Breaking the Law of Reciprocity
The rules of photography say that if you lengthen shutter speed, you have to close down the aperture to compensate, right? It's known as the law of reciprocity and has been one of the fundamentals taught to new photographers since photography first started. But there are ways to break the rules and still end up with the same exposure. And knowing them can help make you a better photographer.
In the history of photographers trying to break the bonds of reciprocity, the first thing people came up with was flash. Add more light and you can decrease your aperture without affecting shutter speed. Flash photography has come a long way since the days of literally setting off a flash of burning phosphorous, but the one thing even the computerized state of the art flashes produced today can't do is decrease ambient light. A flash adds light rather than taking it away so while it can help compensate for smaller apertures it can't help with longer shutter speeds. Macro photographers love flash. People trying to shoot blurred waterfalls need something else.
In this digital age, those converting from film find it liberating that they can change the ISO speed on their cameras between every shot if they so choose. Adding ISO to the equation makes shutter speed and aperture into relatively independent variables. Now you have three things you can adjust to achieve what you are after. If you increase the time the shutter is open, you can now either close down the aperture or lower the ISO setting. Or any combination of the two, so long as the combined number of stops offsets the number of stops you slowed down the shutter speed by. With digital, you can bump up the ISO as the light begins to dim to keep shutter speeds manageable, or lower ISO to allow you to continue lengthening the shutter speed even when the aperture is closed all the way down.
But there are limits and there are trade-offs. Rock bottom on most digital cameras is ISO 100. The sensor just can't go any lower. Going the other direction, noise can become a major problem if you increase ISO too much.
Solid neutral density filters can help slow things down. If you make everything dark enough, you can lengthen your shutter speed as much as you want. I have one, two and four stop solid ND filters that I bought for just this purpose. Waterfalls generally look much better blurred at least somewhat to produce a silky effect. It helps convey the sense of motion that the roaring water has when you are actually there in front of it.
But photographing waterfalls with solid ND filters can be tedious at best. With the filter screwed on the lens, the image in the viewfinder is dark so you have to compose before putting the filter on. You then take a shot at the shutter speed that seems appropriate to the circumstances. Not all moving water is the same. Shutter speeds ranging from around 1/15 second on up to several seconds (and beyond) are what you are after. Precisely what speed depends on how fast the water is moving and what the lighting conditions are like, as well as what effect you are after. Anything from a slight softening up to a completely smooth cotton candy effect is possible. With a bit of practice, you may develop a feel for what setting to use, but a bit of trial and error can help fine tune things in any case. After taking a shot, look at the image on the LCD camera back and evaluate how it turned out. Raise or lower the shutter speed as needed and take another shot. But every time you do, you'll also need to adjust the aperture to compensate, or change the ISO, or change to a different strength neutral density filter. And while you are working, you can't easily see through the viewfinder since the filter makes everything dark. If anything moves, you might not notice it unless you take the filter off temporarily to check.
There is an answer though. The folks at Singh-Ray have created the Vari-ND, a variable neutral density filter that allows you to go anywhere between two and eight stops in a continuous range of densities by simply rotating the filter. At a current list price of $340 on up, it isn't cheap, but then quality gear rarely is. If you like shooting moving water or have other needs for breaking the connection between shutter speed and aperture, it may be just what you need. Based on my own experience with it, I would put it in the category of things that, before owning one there's no way anyone could justify that much money for, but after getting one there's no way you would be without it.
Using a Vari-ND couldn't be easier. First, set your aperture based on the depth of field you need for a given shot. Then, set your desired shutter speed. Don't worry about your exposure when setting either of these, since you can fix it by adjusting the filter. After screwing it on the font of your lens, simply rotate it until your exposure falls where you want it and shoot. If it turns out you didn't quite nail the shutter speed, rotate the filter to its least dark setting so you can see through the viewfinder. Then adjust the shutter speed, check your composition, rotate the filter back again to set the exposure, and fire away. No need to take the filter back off, and no need to change filters to change ND strength. You can dial in precisely the density you need to get the shot you want.
The Vari-ND isn't perfect, but does work well under a wide variety of circumstances. Its main limitation comes from its thickness. Due to its rotating design, it's thicker than a standard neutral density filter so a wide enough angle lens will vignette. Singh-Ray makes it in both regular and wide angle versions, the wide angle one lacking front filter threads, but either will vignette at some point. They also make it in both 77mm and 82mm diameters so it is possible to use the larger size with a step-up ring, but then you have to deal with the added thickness of the step-up ring. I have the 77mm standard version and find it meets my needs most of the time.
Singh–Ray also now also sells what they call the Mor-Slo 5-stop ND filter that can be screwed on the front of the Vari-ND to extend its range to as much as 13 stops of density.
Update 11/18/2007 - Several readers have pointed out that you can achieve this same basic effect by screwing together two polarizers, one atop the other. While this is indeed true, this method is not without its problems. Such a setup would be fairly thick since a polarizer is already one of the thicker filters out there and two of them doubles that problem. This can increase the likelihood of vignetting to unacceptable levels, at least with some wider angle lenses. It's also difficult to get truly neutral polarizers, and stacking two may result in color casts that aren't very flattering. The Singh-Ray Vari-ND is designed to minimize both of these issues and although expensive, it really isn't that much different in cost from two high-quality polarizers anyway. You'll also need to be on the lookout for dust between the two filters, something the Vari-ND also prevents quite nicely.
If you do want to try this with two polarizers, the front one can actually be a linear polarizer, even on modern auto-focus cameras. So long as the rear polarizer is circular, the polarized light will still be re-scrambled before entering the lens so your metering and focus systems won't get confused. Be advised though that high quality linear polarizers are even more rare than are good circular ones.
Update 12/04/2007 - If you've tried using a polarizer for control of reflection together with the Vari-ND you may have been frustrated if you didn't put the filters in the right order. The polarizer must be in front, with the Vari-ND behind it, next to the lens. This way, all will be fine, but in the opposite order (Vari-ND - Polarizer - Lens), the polarizer will be rendered useless due to the design of how the Vari-ND works. This may be a problem if you have the slim Vari-ND version without front threads, so you have to decide which you care more about: no front threads means less chance of vignetting, but having front threads means you can screw on another filter when vignetting isn't an issue. You can always hand-hold the polarizer in front too of course.