Although the idea of depth of field can seem confusing it is actually a relatively simple concept once you understand the interaction of the 3 variables that determine how much of your image appears acceptably sharp.
Depth of Field refers to the area of focus that appears acceptably sharp in front of and behind the single spot of perfect focus.
That area of acceptable sharpness will vary depending on the following 3 factors:
- Aperture (lens opening) diameter
- Focal length of the lens
- Distance from camera to subject
Changing any one of the above factors will have an effect on the depth of field.
The choice of aperture will directly determine the diameter of the beam of light entering the lens.
A wider beam (obtained with f2.8 for example) will produce shallower depth of field. A narrow beam (obtained with f16 for example) will produce deeper depth of field.
Deep Depth of Field (35mm @f11; distance about 70cm from camera)
Shallow Depth of Field (35mm @f1.4; distance about 70cm from camera)
Lens Focal Length
The choice of lens focal length will determine the magnification of the subject and in turn helps define the area of acceptable sharpness.
The longer the focal length (for example 100mm) the shallower the depth of field (area of acceptable sharpness) will appear.
The wider (shorter; for example 24mm) the focal length the deeper the depth of field will appear.
Camera to Subject Distance
Increasing or decreasing the distance of your subject from the camera will affect the depth of field (area of acceptable sharpness).
The closer the camera is to your subject the shallower the depth of field will appear (in relation to lens choice and aperture choice). The inverse is true as the subject is placed farther from the camera - depth of field will appear deeper.