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Whitewater Photography #2: What is the right exposure?

Before moving on I need to cover four technical terms that will come up a lot.

The first is "stops". Stops are a photography reference to a measured amount of light that is consistent throughout the range of light and in all equipment. If a shot is under exposed you can either slow down your shutter speed by one stop, or open your aperture one stop, or speed up your ISO by one stop.

Another is shutter speed. This is how fast the shutter opens and exposes the sensor. The faster it moves, the less light the sensor receives. Faster shutter speeds stop action but don't let in very much light. Slower shutter speeds expose the sensor to much more light, but moving objects will blur. As a general rule of thumb I consider 1/500 the absolute minimum while trying to freeze action. I try to keep it from 1/800 to 1/1250, and don't find added benefit in going much faster, it's rarely if ever bright enough to warrant it.

Aperture is the size of the opening in the lens. It is adjustable just like your shutter speed and the second means of controlling the amount of light reaching the sensor. The numbering seems backwards at first, because the smaller the number, the larger the opening (letting in more light) and the larger number is of course a smaller opening. Your maximum aperture will vary depending on the lens, the chosen Aperture is often referred to as F-stop or F plus Aperture number. For example F5.6.

ISO speed. ISO speed is the digital equivalent to film speed. This is the third way of adjusting your exposure. Lower ISO speeds absorb less light than higher ISO. As a rule of thumb keep your ISO as low as possible for the situation. On stop of ISO would be from 200 to 400, or 400 to 800.

Doing any one of these three will increase your exposure the same exact amount. For fine tuning, modern digital cameras can adjust in 1/3 stops. So we have our three methods of adjusting exposure, now what is the right exposure? Any bright day on the river has a large dynamic range. This means that there is a vast difference from light to dark. Our eyes are amazing at taking in large dynamic ranges of light, while cameras are quite limited. Traditionally digital cameras have less dynamic range than their film counterparts, making shooting whitewater with digital tougher to some degree. So if you can only capture some of the dynamic range, what part do you want to capture? The most natural look to the human eye is the classic "expose for the right" technique. Welcome to the world of the histogram. Leave your camera on auto and this is what you will generally see while shooting whitewater where the dynamic range is too big. Look in the top right and you'll see the labeled histogram.

Shot with Nikon D200 and Sigma 10-20mm @ 17mm.

The meter in the camera is trying to average out the whole scene, and in doing so over exposes the water. The histogram is a graph that shows the dynamic range that is covered by the camera, and where the light is in that range. The left side is the dark, shadow end of things. To the right is the bright side of things, like white water. The most natural look for a scene where the dynamic range is too large, is to adjust the exposure so the graph does not go past on the right. It's ok if detail is lost in shadow, this looks natural to the eye, but if it's lost in the highlights it looks "washed out" and unnatural. Here is the shot as it was taken, the light is close to the right edge of the histogram, but does not pass it. Note how the shadows do go past the edge of the histogram. With a dynamic range this large you have to lose some of the range on one end or the other.

Lose it in the shadows and it looks natural to the eye...