Shutter Speed: Waits and Measures
Shutter speed is one of the most prominent variables available to the photographer. It contributes both to determining exposure and to modifying composition. As shutter speeds get longer, the chances of problems increases along with it.
The standard definition of shutter speed as a unit of measure is the length of time the shutter remains open during the exposure of a given frame. Photography is all about "painting with light," and the shutter has to be open for that light to reach the sensor. Leave the shutter open longer and more light gets in, resulting in a brighter exposure. Makes reasonable sense.
But let's consider the terminology here. "Shutter speed" doesn't seem to fit very well. I mean, if we were talking about speed, we would measure this in terms of miles per hour, or kilometers per hour if you're from a different part of the world than I am. Or perhaps warp factor, if you're from either further away. My point being, it's not speed that's the issue here, it's duration. The shutter actually always moves as fast as it can. So, we should be calling this variable "exposure time" or duration rather than speed. Anyway, so much for the name.
But this isn't the only way that shutter speed is upside down. Typical shutter speeds (excuse me: "exposure durations") are less than a second, yet we often dispense with the numerator entirely and simply consider the denominator in the fraction. Old school shutter speed dials were ringed in a series of numbers with values of 2, 4, 8, 15, 30, and so on, not as they technically should with 1/2, 1/4, and 1/8, each value progressively slicing a second of time into ever smaller slices, not ever bigger ones. Each fraction represents an interval that's half as long the one before it. I suppose all those "1/" markings would have cluttered up the dial too much and can be assumed. When you press the shutter release button, all you hear is "click" for the most part.
Only once the exposure time spans a sufficient duration do we hear it as two clicks, one when the shutter opens, and one thereafter when it closes again. By this point, you'll be recording the movement both of your subject and your camera during the exposure unless you take steps to hold your camera stationary for that long. Without your camera held firmly in place atop your tripod or some other stable support, there are limits to how long you can safely hand hold. The traditional rule of thumb for this has been that you shouldn't consider hand holding at shutter speeds exceeding one over the focal length. In this age of lighter weight mirrorless bodies, varying sensor sizes and image stabilization lenses, we can quibble about the exact length of time one can remain perfectly still while holding a camera and to what degree any motion that does occur will show up in the final image, but clearly there are limits.
Sometimes, you might want the camera motion to contribute to the image. Panning while the shutter is open can create some very interesting effects. If your subject is moving too, you can pan with it, so it renders more clearly than otherwise while the background blurs behind that motion. If your subject is relatively stationary, you can pan against it, typically in a direction that accentuates a prominent line already present. With a bit of trial and error or perhaps experience, you can create creative, expressionistic renderings of otherwise ordinary subjects. Think panning vertically in line with tree trunks against a contrasting background. You get the idea.
As exposure times get longer still, things get even more interesting. I can't even count the times I've tripped the shutter and then had something unexpectedly intrude halfway through as I patiently waited for that second click to happen. Sometimes it was a gust of wind that caused foliage to rustle in at least a portion of the frame. Sometimes it was a family with eager kids tromping across the suspension bridge I was standing on to photograph the creek below. I had one shot spoiled by a deer calmly wandering into the frame while shooting in Olympic National Park. That was a surprise. Given enough time, almost anything can happen while you wait. More than once I've actually put the lens cap back on before the shot was finished. Oops. Two minutes can be a really long time while sitting in the dark just waiting for the shot to end. I could have sworn I heard it click.
Most cameras top out at 30 seconds without resorting to the Bulb setting where you have to manually trigger both the start and end of the exposure. Some programmable remotes allow you to extend things further, but without one you'll have to keep track of time on your own. Thankfully, when working with exposures into the minutes, a few seconds one way or the other won't matter much so the timings aren't critical. It takes a doubling or halving of exposure time to alter the exposure by a full stop.
It's not always possible to control everything in your surroundings long enough to get the shot. Sometimes, another try might prove more successful. Sometimes, you may have to admit it simply wasn't meant to be. Or at least not meant to be under the terms originally conceived. Maybe another day or another way. You'll have to think the situation through and do what makes sense. What else are you going to do while you wait?
At least we don't have to worry about reciprocity failure much anymore. If you're unfamiliar with the term, be thankful. Back in the film era, reciprocity failure was a major consideration when shooting long exposures. "Reciprocity" refers to the inverse, or reciprocal, relationship between aperture and shutter speed in determining exposure, assuming other factors remain constant. If you close down the aperture in order to achieve greater depth of field, you also need to increase (lengthen) the shutter speed (exposure time) by an equivalent number of stops to compensate. At typical exposure times, this relationship remained operative, but with longer exposures, that relationship started to break down, or fail. As exposure durations increased, you had to manually add excess time above and beyond what the meter said to compensate. For most films, the different dye layers started to fail at differing rates creating color casts, but that's another matter. Digital solves all of this nicely by being less susceptible to reciprocity failure than film was. And no matter what, you can see your results on the camera back LCD in time to reshoot, and any color cast could be remedied via white balance anyway. Score one for digital.
Indeed, digital has made long exposures much more predictable and manageable, but possible problems do remain. Long exposures can allow time for random noise to contribute enough to the exposure to show in the result. Long exposure noise reduction can help combat this by taking a second shot with the same settings, but with the shutter closed. Spots that show up in this second "dark" frame are then masked out and repaired in the real shot. With two frames involved, the time to take an image doubles, but it's worth it if you have a noise problem. All you have to do is be willing to wait a bit longer. No problem.