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Depth Of Field

Depth of Field (DOF) refers to the zone of apparent sharpness both in front and behind the point where the lens is actually focused. I say apparent because the sharpest part of the image will be the spot (distance) at which the lens is focused. Areas both in front and behind the point of focus that are covered by the DOF may appear clear, but will not be quite as sharp as the point of actual focus.

Of course you would normally select the most important part of the image, your primary focal point and the spot to which you want to draw the viewer’s attention, as your focus point. The eyes in a portrait are a classic example.

Did you know that there are 3 factors that control Depth of Field (DOF)? They are as follows:

  • The Focal Length of the lens employed
  • Aperture selected
  • The Camera-to-Subject distance

Focal Length

Let’s say, like me, you have a Canon 24-105mm L series IS lens. In practice most folks consider focal lengths below 50mm to be wide-angle and above 50mm to be telephoto on a full frame camera. In fact its probably more accurate to say that 35mm and below is wide angle, while 85mm and above is telephoto.

If I was to take two photographs in such a way that the only thing that changed between one frame and the next was the focal length (e.g. 24mm for the first frame and 105mm for the second), then we could say that the wide-angle lens produced a larger DOF, at a given aperture, than the telephoto.

Of course the composition would be completely different due to the significantly greater angle of view provided by the wide-angle lens and the greater magnification of the telephoto lens. However, if you could move backwards, with the lens set to 105mm, until you were able to include in your viewfinder exactly what you had previously with the lens set to 24mm, you would produce the same DOF with each focal length.

 

Aperture

You can think of a lens aperture a little like the iris of your eye. The iris is the color part of your eye. It’s a muscular diaphragm that surrounds the pupil. The iris regulates the size of the pupil, by expanding or contracting, thereby controlling how much light reaches the eye.

When its dark the iris opens up allowing more light to reach the eye, so that you can see the world around you. When it’s bright the iris closes down to protect your eye from the excessive amount of available light.

The aperture in your lens works in a similar manner to your eye’s iris. In the case of the lens a series of overlapping metal strips are adjusted to alter the physical size of the opening (aperture). The average lens has a range of apertures from which to choose and an italics f is used as an abbreviation for the word Aperture.

The most logical methods by which to control Depth of Field are through your camera’s Aperture Priority (‘A’ on a Nikon), also referred to as Aperture Value (‘AV’ on a Canon), or Manual (‘M’) exposure modes.

There are so many Apertures from which to choose that many folks give in to the idea that “it’s all too technical” and use their cameras on Program (‘P’) or other similarly automated exposure modes. Actually it’s really simply, and a little bit of fundamental knowledge and practice will allow even an absolute novice to begin to take control back from the machine. And of course it’s then that the creative juices start to flow.

The wider the aperture the more shallow the resulting Depth of Field. Conversely, a narrower aperture will produce a larger the Depth of Field.

Isolation or Inclusion

A shallow DOF allows you to isolate your subject within the frame. This is a favourite technique employed by wedding/portrait photographers seeking to keep the viewer’s attention on the subject’s face. Under such circumstances an aperture of f 4, or thereabouts, is often considered desirable to achieve a shallow DOF. By de-emphasising any surrounding elements, the shallow DOF will provide the impression of enhanced subject sharpness.

 

Canon 5D Mark II camera with 85mm f1.2 L series Aspherical lens

Canon 5D camera with 85mm f1.2 L series Aspherical lens

Frequently employed in head and shoulder portraits of individuals or couples, I chose to use a shallow DOF to make the front horse the dominant focal point. The light was lovely, producing deep shadows and luminous highlights. I composed the image in such a way to emphasise the shapes, tones and delicate textures inherent to the subjects. I added a touch of fill-flash to bring the eye of the foreground horse to life.

Describing the Scene in Full

A large DOF is commonly employed in landscape photography to provide the viewer with as much, clearly defined information as possible.

A large DOF, achieved with an aperture around f 22, will help keep important foreground elements relatively sharp. Of course such a narrow aperture allows very little light to reach the sensor. To achieve an accurate exposure a slow Shutter Speed is required, particularly indoors or at the edges of the day. And, if the camera is not kept still during the exposure, the image will be adversely affected by camera shake and, as a consequence, DOF will be lost.

This was exactly the challenge I faced when photographing this beautiful Church interior. What’s more I didn’t have a tripod with me at the time. So a steady hand and slow, deliberate breathing helped make a sharp picture despite the slow Shutter Speed of 1/8 second.

 

Canon 5D camera with Canon 24mm f1.4 L series lens

Canon 5D camera with Canon 24mm f1.4 L series lens

Camera-to-Subject Distance

Actually a very narrow aperture, like f 22, is not always required to produce a large DOF. In the aerial photograph below of the stunning Johanna Beach on the Great Ocean Road in Victoria, Australia I was able to achieve a large DOF at a very modest aperture (f 4). The reason for this is that both the foreground and background were a considerable distance from the camera.

 

Canon 5D camera with Canon 85mm f1.2 L series Aspherical lens

Canon 5D camera with Canon 85mm f1.2 L series Aspherical lens

The important point is that, in relation to DOF, you only need to close your lens’s aperture down towards f 22 when you have important foreground and background elements that you want to keep sharp, and the foreground elements are close to the camera. The closer you get to the foreground the harder it is to keep both it and the background within the zone of apparent sharpness (DOF).

A Final Point

Interestingly, Depth of Field covers an area from 1/3 in front to 2/3 behind the point of focus. If you were to focus on a subject that lay 1/2 into the frame, the foreground elements would tend to blur more quickly than background elements situated the same distance from the subject.

 

© Copyright All Rights Reserved

Glenn Guy, Blue Sky Photography

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