Apertures and Depth of Field explained

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When you focus a lens on a subject only one distance is truly in focus but the eye perceives a range of acceptable sharpness in front of and behind the subject. The range of acceptable sharpness is called Depth of Field (DOF).

If a lens' aperture is opened up it produces shallow DOF. As you stop the lens down the DOF increases.

This is a major creative tool in photography

Aperture Priority Mode

In Aperture Priority Mode ('Av' on Canon, 'A' mode on Nikon) the photographer selects the aperture to produce the desired DOF, the camera then automatically sets a shutter speed to give "correct" exposure.

Apertures can of course also be set in Manual Mode. In which case you set the aperture then meter the subject and manually adjust the shutter speed.

Depth of Field Preview

SLRs are designed to give as bright a viewfinder image as possible so the aperture remains wide open until the very moment of exposure. Therefore Depth of Field is shown at its minimum. To gauge the true DOF many cameras are equipped with a DOF Preview button. When the preview button is pressed the aperture is stopped down to the actual f-stop selected to show the true DOF.

Apertures & Depth of Field

Shallow depth of field can be used to separate a subject from its background drawing more attention to the subject. You will often see this effect in portraiture where a wide aperture e.g. f/2 has been used to make the sitter stand out from a blurred background.

Conversely you may want to show the subject in context with it's surroundings. For example if you are in an artist's studio you may choose to show the artist with his or her artworks on the wall as well as brushes and paints in the foreground all in focus. This requires extensive DOF and means using very small apertures e.g. f/22

Generally you will want extensive DOF for landscapes with everything sharp from foreground to background. You will need to stop the lens down to a very small f/stop and therefore you will need to use a slower shutter speed to produce enough exposure. In turn the longer exposure will require the use of a tripod (or other means of steadying the camera).

Depth of Field extends approximately one third in front of the point of focus and two thirds behind it. So to maximize the DOF focus one third of the way into the scene and stop down to a small aperture.

Depth of Field is also affected by the distance from the subject;
The closer the subject the shallower the DOF, when doing extreme close-up photography you will find just how critical focus becomes. If you are trying to photograph a bee on a flower you will find that if the bee's eye is in focus, its backside is out of focus!
With a more distant subject DOF will be much greater.

The focal length of the lens in use will also influence the DOF. Wide angle lenses (e.g. 17mm) produce images with more DOF. Telephoto and tele-zoom lenses (e.g. 300mm) give much narrower DOF.