Depth of field relates to the depth of an area, from beginning to end, within a scene that is in clear focus and the area which is not is obviously the out of focus area. It is determined by three factors - the size of the aperture opening in the lens, the focal length of the lens ( EG: 50mm, 200mm, etc.) and how far you are away from your subject. So basically, the “depth of field” is the area of a scene that appears in focus.
Assume you have your camera on auto, but you have chosen Portrait Mode on the mode dial. When you depress the shutter button half way the auto focus kicks in, after which, we finalise the shot.
Depending on how far away you were from your subject when you took the shot, will determine how much of the area around your subject is in focus. But also how much of the area in front of and behind your subject is blurred, or out of focus.
Let’s have a look at the three determining factors, as mentioned earlier:
1) Aperture size:
The size of the aperture opening determines the amount of light permitted into the lens and the greater the amount of light, the lesser the amount of depth of field you have at your disposal.
By opening the aperture, that is, selecting a larger aperture setting, for example, f5.6, f4, f2.8, etc., or when you select Portrait Mode on the mode dial, you create a large aperture opening, therefore you will have a narrow depth of field.
On the other hand, by selecting a smaller aperture setting, for example, f11, f16, f22, or by choosing Landscape Mode on the mode dial, you have created a smaller aperture opening, providing for a greater depth of field.
Note: The focus control can only adjust the focus within the area that the lens aperture is set at. It cannot control your depth of field.
2) Lens focal length:
A short focal length lens, for example, 20mm or 40mm, will allow for a broader depth of field than a longer focal length lens. EG: 100mm, 200mm, etc. So the depth of field in this case, is measured by how much or how less your lens focal length magnifies the scene.
The closer you are to your subject, the narrower your depth of field. In doing close-up photography you depth of field can be reduced down to a matter of a few millimetres.
So, to be in relative control of your depth of field, simply remember:
To have an increase in depth of field you need to use a narrow lens aperture, a shorter focal length lens and possibly move away from your subject. And the opposite applies if you wish to decrease your depth of field.