Depth of Field Explained

Depth of field is a fun concept to learn when you are first entering digital SLR photography.
When you focus your lens on a subject, that subject becomes the focal plane. The focal plane is a very narrow plane which is surrounded by depth of field. Depth of field has both a near limit and a far limit. Whatever subject matter falls within the given depth of field, will appear as focused in your image.

Focal Plane, Near and Far Limits of DOF diagram

When the depth of field is not large enough, subject matter can quickly fall outside of the depth of field and become blurred.

Depth of field can be expanded by three methods.
1) Stop your lens down from a wide aperture to a narrower aperture. This will increase depth of field.
2) Move further back from the subject you are placing the focal plane on. By reducing magnification of the subject matter, you increase depth of field.
3) Reduce zoom on zoom lenses… Depth of field can be increased by using a wider angle on your zoom lens. This is the same principle as #2, only helps to achieve similar results without actually moving.

When reducing aperture, your lens will initially perform sharper than when it was wide open. Generally speaking any aperture narrower than F8 will begin to have a negative impact on your image. Depth of field continues to be expanded, however when a lens is stopped down to such a narrow aperture as F11 or narrower, you will begin to suffer from diffraction of light. Diffraction will actually cause a loss of detail and softening of your image – so it is best to avoid it.

The following are images intended to show the expansion of depth of field by narrowing of aperture and also an increase in distance to subject. They were focused at the toy dogs nose, pay particular attention to the glass reflections because it illustrates where the DOF is expanding quite nicely.

Below are depth of field example photos. You can quickly switch between the different aperture values by rolling your mouse over the different aperture values below the image.

The following are also estimated distances of depth of field surrounding the focal plane at each aperture.
Depth of Field at F/1.8 approx .95 front, .98 rear
F/2.8 DOF 1.49 front, 1.58 rear
F/4.0 2.09 front, 2.26 rear
F/5.6 2.9 front, 3.25 rear
F/8.0 4.02 front, 4.72 rear
F/11 5.5 front, 6.9 rear

Now we also will go back to F/1.8 and move further away from the subject, which will also have the effect of increasing DOF. Notice how on the original F/1.8 shot the dogs right foot (left to viewer) appears out of focus. Contrast this to the F/1.8 shot with a further shooting distance.

F/1.8 3.26 front, 3.49 rear

Depth of Field Comparison