Image sharpness is one of the most important creative tools for photographers. However, what is in sharp focus or out of focus in an image is not only decided by the focus itself. The aperture also has a significant influence in this area. The aperture controls the depth of field, or in other words the zone of spatial depth, in which objects in an image appear sharp to the human eye.
The size of the depth of field is directly dependent on the aperture. The following applies: the larger the aperture the smaller the depth of field – and vice versa. Therefore, if the aperture is fully open then the depth of field is significantly smaller than when the aperture is closed.
There is a very quick way to practice changing the field of depth using a simple method. Setup the camera with a short telephoto focal length (e.g. 60 mm or 100 mm) on a tripod and focus on an object at a distance of about two to three metres. When you now look into the viewfinder, the sharp main subject should be clearly emphasised in front of the blurred background image. Set the camera to aperture priority (A/Av) and select a large aperture (= small f number). The aperture will only close slightly during exposure, meaning the depth of field is particularly large. Shoot a series of photos, reducing the aperture size by one f number with every exposure – for example f/2.8, f/4.0, f/5.6, f/8 and so on. The camera will automatically adjust the shutter speed for correct exposure. In the case of small aperture sizes, the exposure time may be a number of seconds under certain circumstances.
Aperture, focal length and focussing distance
Now study the images. Pay particular attention to the details in the background, which should stand out more clearly with every image. Which image has the optimal relationship between foreground and background?
The depth of field is not only influenced by the aperture but also by the focussing distance (distance between the camera and object) and the focal length. The following general rules apply in this process:
You can observe this using the example of portrait photos: Achieve the best cropping effect for the main subject using a fast telephoto lens.
Aperture range from f/2.8 to f/11: The smaller the aperture, the more details will be visible in the background.
Photos: Christian Schaffrin
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