Depth of Field (DoF), commonly referred to as background blur in practical scenarios, is the range of distances in front of and behind the camera focus point where objects appear reasonably sharp in the image. In optics, especially for video recording and photography, it describes the spatial distance range that can form clearly‑focused images. A lens can only converge light sharply at one fixed distance; objects further from this point gradually turn blurry. Within a certain distance interval, however, image blurriness remains imperceptible to human eyes. This interval is defined as depth of field. Certain shooting scenarios such as landscape photography benefit from large depth of field, while portrait photography frequently employs shallow depth of field for compositional effects.

Depth of field is primarily determined by object distance, lens focal length, and lens aperture value (aperture size relative to focal length). Except for close‑range shooting, depth of field is governed by object magnification and lens aperture value. With a fixed aperture, higher magnification — achieved by moving closer to the subject or using a longer‑focal‑length lens — reduces depth of field. Lower magnification increases depth of field. At fixed magnification, increasing the aperture value (stopping down / closing the aperture) enlarges depth of field; decreasing the aperture value (opening up the aperture) reduces depth of field.

When focusing at the hyperfocal distance, depth of field extends from half the hyperfocal distance all the way to infinity. This yields the maximum achievable depth of field for a given aperture setting.

Thanks to advances in digital imaging, image sharpness can be adjusted via post‑processing on computers, which also enables simulated modification of depth of field in post‑production.

Depth of field refers to the relatively sharp imaging range in front of and behind the camera focus point. Subjects within this range appear sharp, while objects located before or beyond this range look blurry.

Relationship Between Image‑sensor Size and Depth of Field

Under identical conditions (equivalent focal length, focus distance, aperture, circle‑of‑confusion diameter, and aspect‑ratio proportion), smaller image sensors produce greater depth of field. The relationship is roughly reciprocal: the ratio of depth of field approximates the inverse ratio of the long‑edge dimensions of image sensors. For instance, doubling the sensor size halves the depth of field. Variations in depth of field represent the most direct visual impact brought by image‑sensor format.

Achieving Maximum Depth of Field

For the same camera, a smaller aperture delivers larger depth of field. Across different cameras, a smaller image‑sensor size yields larger depth of field.

Depth‑of‑Field Scale

Many small‑to‑medium‑size lenses (e.g., the 35 mm lens illustrated) are equipped with depth‑of‑field scales calculated based on focal length and aperture values. These scales display measurements in both meters and feet. When a distance mark aligns with the central white index line, objects at that distance are sharply projected onto the focal plane. On either side of the white index line below the scale are aperture‑value markings. Once the lens is set to a specific aperture, the depth‑of‑field range corresponds to the two distance readings lined up with that aperture’s markers.

Hyperfocal Distance

Hyperfocal distance is the distance from the lens center to the focus point when depth of field extends to infinity. Focusing the camera at hyperfocal distance achieves maximum depth of field for a given f‑stop. Focusing beyond the hyperfocal distance does not extend the far depth of field (which already reaches infinity), yet it shortens the near depth of field and thus shrinks the overall depth‑of‑field span. Some photographers regard this as wasting available depth of field; nevertheless, the object‑field method provides justification for this practice. If a lens features a depth‑of‑field scale, hyperfocal distance can be set by aligning the infinity symbol with a selected f‑stop mark. For example, to set hyperfocal distance at f/11 on the aforementioned 35 mm lens, line up the infinity marker with the “11” marking. Focusing at hyperfocal distance is a special case where the far depth of field stretches out to infinity.

At f/5.6 with short working distance, depth‑of‑field layering can be observed: the background is heavily blurred, figures further back are moderately blurred, and the central subject remains sharply rendered.

Selective Focus with Limited Depth of Field

To capture images with limited depth of field, the opposite principle applies: cameras with larger image‑sensor sizes deliver more pronounced shallow‑depth‑of‑field effects.

Depth‑of‑Field Formulas

Hyperfocal Distance

Let f = lens focal length, N = lens aperture value, C = circle‑of‑confusion diameter for the sensor format. Hyperfocal distance H is defined by the formula:

Medium‑to‑Long Distances

Let s = object distance from camera to focused subject. \(D_N\) = distance from camera to near limit of depth of field. \(D_F\) = distance from camera to far limit of depth of field. When s is much greater than the lens focal length:

At hyperfocal focusing (\(s=H\)):

Total depth of field:

If \(s \ge H\), the far depth of field extends to infinity. In other words, sharp imaging is obtained only for objects lying beyond the near depth‑of‑field limit.

Substituting H, depth‑of‑field can be rewritten as:

Accordingly, for a given sensor format, depth of field depends on three parameters: lens focal length, aperture value (aperture size), and object distance.

Close‑Range Shooting

The formulas above generate substantial errors when object distance s approaches the focal length. Hyperfocal formulas are not suitable for close‑focus scenarios. Depth‑of‑field calculations are better expressed using magnification. Let m denote magnification. Where object distance is far smaller than hyperfocal distance:

At fixed magnification, depth of field becomes independent of focal length. That is, images shot at identical magnification show nearly equal depth of field regardless of lens focal length. This holds true only when object distance is less than hyperfocal distance.

Foreground and Background Bokeh

Blurring occurring in front of the focus plane is called foreground bokeh. Blurring behind the focus plane is known as background bokeh.

Aperture’s Influence on Depth of Field and Circle of Confusion

Point (2) forms a sharp image on the image plane (5). Points at other distances such as (1) and (3) project blurred spots larger than the allowable circle‑of‑confusion. Reducing aperture size (4) shrinks the circle‑of‑confusion for out‑of‑focus points. Blur becomes less noticeable, and those points appear to fall within the depth‑of‑field range.

On the depth‑of‑field scale, aperture is adjusted on one side with corresponding depth‑of‑field markings on the other. Aperture‑value colors match color‑coded depth‑of‑field indices on the focus ring. For instance, setting aperture to yellow‑marked f/11 gives a depth‑of‑field range bounded by the two yellow index lines above, indicated in both meters and feet.

Note: Mathematical formula placeholders are retained as in the original Chinese text. Bokeh is adopted as standard technical term for optical blur effect.