Copyright Michael Karbo, Denmark, Europe.
Chapter 16. The shutter
A shutter is a mechanism, which determines how long an exposure is exposed. We are talking about fractions of seconds, i.e. 1/500 second. An exposure doesn’t need any more time, if it is taken in good lighting conditions. This is a good thing because the shorter the shutter speed is, then the less the chance of an image being shaken. But the shutter speed has to fit the lighting conditions, the diaphragm and the camera’s sensitivity.
When a camera is set to automatic, it will itself choose a shutter speed, which is right for the lighting. If a photographer wants to determine the shutter speed, then he can do it with the help of a manual setting, which will be described later. Right now, we are going to look at the shutter and how it works.
You can easily take photographs without worrying too much about shutter speed. A camera is automatic and so it sets both the diaphragm and the shutter speed by itself and makes sure that a suitable dose of light gets into the image sensor.
Shaken images usually appear when there isn’t enough light. The camera has to work with slower shutter speeds, which means that the photographer must hold the camera very steadily in his hand. Or otherwise the image will be shaken. Some cameras give a warning about this with a little symbol on the LCD screen:
The problem seldom appears in broad daylight on a summer day, when there is lots of light and images are taken with a very short shutter speeds (i.e.1/250, 1/500 second, etc.). But when the light is weaker, in the evening, in the winter, indoors, etc. then shutter speeds are very important.
Figur 57. You can always see the shutter speed in the camera’s information system; it can either be read on the big LCD screen or seen on the little LCD display. It’s a good habit to check the shutter speed every time you take a photograph. Here it is set to 1/90 (Tv 90).
If you use the camera’s flash, the images will seldom be shaken. The shutter speed is usually 1/60 second, when a flash is activated. But with a longer shutter speed shaking appears when the camera is held in your hand. You can’t go much under 1/60 second, before many photographers start shaking their hand. And if you have zoomed into the subject, the shaking will be even worse; it will increase the more we zoom in.
So we have to get used to keeping at eye on the shutter speed – even though the camera is set on automatic exposure.
Figur 58. Keep an eye on the shutter speed. Here you can see four different settings, which all give the same exposure in the sensor but which are very different with relation to shaking risks. When you take exposures with a telelens (for example, 3X zoom) then the shutter speed should be shortened to avoid shaking.
Traditionally, a shutter is a mechanism, which can shut down for light in an lens after a certain amount of time. The amount of light is simply dosed by opening for light in shorter or longer periods of time with variable shutter speeds.
The diaphragm can also be set so that more or less light gets in to the image sensor. But a shutter can give doses of light in far more variable portions. A typical shutter can, for example, open up for light for a whole second when there isn’t very much light and maybe just for a 1/2000 second on a summer day in broad daylight, when the light has an enormous intensity.
In between these two extremities there are a number of other periods of lighting, which can be used in different situations. Traditionally, we list shutter speeds, where the exposure times are halved with every step:
1/1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000 1/8000.
Figur 59. A camera continuously gives information to the photographer about the shutter speed and the diaphragm that is being used for an exposure. Here are shutter speeds and diaphragm steps on the LCD screen for four different cameras (from the top left clockwise: Canon, Fujifilm, Olympusand Pentax)
What is smart is that the diaphragm scale also works in steps, which halve and double the amount of light. This makes it easy to “work out” diaphragm and shutter speeds; the lighting factors work in the same way.
Figur 60. Here you can see four different camera settings, which give the same amount of light. When the aperture size is increased (as with F2,8), then the shutter speed can be correspondingly increased.
Automatic selection of shutter speed
So the camera has to be set with a specific shutter speed and a specific diaphragm. At the same time a camera is also light sensitive. These three values make up the exposure parameters that are set for a particular light. Every time you take a photograph, these three values are set in proportion to each other, so that the image is exposed under optimum conditions.
Luckily, the photographer can get some help. You don’t have to try to guess which diaphragm and shutter speed will be the best right now. The camera has, of course, a built-in light meter, and there is a computer, which helps you find the ideal exposure. And the camera’s various automatics can set the diaphragm and the shutter speed all by itself, if the photographer prefers it.
So everything can be done automatically! All these things function, by the way, in almost the same way in all cameras – analog or digital.
Figur 61. The camera’s automatics enable us to just press a button when we take images. In a fraction of a second the light meter will determine the light conditions and deliver the information to the image computer. The correct settings for the exposure are calculated here. Diaphragm and shutter speed are set according to the calculations and the image is taken. It all takes under a half a second!
Several sorts of exposure
As both the shutter speed and the aperture are variable values, several variations of the settings can give the same result. When a camera is set on automatics, it selects more or less by chance from the available settings.
So, we ask, is the image the same, irrespective of which sort of exposure has been selected. In some cases, the result will be the same – in others not. We have seen that long shutter speeds can produce shaken images. But the aperture is also important for the image quality; a big diaphragm gives little focus depth (more about this later on in the book). This is also something a photographer has to be aware of.
Figur 62. Four sets of exposure parameters, which give the same amount of light to the image sensor.
Figur 63. In the semi-automatic exposure program Tv the photograph selects a shutter speed himself, and then the camera selects an appropriate diaphragm.
An image sensor’s sensitivity can vary, and this has a great influence on the shutter speed. The higher the sensitivity, the shorter the shutter speed.
If a camera is set to ISO AUTO, the sensitivity can be changed from exposure to exposure; the camera manages this itself and it always attempts to find a shutter speed, which doesn’t give shaken exposures.
But we can select a higher ISO value ourselves. This can be necessary if you are working in a bad light and without the opportunity of using a flash or a tripod. In which case, increasing the ISO-value might be the only way of avoiding a shaken exposure – with a risk of “grit” in the images, which often comes with high ISO values.
Figur 64. These three settings give the same exposure of the image sensor.
The mechanisms of a shutter
In traditional cameras the shutter is a mechanical arrangement using a few metal slats, which can be opened as a hole in the objective. The hole is shut while there is no photographing so that no light at all can force its way into the film. When an image is taken, the slats flick quickly to the side and back again. In this short space of time, while there is light in the objective, the film is exposed.
We also find a shutter function in digital cameras, although it doesn’t work in quite the same way. There is normally a constant “hole” in the objective, so that the image sensor is exposed all the time. If this wasn’t the case then we wouldn’t be able to see the subject on the LCD screen. It continuously receives signals from the image sensor so that we can see the subject through the lens (TTL).
In small ordinary digital cameras a mechanical shutter does not, in fact, regulate exposure time. The image sensor does this by itself; it can be programmed to gather light for a certain period. There is quite a lot of flexibility here, for example, many cameras work with a stepless variable shutter speed such as 1/148 sekund.
The shutter speed can be 1/10.000 second, when an image sensor controls the time. Few mechanical shutters can work as fast.
It is first when the exposure is complete that the mechanical shutter closes. Then the image sensor can discharge its data in the dark. A shutter closes too, by the way, if the LCD screen is shut down, and when the camera is turned off.