Light

Under water light behaves much differently then it does on land. It is fainter, colours disappear, backscatter is common, and on top of all that water can distort light! Understanding the behavior of light underwater is central to taking the best photo's...

Water Absorbs Light:
The first thing to know about light underwater is that light acts like a giant filter. As such there will be less light underwater then at the surface, and as you go deeper the amount of light will drop off. This is why you need a camera with a flash - the flash will give you enough light to take decent photo's underwater.

Exactly how much light is absorbed depends on many factors. First is the time of day - during the morning of afternoon the sun hits the water at an angle. As a result a large portion of light never enters the water. Instead this light is reflected off of the water, back into the sky. This effect is minimal when the sun is nearly overhead. This occurs from about 10AM to 2PM. So the best day-time pictures will be taken during this time, when the most light enters the water.

But other factors also affect the amount of light that makes it to you camera. For example, rough sea conditions on the surface can decrease the amount of light which enters the water, as can clouds. The clarity of water can also have an effect - particles in the water will absorb light, the more particles (i.e. the less clear the water), the more light will get absorbed.

Lastly, your depth will play a major role in the amount of light which you see. On average, for every 10m/30' in depth there will be a loss of half the light (i.e. at 10m/33' there will be 50% of the light found at the surface, at 20m/66'' there will be only 25% as much light as there was at the surface, and 12.5% as much light at 30m/100'!

Unfortunately we cannot control the light, but in general you can get the best light if you do the dive around noon, on a sunny day, and keep your depth to less then 10m/30'. And always use a flash.

Water is a Filter:
Another problem with water is that it acts as a giant red filter. What I mean by this is that is that water preferentially absorbs red light, leaving blue and green light behind. By 3m/10' in depth there is almost not red light left from the sun. By 5m/15' orange light is gone, by 10m/30' most yellow is also gone. By the time you reach 25m/80' only blue light will remain.

As a result of this uneven filtering of water most underwater pictures will take on a blue-green colouration. There is only two ways to deal with this. First of all stay as shallow as possible - this will allow you to get the most colour out of the natural lighting. Secondly, use a flash. The flash can restore some of the colour, at least to subjects close to the camera.

Note on flashes: Water will also absorb colours from your flash, so make sure you get close to your subject. For example, if you are 1.5m/5' away from your subject the light from the flash will have to travel through 3m/10' of water before it reaches the film (i.e. it goes to the subject and back). As a result most of the red light will be filtered out of the image. So get as close as you can to maximize colour.

Note on filters: Some people try to correct for waters tendency to remove red light by putting a filter on their camera. This DOES NOT WORK. Filters can only enhance light which is there. Since there is no red light at depth, a red-enhancing filter can do nothing. The only way to restore colour is to use a flash.

Absorption of light by water. Left: Total light absorption. For every ~10m/33' increase in depth the brightness of sunlight will drop by 1/2. Right: Colours are absorbed at different rates by water. Nearly all red light is gone by 3m/10', oranges are gone by 5m/15', yellows last to ~10m/33', greens to 25m/80'. Blue continues to great depth.

Water has Particulates:
Even the clearest ocean water is filled with small partials. These can be dust, algae, and even microscopic animals. If you are using a flash these particles can cause a serious problem known as backscatter. Simply put, backscatter occurs when light from your flash illuminates these particles. The resulting image can often look like it was taken during a snow storm. There are several techniques which can be used to reduce the amount of backscatter which occurs - these are covered in the composition page.

Water Bends Light:
A final problem water can cause is through bending of light. Actually this bending occurs when light passes from the water into an air filled space (i.e. from the water into your mask, or into a camera's lens). This bending causes two effects. Firstly, it causes the subject to be closer (or appear larger) then it really is. Secondly, it may affect you camera's ability to focus. The first effect is not a problem. The second can be - but it depends on your camera.

Where water bending light can become a problem lies with how the auto-focus of some camera's work. There are three main ways camera's auto focus's work. One (usually found on cheaper point-and-shoots) is the focus set at infinity - or in other words the lens is setup such that everything past a certain distance out is in focus. The bending effect of water has no impact on this type of focusing, other then allowing you to take pictures of things closer to the camera the you can on land. A second form of auto-focusing uses an infrared light. This light is projected from the camera, and the camera focuses on this light. This method is also unaffected by water, as the water bends the infrared light the same as all other light, so the system is essentially "self-correcting". However, the third form of auto-focus can be a problem. This technique, which thankfully is very rare, uses a laser to directly measure the distance to an object, and then sets the focus appropriately. Unfortunately this technique does not take into account the bending of light, so all of your photo's end up out of focus. Unfortunately the only way around this is to manually focus the camera for every shot.


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