Shutter Dragging for Time Lapse Videos
There's a term you may have heard photographers casually drop when they're taking about how they achieved a certain effect: “dragging the shutter”. It's not a name for a complicated technique; it simply refers to using a slower shutter speed than a scene requires to achieve a specific outcome, normally to blur a portion of the scene. This term and the technique also apply to shooting image sequences for time lapse presentations, to create more natural-looking transitions between frames.
The concept of shutter dragging for time lapse videos is a simple one. Using a slower shutter speed creates motion blur. Since time lapse videos simulate motion with still images, inducing motion blur in those images helps our brains “smooth out” the transitions. It also helps emphasize the direction of movement within a scene, which can, in turn, add impact to any camera movement in a different direction. That makes it very effective when using a camera dolly or motion slider.
Not only is the concept simple, so is its application. It does require at least a reasonable understanding of exposure, and the best way to develop that is by getting to know the Exposure Triangle. If you're a photographer or videographer, you should know it intimately. Here's an article I highly recommend for those of you that don't understand it or simply need to brush up: Introduction to the Exposure Triangle.
Now, since I know you read that article (Right?), you understand that, if you select a slower shutter speed for a scene, you're going to need to increase your ISO setting, your aperture size, or both. While you might get by with simply switching your camera to Shutter Priority mode and letting its metering system do all the work, the odds of creating several properly-exposed images over a long period of time become much lower. It's important to keep in mind that lighting conditions are likely to change over time and will eventually move out of the range of your camera's ability to adjust the other settings.
If you're shooting with Auto ISO enabled, the camera may increase the setting to the point that digital noise becomes more pronounced and noise doesn't translate well in video production. Don't forget that, as things move within the scene or your camera moves, the metering may also be based on a different area of the scene, which can contribute to flicker in the final production. While there's no hard, fast rule when it comes to the best method for calculating exposure for time lapse sequences, retaining as much control as possible will probably give you the best results and save post-processing time.
No matter what method you use to calculate exposure, a slower shutter speed will almost always provide you with smoother motion in the video. As a general rule, using speeds lower than 1/100 second will yield the best results. The lower the speed, the more blur will occur, and at some point fast-moving objects will cross the frame fast enough to be unrecognizable or not recorded at all, so some experimentation will be needed to find the effects you like.
While you're determining your settings, do your best to anticipate upcoming changes in light and camera angle. This isn't always easy and you may find yourself doing a lot of babysitting and manual adjustment during your shoots. As always, experience is going to be your best teacher, so get out there and shoot!