By Dave Hecei
Now that we ve learned a bit about the aperture, it s time to learn about shutter speeds. While the aperture controls how much light comes through the camera lens, the shutter controls how long the sensor (or film) is exposed to light. Remember that it is the combination of aperture and shutter speed that controls exposure, which is dependent on the ISO setting.
The Shutter Speed. As the name implies, we are talking about speed. When you press the shutter button on your camera, the sensor is exposed to light for only a split second. Camera shutter speeds are listed by a whole number x . In actuality, they are a fraction of 1/x. If your camera says it is set to 125 it is actually 1/125th of a second.
Back in the days of mechanical cameras, shutter speeds normally went from 1 full second to 1/1000 of a second. Starting at 1 second the next step faster was 1/2 sec, then 1/4, and then 1/8. The progression, each one full stop, continued to 16, 30, 60, 125, 250, 500, and then 1000, which of course is 1/1000 of a second. With the shutter it s a bit easier to see that each stop really is letting half (or double) the amount of light through to expose the sensor (or film). The change from 1/30th to 1/60th is half, as is 1/125th to 1/250th. Much easier than knowing that f/2.8 to f/4 lets half the light through the lens.
Now that we ve run the numbers, it s time to learn what the shutter does to the image. It is possible to choose a combination of f/stop and shutter speed to give you absolutely perfect exposure, but still give you a poor image.
Last time we talked about how the aperture setting also determines the area of sharpness in an image what is called depth-of-field. The shutter speed you choose will also affect the image, but in a different way. Since the opening of the shutter is timed, it can open and close very quickly (a fast shutter speed), or open and close for a long period of time (a slow shutter speed.)
The subject matter you are trying to photograph will determine the shutter speed you should choose. The focal length of the lens also needs to be taken into consideration. The ideal shutter speed is one that is fast enough to stop the motion in the scene you are shooting.
If you are shooting a landscape, and have the camera mounted on a tripod, you can actually shoot with very slow shutter speeds. As long as there is no movement in the scene, like wind blowing any foliage, the image will be sharp. But then there are times when you may want to blur the subject. One scene that I love to photograph is flowing water. By using a tripod and a slow shutter speed, the flowing water will take on a cotton candy look to it.
If you are handholding the camera, then you need to shoot with a fast shutter speed. This is necessary to eliminate any camera shake that you the photographer can induce. If you are shooting fast moving scenes, sporting events, racing cars, or even running children, then you want to shoot with a fast shutter speed. The speed is also dependent on the focal length of the lens you are using. If you have a wide or normal lens, then you don t need as fast of a shutter as you would for a telephoto lens. Telephoto lenses (along with macro lenses) magnify the subject but this also magnifies any camera jitter.
When handholding a camera, the rule of thumb is to set the shutter speed to 1/ focal length or faster. An example would be shooting with a 300mm lens you need to set a shutter speed no slower than 1/300th of a second. If the subject you are shooting is moving very fast you can pretty much double this number. Remember that the faster the shutter speed, less light is hitting the sensor. This means that you may need to use a wider aperture (like f/4 or f/2.8).
This might not be possible since many 300mm lenses have an aperture starting at f/5.6. If you cannot set a fast enough shutter for the aperture of the lens, then you will have to increase the ISO setting on the camera. The ISO setting changes the sensitivity of the sensor. If you are shooting at ISO 200 you can change this to ISO 800, which should allow you to shoot the faster shutter speeds necessary for moving subjects.
There will be times when you want to shoot with a slower shutter speed. This can be used to intentionally blur the subject, thus invoking a feeling of motion, but can also be used for a blurring effect with subjects like waterfalls, clouds, sparks, and waves. Just remember that when shooting longer shutter speeds, any camera movement will also blur the image. Most of the time you will be shooting slow shutter shots on a tripod. I always recommend buying a good tripod. I m not talking about the $40 video tripod you can get at the big Mart stores. I m talking about a more pro level tripod. They are available from Bogen (Manfroto), Gitzo, or Giotto. A good tripod will set you back about $200, but it will last you decades. I still use the same Bogen from 1986 and plan to use it for many more years.
Together at Last. When shooting a scene with a modern DSLR, the camera will look over the entire scene and choose what it thinks is the proper exposure. Most of the time this will be pretty accurate. There are times when this won t be true, but we'll go over that some other time. What you, as the photographer, need to do is look at what the camera has set for aperture and shutter speed and either use it or override it.
If you want to show more motion in a scene you adjust the shutter speed down to a slower setting. As you do this you will see the camera compensate by choosing a higher f/stop setting. If you need to use a faster shutter, then as you move the setting up and the camera will open the aperture to a lower f/stop. The same is true when you want to work the depth-of-field in a scene. If you want more DOF you need to set the aperture higher, say f/16, then the camera needs to lower the shutter speed. If you want less DOF then a lower f/stop needs a higher shutter speed.
The idea here is that you are in control of the camera. It s up to you to know that you want to blur the subject or make it sharp, or that you want to make the background out of focus or sharp. Knowing what shutter speeds and apertures to use and how they affect the image you are making is an important part of photography.