Shutter speed denotes the time that the shutter remains open when taking a picture. It is measured in fractions of a second (so you can imagine that each number has a 1/ in front of it): the bigger the number, the faster the speed. Just as with aperture stops, shutter speeds are set as stops on a scale. The typical standardized shutter speeds are:
1248153060125250500
Each of the numbers in this sequence (each stop) represents a halving to the right on the scale, or doubling to the left on the scale, of the duration the shutter remains open.
Shutter speeds weren’t universally standardized until the late 1950s, so earlier cameras may have a slightly different scale, such as 10, 25, 50, 100, 200.
Fast speeds freeze moving subjects, and slow speeds record motion. If you want to avoid motion blur and record sharp images, assuming a normal camera and a steady hand, you should use a minimum shutter speed of as close as possible to 1 over the focal length of the lens: a 28 mm lens, for example, can be held steady at 1/30. If your shutter speed is slower than the reciprocal of the focal length of your lens, you should use a tripod to get a sharp image.
As a general rule you can’t achieve sharp, handheld shots at speeds slower than 1/60. With rangefinder cameras, however, you can go down to 1/30 or even 1/15, even though this is below the reciprocal of a normal lens.
When the shutter speed ring or dial is set to B, the shutter is operated manually and will remain open for as long as the shutter button is depressed. This is used for exposure time in excess of the longest shutter-speed setting.
For very long exposures, for example at night, a tripod should be used to keep the camera steady, with a cable release to open and close the shutter, in order to not move the camera while exposing the film (which would blur the image).
“Bulb” is derived from “air bulb release,” a pneumatically actuated shutter release that has a small rubber bulb at the end. Squeezing the bulb opens the shutter and releasing the bulb closes it. The B can also stand for Beliebig, the German word for “any,” and may also in some cases be labeled with a Z for Zeit, German for “time.”
Like the B setting, T is used for long exposures. However, when the shutter button is pressed at the T exposure setting, the shutter will remain locked open even when pressure is taken off the button. It is canceled by pressing the button again, or by turning the dial away from the T setting (either on the slow shutter speed or a dedicated dial).