Depth of Field

Depth of field

by John VanSteenberg

What is depth of field?

Depth of field (aka: D.O.F.) conceptually is really simple.

It is defined as the “nearest to the farthest points in uniformly sharp focus” in a photograph. In practice, controlling depth of field is a bit more ticklish.

Depth of field control is important to photographs where:

1) The photographer may want to record great depth (in a large group of people like, team photographs, marching bands, etc.; or critically sharp front to back landscape images, or in very close macro images).

2) Photographs where the photographer wants to either include or exclude some object within the frame from the consciousness of the viewer; or to increase or minimize the degree of attention a part of the image receives from the viewer (as in throwing something out of focus to a degree to control the attention it requires, or make it less important).

Controlling Depth of Field

If controlling depth of field is important to you for a particular image, then it is likely you might start by using either the ‘aperture’ or the ‘manual’ modes on your camera to determine exposure.

Generally, what affects depth of field:

Basically, the captured depth of field depends on a five factors when you make a photograph.

a) The distance from the camera to the point of focus. Specifically, from the focal plane (where the sensor is located) to the specific distance the lens is set at. This is particularly important in macro where the focal distance is very short as the depth of field becomes very shallow, or in covering a specific distance from near to farthest points in focus.

b) The aperture you use, ultimately determines the depth of field at the distance and focal length settings in use.

c) The focal length you use (to cover the field of view).

d) The uniformity of sharpness (to make an area of the image more or less attention getting).

e) The degree of enlargement in the final print (During taking the image, the degree of enlargement is usually not in great consideration as there are many possible output sizes.)

Uniformly sharp focus

The area outside the D.O.F. begins to fall off and becomes progressively less sharp (more blurred) as the distance from the point of focus is increased. As this distance from the focal point increases, the falloff becomes more noticeable. Technically, uniformity is often measurable before it is easily visible.

How the distance from the camera changes depth of field

As distance from the camera to the focal point varies, the absolute depth of field (in inches) varies, if all of the other factors remain equal. In other words:

With a prime (single focal length lens) at every individual aperture, as the camera & lens are moved away from the subject’s focal point, the depth of field increases, in inches. Conversely, if the camera is moved closer to the subject, the depth of field becomes narrower, in inches. The specific depth of field at each aperture/focal length combination depends on distance.

As to zoom lenses: the same is true, except that you can vary coverage field by changing focal length without changing physical distance. Doing so also changes depth of field. So, when composition changes due to zooming, the depth of field also changes. (See below.)

How aperture changes depth of field

Changing aperture will expand (widen) or contract (narrow) depth of field.

For any given lens focal length:

The lower the f/number (larger the opening) the narrower (shallower) the depth of field.

The higher the f/number (the smaller the opening) the greater the depth of field.

How focal length changes depth of field

For any given camera, as the focal length of the lens becomes longer/higher (more toward telephoto), the depth of field narrows. As the focal length becomes shorter/wider (towards a wider angle) the depth of field increases at any given focusing distance.

This applies to both prime lenses and zoom lenses.

Suggestions and implications:

Ways to increase depth of field:

1) Raise the ISO to allow for higher apertures at any volume of light, while keeping the same shutter speed.

2) Lower shutter speed to increase aperture at any volume of light, at a specific ISO.

3) Move farther away from the subject and keep the focal length and other exposure settings the same. (You may wish to crop to the smaller field of composition to permit the depth of field and field of coverage you wish.)

What depth of field is not

1. Blurring caused by camera movement.

2. Movement caused by subject movement.

17mm at F/2.8