surface look better without it. Of course this number is not something that one can apply to all parts of the ocean, as other things like water clarity/turbidity and time of day will affect the lighting when taking photos underwater. This is something that I needed to experiment with more, so I took it on some diving trips.

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A shallow photo that I took using the red filter, which turned the white sunlight at the surface purple.

As expected, the red filter was much more appropriate for the deeper depths reached on SCUBA. The colors of fish, corals, and other brightly-colored underwater organisms popped much more with the red filter. However, the filter does exactly that: it filters out some of the light (the blue and green light) to achieve a more balanced color scheme, meaning less light overall is reaching the camera lens. With the lower light conditions, some of the photos turn out blurry, as my GoPro (Hero 3+ black, a slightly older version) isn’t able to capture photos that fast in low light conditions. However, I do get some very nicely colored shots (like these little Nemos below).

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Anemone fish seen on a dive in the Red Sea, shot with a red filter over the lens.

So my advice for those of you planning to use a red filter for your next underwater photography adventure is to slap it on if you are SCUBA diving or doing slightly deeper free diving. But if you are splashing around within the first few meters of the surface, you probably will not need one. You can also find different kinds of red/orange/pink filters that are appropriate for different depths (filtering out more or less blue/green light), but I have yet to try. If anyone has had experiences with different kinds of filters for underwater photography, I’d love to hear about them!

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