Shutter speed and exposure time
The purpose of the shutter is to control the specific duration that light from the lens projects on to the image sensor. Assuming all other parameters of exposure are equal, using a “slower” (longer duration) shutter speed will increase exposure and result in a brighter picture, and using a “faster” (shorter duration) shutter speed will reduce exposure and result in a darker picture.
Most cameras with fully manual and semi-automatic controls permit photographers to set the shutter speed within a predefined range. The available range varies depending on the make and model of a camera. A typical sequence of shutter speeds is (in seconds) 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, and 1/8000. This sequence follows a geometric progression where the difference in exposure between adjacent values is one stop (1 EV), each exposing the image sensor for either double or half the duration of its neighbour. Most cameras will also permit the selection of intermediate steps in one-half and one-third stop increments.
The relationship between the duration of light striking the image sensor and the resulting total exposure is directly proportional. For example, a shutter speed of one second is double (+1 EV) both the duration and brightness of a one-half second exposure. Meanwhile, a shutter speed of 1/500 second receives half (–1 EV) the light of a 1/250 second exposure, and one-quarter (–2 EV) the light of 1/125 second exposure. Beyond being proportional, this relationship is also straightforward, especially when compared to aperture stops.
The most commonly used shutter speeds are measured in fractions of a second. Some cameras simplify those fractions by showing only the denominator. It’s especially true in the optical viewfinders and top LCD panels of DSLRs and on the dedicated shutter dials of retro-styled cameras. For example, 1/8 second is shown as 8, 1/125 second as 125, 1/1000 second as 1000, and so on. The practical reason for such shorthand is that it saves space; however, as a consequence, it’s simple to mistake your slow fraction-of-a-second exposure for one that’s several seconds long. If this describes your camera’s behaviour, familiarize yourself with how it distinguishes between fractional and multi-second exposures. For instance, Canon and Fujifilm cameras indicate full seconds by following the number with straight double quotes, as in 5” for five seconds.