Long exposures of moving water in bright light

PSA Journal, Jan, 2007 by Joseph Hearst

Moving water is a fascinating subject. We can find crashing waves, roaring waterfalls, or tranquil reflections in a pond or a stream. As photographers, we can show these features in ways that cannot be perceived with the naked eye. We can turn a view of the ocean from suspended spray to misty clouds, we can change waterfalls from curtains of drops to sheets of foam, and we can vary reflections from precise shapes to colored washes, merely by changing our shutter speed--but it isn't quite as easy as it sounds.

Most all photographers know how to freeze action with a fast exposure: all it takes is plenty of light or a high ISO. And in low light, long exposures work well, but it is harder to slow the exposure down to several seconds when the light is bright. To make long exposures in bright light, the photographer needs a very dense neutral-density filter. It is, of course, possible to buy such filters, but the truth is that we need different ones for different conditions. Moreover, it is hard to compose a picture while looking through a very dense filter. A variable neutral-density filter that can go very dark is available, and with that one it is possible for the photographer to first compose the scene and then darken the filter. Unfortunately, such a filter costs over $300.

There is a less expensive solution, at least for the digital photographer: crossed polarizers. Unpolarized light can be thought of as a mix of light vibrating, or "polarized," in two directions, 90[degrees] apart. A polarizer is made so that it only allows light that is vibrating in its polarizing direction, or "axis," to pass through it. If light passes through two polarizers in succession, the first one will remove all of the light vibrating in the direction 90[degrees] to its axis IF the second polarizer has its axis turned 90[degrees] to the axis of the first one, then it will remove the rest of the light.

If the incident light is already partially polarized, for example by reflecting from water or being scattered in the sky, and we turn the polarizer to remove the glare or darken the sky, what we are actually doing is setting its axis at 90[degrees] to the angle of polarization of that light, and so that is the light removed by the first polarizer. If we add a second polarizer, with its axis 90[degrees] to the axis of the first one, no light will get through. If, however, the axis of the second polarizer is at some other angle with respect to the first, only part of the light will be removed, and the amount can be controlled by changing the angle. (In fact, the polarizing material normally used in photographic polarizers actually allows a small amount of blue light through, even when they are crossed at 90[degrees].)

If your regular circular polarizer has front threads, all you need to do is buy an inexpensive linear polarizer of the same size and attach it to the front of the circular one. A second circular polarizer will not work, because a circular polarizer is actually a linear polarizer with a device called a quarter-wave plate attached to the back. This plate converts the linear-polarized light to what is called circular-polarized light, which behaves as a mix very much like unpolarized light, so the second polarizer cannot remove all of the light, even at 90[degrees] to the first one. (A second circular polarizer will work, however, if the front one is reversed so the light hits its quarter-wave plate before it hits the polarizer itself.)

To make a long exposure, first go into aperture-preferred mode and stop the lens down as far as you can, and set your ISO to the minimum your camera allows. Set the camera up on a tripod. Then, to control the amount of light reaching the camera, first set up the circular polarizer as you usually would for the scene, which will remove any polarized light, and then add the linear one and rotate it in either direction and watch the image as it changes. It may get darker or lighter, depending on how the linear polarizer was originally oriented to the circular one. If it gets lighter, just go the other way. Watch the exposure time as you rotate. If using an SLR, cover the viewfinder with your finger if you are not looking through it, to prevent light from the back interfering with the meter. If the image starts to turn blue, or you can no longer see the scene, you have gone too far. When the exposure is as long as you want, or as long as you can get it without the image turning blue, shoot a picture (with the viewfinder still covered), and look at the histogram. If the histogram is crammed towards the left, use a positive exposure compensation (which will increase the length of the exposure still more) until the histogram looks good. If you can't fix the histogram, rotate the polarizer back a bit and try again. Be careful not to overexpose: you may want to check the "blinkie" view.

Waterfalls are the easiest subjects to practice on, because they don't change much in short times, so you can try different exposure speeds and see what you prefer. You can go from slight smoothing to what one judge disparagingly calls "cotton candy." When you are comfortable with the technique you can go on to reflections and waves. The changes in reflections depend on the wind; sometimes long exposures won't do much, and other times there will be a lot of difference. They are, however, fairly predictable. Waves are the hardest, because each set of waves is different, and you need to time the shot for the best wave action. And of course the action may be different in different parts of the frame. Make many exposures and vary the exposure time.