Depth of Field Explained

Depth of field can be daunting to get to grips with, but is actually a very simple concept to understand. Use it to take your photos to a new level.

There are many technical aspects to photography which can seem daunting to the beginner photographer, and depth of field is one which causes much confusion.

Thankfully, it is actually a very simple concept - the term "depth of field" simply refers to the area of the scene which appears well focused.

It is the product of three factors - the lens aperture, the focal length, and how far away from the camera the subject is positioned.

This article covers each of these three factors and looks at how we can use them to control depth of field in our photography, using it to our advantage to isolate our subject or bring everything into sharp focus.

What is Depth of Field?

When we adjust our camera's lens to focus on a subject it will only achieve perfect focus at one particular distance; anything in front or behind this point will be blurred to a greater or lesser degree.

Diagram illustrating depth of field

Depth of field refers to the area around the perfect focal distance which appears acceptably sharp.

What Affects Depth of Field?

Depth of field is affected by three main factors:

  • Lens aperture diameter
  • Focal length
  • Distance from the subject

Aperture and Depth of Field

The aperture determines the diameter of the beam of light that the lens admits. The wider the aperture, the wider the beam of light. A wider beam is more susceptible to depth of field effects than a narrower beam.

Line of dominoes

Opening the aperture creates a narrow depth of field. Image by Marc Dezemery.

Using a wider aperture produces a shallower depth of field; using a narrower aperture gives a greater depth of field.

Focal Length and Depth of Field

Focal length is a measure of how much the lens magnifies a scene. The lens also magnifies differences in focus.

A longer focal length magnifies focus differences, resulting in a shallower depth of field.

Focus Distance and Depth of Field

The closer the subject is to the camera, the greater the relative distance from the front to the back of that object. A high relative distance gives a corresponding reduction in how much of the object appears in focus.

Depth of field in effect on a book's pages

Getting close to the subject narrows the depth of field. Image by Karl Randay.

Controlling Depth of Field

If, for whatever reason, you want to figure out your depth of field exactly, you can use an online depth of field calculator. However, this is usually unnecessary, and you can achieve the results you want much more easily by applying the following rules of thumb:

To Increase Depth of Field To Decrease Depth of Field
Narrower aperture Wider aperture
Shorter focal length Longer focal length
Move away from subject Move towards subject

 

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