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Understanding Shutter Speed and Using It Creatively
Shutter speed is the next point on the 'exposure triangle' that we're going to look at. We know that the aperture controls the size of the lens entry pupil and in turn determines the f ratio, which impacts the depth of field. Let's now see how shutter speed is involved in the process of exposing a photo, and learn about the effects it has on the character of images.
What does the shutter do?
The shutter is closed all of the time, apart from when a photograph is being taken. So it controls when light is able to get through to the camera sensor and create an image. It also determines the length of time for which the light passes into the camera: the exposure time.
The exposure time is really important, because the longer it is the brighter an image will be. Used in conjunction with aperture and ISO, shutter speed is used to set the exposure value of an image. But it doesn't just influence the exposure. In the same way that aperture size causes differences in depth of field, shutter speed causes differences in the appearance of motion (more below).
What actually is the shutter?
You might think the shutter is a single door, or 'curtain', that opens for a set amount of time and then closes again. In reality it is actually 2 curtains, 1 in front of the other, that work together. This is a really efficient system that enables some very fast shutter speeds. Here's how it works:
First, you press the shutter release button. This causes the rear curtain to lift up, although the exposure has not yet begun because the front one remains in position. Then the front curtain drops down, exposing the sensor to light coming through the lens. When the time that you have selected for the shutter speed has passed, the rear curtain drops down, ending the exposure. The front curtain then falls back into position, ready for the process to start again.
So why is the shutter built like this? Well, having 2 curtains instead of 1 makes it possible to set some incredibly fast shutter speeds. Sometimes, the speed is so fast that the shutter is never fully open. The rear curtain begins to drop back down, just as the front one is falling away. This creates a thin slit of light that passes across the face of the sensor.
The below video is a good slow-motion illustration of the shutter. You can see how at 1/200th of a second the sensor is fully revealed. Then, at 1/1000th of a second only a slit of light passes across the sensor. (The first thing to move in this video is the reflex mirror, which is another subject we'll learn about elsewhere)
Measuring shutter speed
Shutter speed is measured in seconds, or more often than not fractions of seconds, for example 1/100th of a second. The bigger the number the faster the speed, i.e. 1/1000 is faster than 1/60. Most of the time we select a shutter speed before taking a photo. But it's also possible to keep the shutter open for as long as the shutter release button is held down for, using 'Bulb' mode.
Interaction with aperture and ISO
When 1 point on the exposure triangle changes, at least 1 of the others needs to change too to keep the same exposure value. So, when you slow the shutter speed down, increasing the exposure time, either the aperture size or ISO speed would need to be reduced. Conversely when the aperture size is reduced to increase depth of field, either the shutter speed or ISO speed would need to be increased.
Shutter speed and camera shake
One of the most important considerations when choosing a shutter speed is camera shake. Camera shake is the movement of the camera during a hand-held exposure which can cause blur in a photo. So it's not just the motion of the subject that is recorded by the sensor, but also the movement of the camera itself.
It's very difficult to hold the camera completely still without using a tripod. So when we need a fairly slow shutter speed, either because of low light or the need for a small aperture, the movement of our arms or hands can result in a blurry photo.
For this reason, it's important to be aware of the 'tipping point' at which camera shake begins to occur. This tipping point is when shutter speed drops below the reciprocal of focal length. In other words, when the selected shutter speed is not at least the equivalent number to the selected focal length. For example, when shooting at 80mm focal length you should always use a shutter speed of at least 1/80 to ensure sharpness. This is often referred to as the 'reciprocal rule'.
1. Even the tiny vibrations caused by the shutter mechanism itself can create enough of a disturbance to register as blur in a photo! For this reason, using a remote release device when shooting on a tripod is a great idea to ensure pin sharp results.
2. Because camera shake is a greater risk with big focal length lenses, many of these lenses have an Image Stabilization (Canon)/Vibration Reduction (Nikon) feature which you should use - but only when hand-holding.
The reciprocal rule stops being meaningful once you get into shutter speeds of about 1/50th of a second and below, because it's extremely difficult to prevent camera shake at those low speeds. So with shutter speeds of 1/50th of a second or below use a tripod when possible, or prioritze changing the other points on the exposure triangle to get the shot when hand-holding.
Bear in mind that if your camera has a crop sensor (i.e. 1.5x, 1.6x etc) then focal length is greater than that shown on your lens, and shutter speed needs to fall into line accordingly. So, if you use a camera with a 1.5x crop factor and are using a 50mm lens, you will need to use a shutter speed of at least 1/75, because the effective focal length is 75mm.
As we have seen, the shutter speed is one part of the exposure triangle, influencing the brightness of an image by exposing the sensor for a set period of time. But we've also seen that the exposure time determines whether an image id sharp or blurry. The ability of different shutter speeds to capture motion in different ways provides lots of creative potential:
- Capturing action
A really fast shutter speed, like 1/2000, can capture fast action - like wildlife or sport - and create a moment in time that we would normally never get to see.
- Misty waves
One of landscape photographers' favourite tricks is to capture the sea on a long exposure, which produces a serene, misty effect. Set your camera up on a tripod at the beach and experiment with exposures from about 1/4 (for gentle blur that preserves some detail) to 30 seconds (for complete blur and that 'misty' look)
- Atmospheric street scenes
You can use long exposures to capture the sense of the hustle and bustle of urban life. It's important to choose a speed that preserves enough detail so that people are recognizably human, but that also shows they are on the move.
- The night sky
Long exposures are needed to capture night landscapes, which usually feature the starry night sky. These shots can be really stunning, and often involve 'painting' light on to a foreground object during the exposure.
Panning is a fun technique that involves 'tracking' a moving subject with your lens, so that it remains fairly sharp whilst the background is turned into blurred lines (horizontal or vertical depending on the direction your camera is moving). The shutter speed should be slow, but not too slow. Something between 1/50 and 1/5 normally works.
Shutter priority mode
We saw in the previous tutorial how aperture priority mode enables us to control the aperture without being in full manual mode. Well there is a priority mode for controlling the shutter too: shutter priority. In shutter priority mode we have the freedom to set the exposure time, whilst the camera will automatically select an aperture speed to create a correct exposure. In Nikon camers the shutter priority mode is marked as S, whilst in Canon cameras it is Tv.
I hope this has helped you make sense of the key points about shutter speed in digital photography. You now know: what the shutter's role is, how the mechanism works, how it effects the appearance of motion, how it can be used creatively, how it interacts with the rest of the exposure triangle and how to use shutter priority mode.