07 February 2010

Using shutter speed

Water with motion blur This is the third article in my series on taking control of your camera. In this article I'll explain how to control shutter speed, and the effect it has on the photo.

In the introduction to this series, I explained that there are four fundamental controls on a camera. The first of these is shutter speed.

All cameras have a shutter, which prevents light from entering the camera until the moment you take a photo. When the shutter button is pressed, the shutter opens momentarily and exposes the sensor (or film in the pre-digital age) to light. The length of time for which the shutter stays open is referred to as shutter speed, and is measured in fractions of a second. The exposure is, therefore, directly controlled by shutter speed: The longer the shutter is held open, the more light exposes the sensor.

Take a typical shutter speed of 1/60th second. By changing the shutter speed from 1/60th to 1/30th second, the sensor is exposed to twice as much light, i.e. the exposure is doubled. Remember I described "stops" in the previous article? A doubling of exposure equates to +1 stop. Similarly, going from 1/250th to 1/1000th is a quarter of the light, equating to -2 stops.

It might seem confusing, but it's a lot easier to understand in practice. Just remember that if you make the shutter speed slower, the exposure is increased. If you make it faster, the exposure is reduced. As with all exposure controls, the amount by which it changes is measured in stops.

Water with motion blur

Use a long exposure to create a sense of movement in water. This was taken with a shutter speed of 8 seconds.

How to control shutter speed

You don't have to go to full manual control to set shutter speed. Most cameras have a "program shift" setting, usually labelled "P" on the main dial. Think of this as "semi-automatic". It allows you to override the shutter speed by changing it up or down from the default that the camera has chosen automatically. Just check your camera manual for which dial or buttons are used to make the changes.

Although changing shutter speed has a direct effect on exposure, you won't actually see that effect when using the camera in "P" mode. That's because when you change shutter speed in this mode the camera will automatically change other settings to keep the exposure the same. This allows you to set the shutter speed manually wihout having to worry about overall exposure. The reason that you might want to do this will soon become apparent. If you actually want to change the exposure in this mode, to lighten or darken the image, then use exposure compensation as described in my previous article.

Another way to set shutter speed, similar to "program shift," is to use "shutter priority", usually appearing as "S" or "Tv" on the main dial. (Tv is short for Time variant.) This does roughly the same thing as "P" mode, except that the initial shutter speed is set by you not the camera, and is remembered from one photo to the next, even if you turn off the camera.

Choosing the right shutter speed

Blur is the photographer's nemesis: we call it a "still" camera for a reason.

I previously explained how shutter speed has a direct effect on exposure, but changing shutter speed also has a very important creative effect on your photos. It is for this reason that we often want to set the shutter speed manually.

The longer the shutter is held open - i.e. the slower the shutter speed - the more a moving subject will be blurred. This is called "motion blur," and is simply caused by the subject moving whilst the exposure is made. Motion blur can be a good thing if you use it to artistic effect. The photo above of a waterfall was taken with a long exposure (a slow shutter speed) of 8 seconds in order to blur the water's movement.

Most of the time, though, blur is the photographer's nemesis: we call it a "still" camera for a reason. You'll normally need to ensure that you set a shutter speed fast enough to freeze any movement of your subject, and so eliminate blur. This is especially true of action shots, whether that's a bird in flight or Usain Bolt running the 100m. The following table gives a rough guide of what minimum (emphasis on minimum) shutter speed you'll need to get sharp results:

Person walking or talking1/90th second
Person running or cycling1/250th second
Cars, Action1/500th second
Explosive action1/1000th second or faster

The photo below was taken in Ngorogoro Crater, Tanzania, using a 1/500th second shutter speed to freeze the action. Even at this speed there is some motion blur in the legs and tails.

Fast shutter speed

A shutter speed of 1/500th second was used to freeze these two monkeys in a fight.

Remember, though, that all rules are made to be broken. Sometimes a little motion blur adds to the sense of movement in a photo, so don't be afraid to experiment.

Avoiding camera shake

The other important reason to set shutter speed manually is to avoid camera shake. This occurs if the camera moves whilst the shutter is open. A sure sign of camera shake is when everything in a photo looks blurred, not just a moving object. Camera shake should generally be avoided like the plague, and is the reason landscape photographers take to carrying heavy tripods up mountains. A platform like a tripod means that the camera can be held steady and the problem of camera shake (mostly) goes away. If you don't have a tripod then a fence post, rock or wall can help.

Hand-holding the camera

Photography isn't exactly zen meditation, but you still need to breath properly.

When hand-holding the camera, eliminate camera shake by using a fast shutter speed. For normal shots, a shutter speed of at least 1/60th second is needed. For telephoto shots, you'll need a faster shutter speed because the telephoto effect "amplifies" the camera shake. 1/250th or even 1/500th may be required. Conversely, with wide-angle shots you can get away with a slower shutter speed of around 1/30th second, because the camera shake is less apparent.

Even if you use a fast enough shutter speed, it is still important to hold the camera properly. This can make a surprising difference.

  1. Stand with both feet firmly planted and shoulder-width apart.
  2. Hold the camera with both hands, using your left hand to support the underneath of the lens. (Even for a compact, it's effective to rest the lens on the fingers of your left hand.)
  3. Photography isn't exactly zen meditation, but you still need to breath properly. Breath out gently (or even hold your breath) the moment you press the shutter and you'll reduce any movement.
  4. Finally, avoid the novice mistake of poking the shutter button with your finger so that the camera moves at the critical moment: you should always gently squeeze it.

Expert tip: For SLR (single lens reflex) camera users, when hand-holding the camera a common rule of thumb is to shoot with a shutter speed at least one over the focal length of the lens. So, for instance, a focal length of 90mm requires 1/90th second or faster. (If you are unfamiliar with focal length I'll explain it in my next article.) This applies to effective focal length, so if you have an SLR with a "cropped" sensor, you'll need to know and apply the crop factor (or a safe bet is to shoot at least one stop faster than the above rule of thumb).

Image stabilisation

Many cameras and/or lenses today have image stabilisation (IS). This uses clever technology to help eliminate camera shake, and can allow you to shoot at one or two, even three stops slower shutter speed. That can make a big difference to where, when and how you can shoot. It is no substitute for good technique, however, and IS cannot compensate for motion blur (described above), only camera shake. If you want to shoot with longer exposures, such as for the waterfall photo above, then you have no option but to use a tripod or similar support.

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