The Shutter
The shutter’s function in the camera is simply to block light from getting into the camera, unless a photo is being taken, and then to allow light in only for as long as needed to achieve a correct exposure. The name shutter is a little misleading, however. Rather than being a door that opens, and then closes at the end of the exposure, it is more like a gate, with a pair of doors that slide together across the sensor. This arrangement allows for much shorter shutter speeds than could be achieved by moving a single door. The animation below explains this more clearly:
For the sake of clarity, the above diagram excludes the mirror and all other camera mechanics, only showing the sensor, shutter and some representative surrounds to give a context for the illustration.
When you press the shutter release on the camera, a complex sequence of events occurs, including setting the aperture in the lens, flipping up the mirror of an SLR and a lot more. We’ll just look at the shutter however. So referring to the animation above:
- The rear of the two shutter “curtains” as they are called, opens behind the front curtain, and stays up out of the way. At this stage no light has entered the camera, as the front curtain is still closed.
- The front curtain drops away, revealing the sensor behind, and allows the light to enter the camera, beginning the exposure.
- After a period determined by the set shutter speed, the rear curtain now drops to close the camera to light and end the exposure.
- To prepare for the next exposure, the front curtain returns to its normal closed position.
There is an outstanding interactive movie of the inside of a Nikon D3 showing the complete process in incredible detail. I highly recommend taking a look.
Fast Shutter Speeds
When the shutter has been set for a very short exposure period, such as 1/200th sec or faster, the twin curtain system comes into its own. A single “door” shutter wouldn’t be able to open and close this fast. For such short exposures, the rear curtain starts to fall, covering up the sensor before the front curtain is even fully open. The result is a moving open slit passing in front of the sensor. See the animation below:
This method of exposing the sensor (or film) is important when shooting with flash, but the topic of flash sync speed is best covered in another post. In the very early days of photography, when films had poor sensitivity to light, the rolling gap effect of this kind of shutter led to problems with moving subjects, but today there is no disadvantage at all. Most high end DSLRs can achieve incredibly short shutter speeds, such as 1/8000th sec with this shutter design.
The Shutter’s Effect on your Photos
It may seem like your camera is capturing an instant in time, but in reality, it’s capturing a duration of time equal to your shutter speed. This is especially noticeable with long exposures. Anything in the image that is moving while the shutter is open will appear blurred. This also includes the camera itself. Accidental movement of the camera during an exposure is called camera shake and has the effect of smearing the image in the direction of movement. This can be done deliberately for artistic effect, but in most circumstances you’ll want to avoid this.
A good rule of thumb to minimise camera shake is to set the shutter speed at a minimum of one second divided by your focal length. So for example, if you are using a 50mm lens, then to minimise camera shake, you should shoot at 1/50th sec at least. If you are shooting at 200mm, then set the shutter to 1/200th sec. The reason for this is that as we know, longer focal length lenses magnify the image, but they also magnify the camera shake.
Some examples of how choosing your shutter speed can affect your photos:
- Choose a high shutter speed to freeze motion or action, for example in sport photography.
- Create the appearance of motion in your photo by choosing slower shutter speed and deliberately controlling camera or subject movement during the shot.
- A longer exposure on a tripod can make moving water fluffy and white.
- Very long exposures on a tripod at night can capture very dim scenes, and even stars in the night sky fainter than you can see.
