Physics and Photography: Red Filter 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 pus, and when I got back to the surface, that stream of dark bodily fluid turned bright red and looked more like normal scrape than the start of the next zombie apocalypse outbreak.

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 recently, I never bothered buying the red piece of plastic that fits on the front of your camera lens because I’m frugal as all get out and after paying more money that I would care to part with for my camera alone, I was in no mood to pay more for additional accessories. But as I mentioned in my post about DIY GoPro accessories, I do like to put things on my camera. So before coming to Saudi Arabia for the second time, I got a red filter for my GoPro, and this week I was able to test it out on a snorkeling trip. Here’s what I learned:

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Source: Kyle Carothers, NOAA-OE http://oceanexplorer.noaa.gov/explorations/04deepscope/background/deeplight/media/diagram3.html

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 this 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 am going to have to experiment with more, and I am happy that the red filter I bought is one that easily pops on & off. Italso has a leash that tethers it to the body of the camera, so I can swim around deciding whether I want to use it or let it dangle on the side of my camera depending on how deep I am.

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

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.

 

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