The Effects of Changing the Shutter Speed
The shutter in the camera is a mechanism to limit the amount of light that reaches the sensor (or film). This amount of light is directly proportional to the time the shutter is open – the shutter speed. In low light conditions such as at night or indoors the shutter needs to be open for a longer time in order for sufficient light to reach the sensor. Outside on a bright day the shutter needs to be open for only a very short time to capture that same amount of light.
In the early days of camera design when the photographic medium was very slow shutters were not necessary: the exposure was controlled by removing and replacing a lens cap or even shielding the front of the lens with a hat. The exposure time was in the order of several seconds and a slight variation in this duration made little difference.
With the invention of film the medium became faster and any variation in exposure time resulted in huge differences to the picture. Some form of mechanism was needed that could accurately expose the medium for very short periods and the durations of these needed to be consistent. Any type of “guillotine” in which a single curtain slid open to expose the film and then slid back after a predetermined time had a huge disadvantage because the inertia in the stopping of the opening movement followed by the subsequent reversal of direction took a finite time and thus prevented very short exposure times from being achieved. I do not propose to go through the history of shutter development – there is a good article on this in Wikipedia – but will mention three. The Leaf Shutter; the Focal-Plane Shutter; and Electronic Shutters.
In the Leaf Shutter, when the shutter-release is pushed a set of five overlapping blades, arranged more-or-less like the aperture control mechanism of a lens and powered by a spring, begin to open. When the exposure time is reached the blades close again. From the commencement of opening to the final closure all the light coming through the shutter hits every part of the film or sensor. Yet, as in the simple guillotine type mentioned above, this mechanism has to stop and reverse, thus the Leaf Shutter is not capable of allowing very short exposure times – typically 1/500 second and slower. In addition, Leaf Shutters are typically placed within the lens which requires all other lenses to be used by that camera to have shutters as well.
The Focal-Plane Shutter sits immediately in front of the film or sensor within the camera body itself. With this type of shutter protecting the medium the user is able to look through the lens at any time without exposing the film or sensor to the light. In addition the choice of lenses one can use on the camera is much wider since there is no requirement for a lens to have its own shutter. The Focal-Plane shutter has two screens or curtains, placed very close to each other, that both move in the same direction . Before the shutter release is pushed the first screen is in its closed position and the second screen is open. When the release is pushed, the first curtain begins to move across the medium opening up more and more of the image. When the selected exposure time has passed from the start of the first curtain’s movement, the second curtain begins to move across in the same direction stopping any further light from reaching the medium. Since these two curtains are independent much faster shutter speeds are possible. But, since at fast shutter speeds the second curtain may start to close before the first curtain has completed its travel, not every part of the medium is exposed at the same time. The image proceeds across the medium in a stripe. This can in some instances cause distortion of objects that are moving relative to the camera, however this is a relatively rare phenomenon.
Now with modern cameras that use a sensor rather than film there is obviously no need for a mechanical shutter: an Electronic Shutter could be used. But since the sensor is also the source of image for the viewfinder it cannot simply be powered on and off. Instead, when the shutter release is pressed more power is sent to the sensor and the software captures the image. Yet with increase in power to a sensor electronic noise is generated -- just as with increasing ISO levels. So with current engineering cameras with electronic shutters tend to produce noisier images.
The Shutter – Controller of Movement
The shutter speed – the period during which the medium is exposed to light – is crucial in managing how moving objects appear in a photo. If anything in the shot is moving, a too slow shutter speed may make it appear blurred. Even in apparently static shots such as landscapes the photographer needs to be aware of objects such as branches of trees, leaves, and blades of grass (especially if they are in the foreground) which can sway or flutter in a breeze.
Yet this blurring effect of a slower shutter speed can be used intentionally by the photographer to introduce “smudging” of objects such as people, animals or vehicles to give the impression of speed, or smoothing out the natural movement of water and clouds to produce a more artistic image. This is demonstrated in the second series of photographs below.
Caleb Ward (2016) How Do Camera Shutters Work? Retrieved from https://www.premiumbeat.com/blog/how-camera-shutters-work/
Time-Life Books (1971) Life Library of Photography - The Camera USA: Time Inc.