Aperture, shutter speed and ISO sensitivity are the three pillars to control the amount of light reaching the imaging sensor and to obtain a correct exposure. For more information about achieving a correct exposure, please take a look at the article "Exposure Value".
But in addition, aperture and shutter speed are the two most important tools of a photographer to influence the look of the image.
The aperture affects the depth of field and the shutter speed controls the motion.
Let’s have a look at the aperture first. The aperture is located inside the lens and can vary its size in order to regulate the amount of light passing through. A larger aperture (smaller f-number) allows more light passing through the lens compared to a smaller aperture (higher f-number).
The illustration below show examples of typical aperture values. As you can see, the opening at f/4.0 is a lot larger compared to f/5.6 and f/8. In photography, a shallow depth of field is achieved by opening the aperture (selecting a relatively low aperture value).
The examples below are taken from the same shooting distance in aperture priority mode. With each shot, the aperture has been stopped down by one full f-stop and the camera automatically adjusted the shutter speed to keep a constant exposure over the entire series of images. When looking at the first image (f/2.8), you will notice that only a very small section of the image is in focus. The background is completely blurred, putting emphasis on the first metal flower of the fence. With each image, the depth of field extends further throughout the whole image. In the last image the entire fence as well as more flowers in the background are captured sharply.
Typical applications for using a shallow and a large depth of field
Shallow depth of field
A typical application for a shallow depth of field is portrait photography. In this case the main subject should be isolated from the background in order to draw the viewer’s full attention to it.
Note: In addition to opening the aperture, a larger focal length and/or a shorter shooting distance will also contribute to achieving a shallow depth of field.
Caution: When working with a very shallow depth of field, it is important not to move the camera back or forth, because any decrease or increase of the shooting distance can lead to important parts of the image being out of focus. The same applies to the subject itself. If the subject is moving towards or away from the camera, it can move out of the focused area. In this case it can be recommended to stop down the aperture and sacrifice the shallow depth of field in favor of important parts of your images being reproduced with sufficient sharpness. Alternatively, you can take several successive single shots or use continuous advance mode in order to select the image with the best focus afterwards.
Large depth of field
A typical application for a large depth of field is landscape photography. In this case the entire image should be captured sharply from the foreground to the background in order to emphasize the vastness of the scene.
Apart from controlling the amount of light reaching the sensor, the shutter speed is used to control motion. Shorter shutter speeds freeze movement, while slower shutter speeds capture movements of fast moving subjects.
Looking at the first image below, you will see that the motion of the water is nearly frozen, due to a fast shutter speed of 1/320s. In the second photo the surface of the water looks smoother due to a relatively slow shutter speed of 1/25s.
Caution: You also have to be careful when selecting slower shutter speeds, because they can lead to motion blur or camera shake. While the first one can be used to emphasize the speed of a subject and to give the photo a more dynamic look, camera shake occurs because the camera was not held steady during exposure. In this case it can be helpful, if the camera or the lens is equipped with an image stabilizer. But also image stabilizing systems have their limitations. Once the shutter speed is too slow, only a tripod or another sturdy surface to place the camera on, can help prevention camera shake.
Typical applications for fast and slow shutter speeds
In the first image, the car is slightly blurry, because its movement has been captured by the slower shutter speed. This gives the image a more dynamic look. If this image was taken with a fast shutter speed, it would look as if the car was parked on the side of the road. In the second image the movement of the car spinning off the race track is frozen and you can see the gravel flying around.
This night shot is an example of heavy camera shake. Due to the low light, the camera needs to select a pretty slow shutter speed in order to capture enough light. In those extreme situations camera shake cannot be compensated by the image stabilization system of the lens or the camera. Only a tripod would help to capture a sharp image in this case.