Depth Of Field

Depth of field is, in simple terms, the distance between the nearest and furthest objects in a photograph that appear to be in focus.

Why is depth of field important?

Depth of field defines the "feel" of a photograph. A shallow depth of field will direct the eye onto particular details within a photograph and make a subject stand out against a blurred background. They also prevent the eye from being distracted from a busy background that is not relevant to the subject matter. Portraits or close up shots of flowers or insects tend to use shallow depths of field. A larger depth of field ensures more elements within the photo are in focus. Landscape and cityscape shots typically use a larger depth of field, especially when you want to lead the eye through a scene from front to back, say in a perspective image.

How do you control depth of field?

The depth of field depends on three factors: - The f-stop number, or aperture size. - The distance from the subject. - The focal length of the lens itself. The smaller the aperture (the larger the f-stop number), the larger the depth of field, and vice versa. The greater the distance between the lens and the subject, the larger the depth of field. The shorter the lens focal length, the larger the depth of field. For example, a 28mm wide angle lens has a larger depth of field than a 75mm lens at the same aperture setting and shooting distance.

What should you be careful of?

Using a wide angle, short focal length lens close to a subject to reduce the depth of field, say for a portrait shot, can sometimes result in a distorted and unflattering image. To avoid this and at the same time achieve a blurred background it is better to use a longer focal length with a large aperture (low f-stop) setting. Be aware also that the greater the distance from the subject, the greater the potential for a drop in sharpness, especially in dusty or humid conditions. Increasing focal length with a zoom lens to capture a close-up of a subject from a distance also risks magnifying the effects of "camera shake", especially in low light conditions. Image stabilisers help greatly here, but it is better to get closer to the subject and minimise use of the zoom to guarantee a sharper image. Using small aperture (high f-stop) settings for maximum depth of field can also result in underexposure or blur due to camera shake in low light conditions. First try increasing the aperture size slightly and check the exposure. If not, consider either a higher ISO setting (higher light sensitivity) or use of an image stabiliser or tripod.

Auto or manual control?

Most cameras have "portrait" and "landscape" functions to achieve shallow and infinite depths of field. In order to develop your photography skills, it recommended that you try avoiding the auto settings and instead use the aperture priority setting, often labelled 'A' or 'Av' on the camera function dial. This allows you greater control over the outcome of the photo, plus you are more likely to get a better feel for what does and doesn't work.

How can you calculate depth of field?

Here is a link to a good site which enables you to calculate the depth of field for various camera models, focal lengths, aperture settings and shooting distances: http://www.dofmaster.com/dofjs.html