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Tuesday, August 14, 2012

Misinformation: Video Tech

The relationship between shutter speed and frame rate

Shutter speed is one of the two most basic concepts in still photography, yet when it comes to video, there's a lot of confusion as to what the shutter speed will bring to a motion image and what the difference is between shutter speed and frame rates, as well as which is the best shutter speed and frame rate to use when shooting video. Quite simply, in video the difference between shutter speed and frame rate is that a shutter speed denotes how long the sensor is exposed to light during each exposure, while the frame rate sets how many individual frames are exposed during each second; additionally, in the case of motion images, each frame is also a fraction of your frame rate.
Video-capable DSLRs provide a basic set of frame rates for photographers to explore. The available shutter speed rates are as follows: the popular cinematic-looking 24 fps, which is the standard rate for capturing film; 25 fps, which is a cinematic look that's proportional to the PAL European standard frame rate of 50i or 25p for broadcast television; 30 fps, which is faster for less judder than 24 fps, but still offers a filmic look while also being directly compatible with NTSC (most of the Americas) broadcast standards; and 60 fps, which is a much faster, sharper frame rate that fills memory cards quite quickly, which is why it was initially offered by DSLRs in 720p resolution. You'll also see a smattering of frame rates that are often referred to by averaging to the next whole number. The 30 fps NTSC broadcast frame rate is actually 29.97 fps, for example, and 60i translates more accurately to 59.94 fps. These rates address broadcast limitations and the variety of pulldown methods for converting alternating frame rates into broadcast and playback standards. These pulldown methods will also introduce pulldown judder, so it's best to know your final output format when first capturing your video. Of course, you also have your choice of progressive (p) frame rates, which capture the full-resolution image, and interlace (i) frame rates, which "scan" in an image one line at a time for a compressed file that's easier and faster to transmit and broadcast.

Just as in still photography, setting the shutter speed will determine the amount of sharpness and, conversely, the amount of motion blur in each frame (as well as exposure in concert with aperture). However, unlike stills, sharpness isn't necessarily the goal. Motion playback looks better to the human eye when there's a subtle degree of blur between frames, which is why time-lapse photographers often use a technique called "dragging the shutter," in which they use slower shutter speeds to make subject movement appear more natural. It may be easiest to think of these two terms as entirely separate functions. For photographers, the concept behind shutter speed is readily understood, so if you think of a video clip as a series of stills (which it is), you'll quickly realize that shutter speed can be changed and it won't affect the frame rate.

As it is with still photography, sometimes bright ambient conditions will override your chosen shutter speed and aperture. In these situations, a variable neutral-density filter will be your best option for gaining an appropriate shutter speed. Almost all professional camcorders offer a basic built-in ND filter, but DSLRs do not.

The common understanding for achieving natural-looking video is that you should use a shutter speed whose denominator is roughly twice the frame rate (i.e., a shutter speed of 1⁄60 sec. when shooting at 30 fps, 1⁄50 sec. with a frame rate of both 24 fps and 25 fps since no DSLR offers a 1⁄48 sec. shutter, and 1⁄125 sec. with 60 fps).

This 180º shutter rule is inherited from film cameras where the shutter would expose a frame to light for only half the exposure so it could feed film for the next exposure. DSLRs operate differently, though, and this isn't a law by any means, just a suggestion for achieving normal-looking video. So feel free to experiment with different shutter speeds and frame rates, and make sure to look at the dramatic difference that your chosen frame rate will have when moving or panning the camera in comparison to more static shots. Often, as is the case with much of photography and video, the choice you make in regard to both frame rate and shutter speed ultimately will depend on your own artistic sensibilities. Even very fast shutter speeds can give you an exciting staccato look (high-shutter-speed strobing) if used correctly in the scope of your project.