Depth of field

Depth of field is the band of the image that’s in focus. Although there is only one true focus point, there will also be a band across the image that’s acceptably sharp, with blurring in front and behind. The band is narrower for shallow depth of field and wider for deep depth of field i.e. more of the image is in focus (acceptably sharp) for deep depth of field and more is blurred for shallow depth of field. Depth of field is generally one third in front of the true focal point and two thirds behind the true focal point.

Depth of field is affected by:

• the aperture (the amount of light allowed into the camera, small f number = larger hole/aperture, more light and shallower depth of field)
• the focal length of the lens (the ability of the lens to magnify the subject, longer focal length = more magnification and shallower depth of field) and
• the distance from the subject (closer to the subject = shallower depth of field).

All of these can be used to manage depth of field when taking a photo. Adjusting aperture is the usual way to affect depth of field but sometimes may not be possible; for example in poor light when slow shutter speed may cause blurring. Sometimes, changing the distance from the subject or the focal length may be helpful to control the depth of field.

In summary, to increase depth of field:

• narrow the aperture (larger f number e.g. f22)
• shorten the focal length (e.g. 12mm wide angle lens)
• move further away from the subject.

To decrease depth of field:

• widen the aperture (smaller f number e.g. f2.8)
• lengthen the focal length (e.g. 200mm telephoto lens)
• move closer to the subject

Shallow depth of field is generally used to help the subject stand out from the background; for example, for portraits or wildlife. A zoom or macro lens with a small aperture is often used, close to the subject.

Deep depth of field is generally used when all of the scene is required to be in focus; for example, for landscapes. A wide angle lens with a large aperture is often used, standing back from the subject.

Portfolio

Same focal length and distance from subject, different apertures

These images were all taken with a 45mm lens, with the camera on a tripod (because of the low shutter speed required with a small aperture) in the same position. Aperture setting was used, with the camera slowing the shutter speed (1/15 sec, 0.5 sec and 2 secs) as the aperture was closed (f2.8, f9.0 and f18) allowing less light into the camera (see the images for details). ISO was raised slightly to 400 because the light was poor. I focussed on the face of the small robin at the front.

In the first image with the largest aperture (f2.8), the worktop in the foreground is blurred, as are the 2 robins towards the back, and the image shows a narrow depth of field. In the second image with the medium aperture (f9.0), foreground and the middle robin are now also in focus. In the third image with the smallest aperture (f18.0), the robin at the back is also in focus and the image shows a deep depth of field.

Same aperture and distance from subject, different focal lengths

These images were all taken using a medium aperture of f8.0, with the camera on a tripod in the same position. Aperture setting was used with the camera adjusting the shutter speed as necessary to allow sufficient light into the camera for correct exposure. The ISO was set at 200 because the room was quite light and I was using a tripod to avoid blurring. I used several different lens (a 60mm macro, a 12-40mm zoom and a 9-18mm wide angle lens) to vary the focal lengths, and I focussed on the centre of the flower each time.

In the first image using the longest focal length (60mm), the radiator in the background and the leaves in the foreground are blurred and the depth of field is narrow. In the third image using the shortest focal length (12mm) the majority of the image is in focus and the depth of field is deep. The central image (focal length 30mm) shows a depth of field in between the 2 extremes, with some blurring of the wall and radiator at the back and the table or leaves in the foreground, but most of the flower in focus.

Same aperture and focal length, different distances from the subject 

These images were all taken with a 45mm lens, and a medium aperture of f8.0. The camera was on a tripod, which was moved away from the subject each time. Aperture setting was used with the camera adjusting the shutter speed as necessary, and an ISO of 200 was used because I was outside and the light was good. I focussed on the large stone at the front.

The depth of field is narrow in the first image, the shortest distance away, and the corner of the brickwork at the front and everything from the rear of the largest stone to the back of the image is blurred. In the second image, more of the stones and the brickwork is in focus but much of the background is still blurred. More of the third image is in focus, when I was standing further away.