Shutter Speed (S); Shutter Value (Sv); Time Value (Tv)
The shutter is activated by the user pressing the shutter button. Once the button is pressed the ‘mirror’ moves out of the way. Next the camera opens the shutter for a calculated period of time. The exposure of the image sensor (previously film) is time critical.
The opening and shutting time of the shutter is is called ‘Shutter Speed’. The shutter speed depends on the other essential settings: ISO and Aperture. If the shutter speed is too fast for the amount of light available the picture is under-exposed. If the shutter is speed is too slow, the sensor gets too much light. Then the picture is over-exposed.
Shutter speed is very precisely controlled. In the various auto-modes the on-board computer on modern DSLRs calculates the shutter speed. The camera senses the light intensity using its ‘Through the Lens Light Meter’. Based on a program to provide the best possible exposure the camera then sets the ISO, shutter speed and aperture. In full auto the camera will estimate exposure based on optimum exposure conditions. In other modes the exposure will be biased according to the type of mode that is set. For example sports mode will tend to have a high shutter speed. Night photography will have very low shutter speed. Consult your camera manual for details on how to use the various mode options.
In manual mode the shutter speed, ISO and aperture are all chosen by the photographer. In this case it is up to the user to balance the exposure according to their own needs for the outcome of the image.
Shutter speed, like ISO and aperture settings needs to be managed according to either a program in the camera (auto-modes) or personal choice by the user (manual modes). Exposure is always trade-off between these three settings. There is no such thing as a perfect exposure. However, camera manufacturers will probably argue that their modes and settings will be optimal for that camera.
Shutter speeds vary according to the camera. The DSLR range of shutter speeds vary from 30 seconds to 1/8000th of a second. However, some specialist cameras go faster. These are not normally found in the general consumer market.
The use of a high or very high shutter speed will tend to fix the movement of anything that the camera has in its optical path. Sharpness is often associated with high speed. This is because at high speed the frozen movement means that edges are well defined. The opposite occurs with long shutter times. A 30 second exposure must be done in very little light or the image will be completely over exposed. So long shutter openings tend to be used at night or with very dark filters to cut down the light. Shutter speeds in the range of 2 seconds to about 1/30th of a second will allow for lots of movement blur in the shot if anything is moving in the optical path.
The use of shutter speed is one of the main aspects of composition. The use of softness with movement or the hard, sharpness of fixed speed is a major way of expressing the ‘moment’ in contemporary photography. It is one of the unique aspects of the art of photography and one of the main ways that a photographer can express their interpretation of the scene.