#Depth Of Field
Depth of field, sometimes referred to as DOF, refers to how much of an image is in focus, specifically the distance between the nearest and farthest in-focus parts of an image. Images with a shallow depth of field exhibit a lot of out-of-focus area (blurred foreground and/or background) while images with a deep depth of field can have nearly everything in focus. Depth of field is related to both the lens' aperture, and the size of the image sensor. Larger apertures (smaller f numbers) will leave less in focus - they give a shallow depth of field. Smaller apertures (larger f numbers) leave more of an image in focus - a deeper depth of field. Larger sensors also make it easier to blur a foreground and/or background at a given aperture, while a smaller sensor makes it easier to get everything in focus, at a given aperture.
Technical details of Depth of Field
Due to the fact that modern digital cameras come in all different sizes, some would argue that understanding and mastering depth of field has become more difficult. However, thanks to the fact that the results of experimentation are instant on digital cameras now, it is easy to get a firm grasp of depth of field rather quickly.
How Aperture affects Depth Of Field
A large, bright lens aperture lets a lot of light into the camera and onto the image sensor, however such a big aperture also results in a very thin plane of focus, and a lot of foreground and/or background blur.
A small, tight, dark lens aperture lets less light into the camera, but because it is smaller, it focuses the light very sharply for a greater depth that extends in front of and behind the actual focus distance.
How sensor size affects Depth of Field
The size of an image sensor, whether digital or film, affects depth of field in a similar way to a lens aperture. This is because depth of field is a product of both the lens aperture and focal length, plus the sensor size relative to that aperture and focal length.
A 50mm f/2 lens on a full-frame camera has a 25mm aperture, whereas a