The shutter is a device to control the light intensity illuminating the sensor. Controlling the exposure, the shutter is one major element in a camera responsible for the brightness of the picture. Digital cameras use various different types of shutters, but all can be classified as either electronic shutters or mechanical shutters. This article will illustrate these two different shutter designs:
The first thing that comes to mind is that the sensor, being an electronic device, should be able to simply turn on and off electronically. Many compact digital cameras already are designed this way. They do not have any moving parts to control exposure. Therefore, some of the benefits from electronic shutters are less vibration, less hardware, more reliability, less cost, more flexible flash synchronization and silent operation.
Although an electronic shutter has quite some benefits, it is not perfect and can still cause problems. To understand the difficulties electronic shutters have to overcome, it can be helpful to compare the two main implementations of electronic shutters:
Rolling Shutter: Using an active pixel sensor (as described in part II of the sensor-chapter) with a three-transistor design (3T), the sensor will need to expose one line of pixels after another to ensure same exposure times for all lines. The reason therefore is because the standard 3T design does not allow to freeze photodiode signals after a certain exposure duration and hold it until readout. If the exposure started simultaneously, the lower lines would be exposed longer than the top lines, because the readout process usually begins at the top line and takes some time to finally address the lower lines. Consequently, the exposure would continue for these lower pixels until the signal is read.
To prevent the sensor from this incorrect distribution of exposure, the individual pixel lines will be activated successively, having the lower lines to collect light slightly later while the top lines are already read out, exposing each line with exactly the same amount of time. This principle of an electronic shutter is called rolling shutter as the “line activation” is rolling downwards, starting at the first line and reaching down to the lowest line.
The rolling shutter solves the problem of uneven exposure times that would occur if all pixels were activated at the same time while the slower readout would follow. However, it has to be considered that the overall exposure time will increase significantly with a rolling shutter. While choosing an exposure time of 1/4000s for an image on a compact digital camera using rolling shutter will have each line of pixels activated for 1/4000s, the entire photo will not be finished after this amount of time but will eventually take 1/100s. This means that the pixels at the bottom of the sensor will do their 1/4000s exposure about 1/100 second later than the pixels at the top. The finished picture still has the same properties as an image with a 1/4000s exposure, but a rolling shutter simply cannot capture quickly moving objects fast enough to freeze the scene. For this reason, some undesirable effects such as the Lartigue effect – also referred to as Jello effect or rolling shutter effect – can occur at rolling shutters. This effect describes an optical distortion where a moving object appears with incorrent dimensions and forms depending on it’s movement. This effect is significantly pronounced when shooting quickly rotating objects. The image below – kindly provided by Flickr User Jason Mullins who shot the scene with his Iphone 4 – shows the rolling shutter effect of an airplane propeller.
This rolling shutter effect does not only manifest in still photography, but also in video, as you can see in this example: The Rolling Shutter Effect – Youtube Video
The left side is moving up while the scanning of the sensor is going downwards, so it appears shorter. The right side is going down with the scanning so it becomes elongated.
This problem could be eliminated if electronic shutters became as fast as mechanical shutters to scan the frame. Unfortunately readout electronics is not as advanced yet and even very high-end cameras still suffer from rolling shutter artifacts. To get some reference values, some cameras currently on the market feature readout times of around 1/20s (Panasonic GH4) while visible rolling shutter artifacts begin to disappear with a shutter speed of 1/1000s or lower.
Global Shutter: Conversely, a global shutter has the ability to start the exposure of all pixels at the same time and freeze their signals in a single instant, regardless of how long the readout process takes. After the pixel signals have been put on hold, readout can start afterwards step by step. The global shutter will therefore not show any distortions associated with the slow scanning of the sensor array. Another important advantage of an electronic global shutter is that it is perfectly suitable for synchronizing flash. Using flash adds a large amount of light to the scene for an extremely short amount of time, and this can only be captured correctly if all pixels are active simultaneously.
As shown in part II of the sensor chapter, this feature to freeze pixel signals is only available on 4T APS designs and therefore has some slight disadvantages. The added electronics necessary to be able to store the charge for each pixel reduces the fill factor of the pixel. In turn, a reduced fill factor means that less light is converted to electrons and must be amplified later, increasing noise levels. Also, 4T APS designs are more expensive and more complex to produce.
A global shutter can also be achieved by using a CCD sensor (either an Interline Transfer [IT] CCD or Frame Interline Transfer [FIT] CCD). The interline transfer process can be used as a shutter, but even with the charges transferred to the non-photosensitive shift registers, more light is still shining on the entire device and can interfere with the signal that is about to be processed. Also, CCD sensors have slow readout capabilities as there is only one analog-to-digital converter performing the neccessary operations. Also, they do not provide random access which is often required for modern compact cameras. Lastly, their higher power consumption and vulnerability to smearing or blooming effects have caused the CCD sensor to step into the background.
There are many good arguments for using electronic shutters. However, as shown above, there are still some disadvantages when relying on these types of shutters in special conditions. My personal assessment is that further improvements in technology will finally allow electronic shutters to replace mechanical shutters in the future. Until then, Digital Single-Lens Reflex cameras will accept no such compromise and will rely on mechanical shutters.
Different from the electronic shutter, the mechanical shutter is a separate device located in front of the sensor to physically block incident light from reaching the sensor. By it’s special design, it can control how long the pixels on an image sensor collect light without actually changing the individual pixel architecture. Sensor electronics can therefore be less complex and provide a larger fill factor to be more specialized in actually capturing light.
There are different designs of mechanical shutters. Diaphragm shutters are located within the lens assembly while focal plane shutters are located directly in front of the sensor close to the focal plane. Camera evolution has widely superseded central shutters and therefore, this article will present the focal plane shutter.
The focal plane shutter consists of two blinds, also called curtains. Both curtains (called first and second curtain) are arranged slightly behind another so they can perform independent movements. Each of these blinds can quickly cover and uncover the sensor by running up and down. For that reason, these types of shutters are also referred to as vertical run focal plane shutters.
The full shutter firing sequence includes the following steps:
- The second curtain slides into a ready-position which is above the sensor.
- The exposure begins as the first curtain moves down allowing the sensor to be illuminated.
- The exposure ends with the second curtain moving down in front of the sensor in the same direction and with the same speed as the first curtain.
- Finally the first curtain moves back into its original position.
This sequence allows all parts of the sensor to be exposed for the same amount of time. During the exposure, the curtains always move in the same direction and at the same speed regardless of what the exposure is. The exposure lenght is determined by the time lag inbetween the motion of the first and second curtain. For shorter exposures, the second curtain can start actually covering the sensor before the first curtain has completed its movement. For really short exposures, this slit can be quite narrow, sometimes as little as a millimeter as it moves down the sensor. The following illustration shows the principle of a vertical run focal plane shutter.
Actually being a relict of former analog cameras, mechanical shutters are still widely used in today’s DSLR cameras due to some advantages. The effect of covering the sensor with shadow most of the time reduces it’s temperature and therefore improves noise performance. Also, the physical curtains protect the sensor from dust, although dust is not a systematic problem.
The shutter speed refers to the duration for which the shutter is open and is measured in fractions of a second. A shutter speed of 1/200 second lets through half as much light as a shutter speed of 1/100 with all other settings unchanged.
The range of shutter speeds is a lot more extensive than that afforded by the range of aperture values on even the most expensive lenses. The top speed on many shutters in digital cameras is around 1/4000 second. The longest automatically set shutter speed is usually 30 seconds – allowing 17 stops of light to reach the sensor (up to 120,000 times more light). If the bulb mode setting is used, the shutter will remain open for as long as the shutter release remains pressed, so the shutter speed can be stretched to whole minutes – or even hours.