Depth of Field - What is it?

Introduction

In order to optimise Depth of Field as a Photographer we need to understand a little bit of theory.  I will try explain it simply here without having to resort to the complicated mathematical models I experienced in my degree course (and have now forgotten).

Plane of Focus

In theory there is only 1 point on a line from the subject through the lens to the digital sensor (or film) that is 'in focus" when it arrives at the sensor.  Optical designers talk about "a plane of focus"; every point on that plane is perfectly in focus, everything else is out of focus.

Fortunately, life is never that precise.  Points either side of the plane of focus are so close to being in focus that we cannot tell the difference.  It all depends on how we view the image, how far away from it we are, and the state of our eye sight.  Photographers therefore talk about "acceptable focus" and a zone of acceptable focus, or Depth of Field.

Depth of Field

In the following diagram the photographer has focused on the head of the cat with different camera settings and gets different results.

In the top example the head of the cat is in acceptable focus and everything else is "out of focus".  It has a "narrow/small depth of field".

In the bottom example, everything is in acceptable focus from the mountain in the background to the trees in the foreground.  There is a large depth of field.    

The Depth of Field is determined by several factors:

• The focal length of the lens
• The subject to lens distance
• The size of the aperture of the lens
• (The sensor size)

In our day to day photography the main factor we can control is the size of the aperture.  A large aperture (small f/No) reduces the DoF.  A small aperture (large f/No)increases it.

It does not follow, however, that all you have to do to maximise DoF in your image is to focus on your subject and set the aperture as small as possible.

• Lenses do not perform the same throughout the aperture range; the best performance is likely to be achieved in mid range aperture values.  Diffraction starts to be a problem at smaller aperture sizes as different wavelengths (colours) of light are bent by different amounts leading to colour fringing and softening in your image.
• You may "waste" acceptable depth of field behind part of your photo.  For example, if you focus on the front of a house the Depth of Field will extend in front and behind the house itself.  You cannot see into the house and so have "wasted" potential for part of the scene to be sharp.  If you change your point of focus to focus in front of the house you may well improve the focus on foreground items.  This is the principle of the Hyperfocal Distance and a technique called hyperfocal focusing.
  

Hyperfocal Distance

This is a difficult concept to understand but I will try to explain it using screenshots from an iPhone app called Hyperfocal.

Example 1 small aperture focused at infinity

In example 1 the camera is focused at infinity with a small aperture (f28) and the calculations show that the DoF extends from 18.8 meters in front of the camera to beyond infinity (not possible but it means that some focus potential has been lost).

In the 2nd example, the aperture has been opened up to f1.8 and the nearest point in focus has moved out to almost 300 meters.

The 3rd example demonstrates hyper-focal focusing to maximise the depth of field in your image.  If the photographer sets a small aperture and focuses in front of his subject he can bring the near point of focus closer to him (9.5 meters) and still keep the mountains in focus.  The nearest point in focus with the camera focused only 19 meters into the scene is now 9.5 meters extending out to infinity. This maximises the amount of the picture that is in focus, increasing it beyond that which is possible if the camera is focused at infinity.

Example 2, large aperture focused at infinity.

This is one reason why focusing on near distance objects is such a good practice when photographing landscapes.  The point to remember is that the optimum point to focus on is far closer to the camera than we would think.

Digital Camera World

Update 16 Feb 22.  Digital Camera World published a "cheat sheet" which is a nice visual reminder of all this.