Depth of field is the limitation of perceived sharpness within a photographic image. The greater the depth of field, the more of the image from front to back that appears sharp and in focus. An image that is said to have a shallow depth of field has a short and more specific depth of sharpness and area in focus.
In photography, careful use of this concept can be a very powerful tool. It can force viewers to focus only on the sharp areas by utilizing a shallow depth of field. This concept is most often seen in portrait photography, where the photographer uses it to create a background that is blurred so that the subject stands out as sharp. Our eyes are not comfortable viewing images that are out of focus. Our eyes then tend to look at the parts of an image that are sharp and in focus. Our gaze will then focus on that part of the image, rendering the other unsharp parts of the image as blurry and not worthy of our attention. This use of a shallow depth of field is particularly well suited to portraiture. As long as the eyes are sharp, most other things can be forgiven if they aren’t pin sharp. People and animals tend to look at the eyes first, and so the eyes really need to be sharp in nearly all portrait photography.
Landscape photography is generally at the opposite end of the spectrum as far as the depth of focus we want to achieve. The vast majority of landscape images require a very deep depth of field, with all the elements in the scene sharp and in focus. This is due to the fact that landscapes generally are trying to emulate an actual scene as we see it. Viewers are usually drawn into the image by its great depth of field, and they tend to look at more details.
Depth of field is controlled in two ways. The most common way to control it is by shooting in aperture priority mode or setting the aperture in manual mode. The smaller the aperture (the larger the number, i.e. F22), the greater the depth of field. The larger the aperture, (the smaller the number like F2.8), the shallower the depth of field. The apertures in-between have a depth of field that is directly proportional to the aperture selected. Most lenses also have a sweet spot where the sharpness is best. This is due to the diffraction of the light as it passes through the lens elements.
The second means of controlling the depth of field is by using a camera or lens that enables the lens to be tilted forward or back. This enables the focal plane of the lens to be more inclined to the plane of focus of the subject. This can provide a much better depth of field without a change of aperture. It is one of the major reasons for using bellows-type cameras or tilt-shift lenses. With such a camera or lens, one can have a huge level of control over the depth of focus at any aperture.
Depth of field is also dictated by the focal length of the lens, and the camera format for which the lens is used. For instance, a wide-angle lens always has a much greater depth of field than a telephoto lens. A very wide-angled lens such as a 14 mm lens has a depth of field so great that it almost doesn’t require focusing. On the other hand, a 600 mm telephoto lens has an extremely shallow depth of field when zoomed all the way. Unless you were focusing on a subject far away at 600 mm, the depth of field will always be very limited.
There are also macro lenses, which are made to be able to focus very close to objects. Once you start moving in and start focusing very closely, the focal plane will become extremely shallow. The closer you get to the subject, the focal plane can become razor-thin. In extreme close-ups, even the slightest movement will cause the image to go out of focus entirely. Although you wouldn’t use a macro lens for landscape photography, it does illustrate the depth of the focal plane and how it works.
So remember with landscapes, you want to use a smaller aperture (higher number) to increase the area that is totally in focus in your image. The greater depth of field will allow you to capture all the elements of the image in sharp focus. You will probably need to use a tripod though because as your aperture decreases your shutter speed will need to increase to get a useful exposure. You will need to increase the shutter speed yourself if you are shooting totally manual. If you choose aperture priority, the camera will choose the shutter speed for you after metering the brightness of the scene. Unfortunately, although modern digital cameras are very good, sometimes they still get it wrong. You should view the histogram on your view screen and make sure the histogram is not jammed against either end of the scale.
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