Controlling Depth of Field

Lesson #3 in Photography for Ignorant Geniuses

Most amateur photographers look at photos taken by people with slightly more knowledge in the subject and try to understand how their backgrounds look blurry, therefore making the subject stand out much more. In this lesson we shall go into the reason the background (or foreground) is blurry and we shall explore the factors that will help you achieve perfect pictures.

The first, and probably most obvious, query is usually to understand what makes the background blurry. That, in itself, is easy enough to answer. The background looks blurry because it is out of focus. The technique used here is sometimes referred to as selective focus – because you specifically select which part of the image you would like to appear in focus.

If left to their own devices, however, most cameras will try and set themselves to avoid a setting which will aid in this for one simple reason: selective focus is a very dangerous game to play. The more you push your camera to achieve a shallow depth of field, the harder you are making it for you to focus well – we shall soon see why.

Focal point and the plane of critical focus

The laws of the physics of light dictate that every optical device has one specific focal point. That, my friends, is that. It is not up for debate (well, surely not by me). The lens in your camera is an optical device and it therefore has one focal point too.

A focal point is simply the one spot at which an object shot through a lens is perfectly in focus. This is what a camera is determining when moving the lens in and out when set on autofocus. Once it has selected the right spot and moves the lenses within it to be at the right distance from each other a camera produces a shot which is in focus and therefore has a sharp subject.

Modern cameras usually choose the object to put in focus based on their analysis of the scene, so if you have a face in the picture they will usually try to focus on it. SLRs have a set of focus points and most of them allow you to either select one particular point to focus on or else set the camera on auto, and it will attempt to choose the right point for you. Sometimes, with an SLR you can also choose to first focus on a certain spot by pressing the shutter half way down and then re-compose the shot before actually clicking all the way down to take the shot. This allows you to have a focus spot which is not on one of the points the camera comes pre-defined with.

Thankfully the one spot that the camera has focussed on is not the only part of the image that is in focus. Imagine seeing a scene from the side. The plane of critical focus would be the flat surface created if you had to slice it at the focal spot. You can safely assume that this is completely sharp too.

Depth of Field

Thankfully most camera lenses are slightly forgiving and together with the plane of critical focus you also get some leeway for sharpness to either side of the focal plane. This area in which objects appear sharp is defined as depth of field.

In the diagram below we can see that the focal spot is surrounded by a short distance to either side of the subject in which the rays of light are still close enough to the focal spot to fool our eyes into thinking that they are in focus. Once an object is outside that space it will appear to be out of focus:

Depth of Field - the physics

The comparison below shows the same subjects at different depths of field:

An example of Depth of field in action

Controlling Depth of Field

By now you should have guessed where this lesson is going. For complete creative control over your photos you need to know how to choose the amount of depth of field you want. To do this you will need to understand what influences depth of field, and ideally why.

There are three major factors that affect the amount of depth of field in a photo.


As you might have realised from the example I showed earlier on, I changed the aperture from one picture to the other to show you how depth of field affects a photo. In the diagram below we can see the same object being shot at different apertures. By closing the aperture in the second example we have made the depth of field significantly longer.

Depth of Field - Aperture
Remember: Larger number denotes smaller aperture!

Distance of the object from the lens

Applying the same rules of physics shows quite clearly why moving an object further away from the camera will give us a longer depth of field. Light has to be bent less and therefore you have an acceptable focal area which is wider.

Depth of Field - Distance

Focal length of the lens

For a lens to be at the same aperture value at different focal lengths it needs to be physically open much wider. This means that at 18mm at f2.8 a lens has to be open much less than it is would need to be open at 55mm at f2.8. This is why telephoto lenses with wide apertures are so bulky!


Focusing at infinity

The rules of depth of field change slightly when you focus at infinity. When the object you are shooting is beyond a certain distance (about 10 meters) and at the end of the focusing range of a lens, the only objects in the picture that shall be out of focus are the ones that are closest to the camera. The rest is considered to all be in focus.


How to use Depth of Field Creatively

Once you have understood how to control depth of field it is high time you put it into good use.

There are times when you need a shallow depth of field to create effect. Portraits are a perfect example, because a shallow depth of field will allow you to isolate the subject. A close up of an object, such as the thorn below, is also a good time to isolate your subject.

A Thorn
Shot at f2.8 for the shortest depth of field available on my lens.

If shooting a landscape, however, you’d want to maximise depth of field to capture as much detail as possible.

For this landscape photo of Monaco I used a smaller aperture (f7.1) to ensure sharpness throughout the image.


Equipment Caveat Alert!

There is also a fourth factor which affects depth of field, however you don’t have much control over it. This is the size of the sensor. The smaller the sensor you use, the more difficult it is to achieve a shallow depth of field. This means that on a micro-sensor (such as the one in your phone), it is nearly impossible to control depth of field. Most compact cameras also have relatively small sensors, which means that the knowledge you’ve learned from lesson will be very hard to implement unless you have a camera which is geared towards the serious enthusiast or professional.

Most cameras in phones are nearly always in focus because they are built to focus at infinity from a very short distance – around 30cm.

Photography for Ignorant Geniuses is a set of tutorials I give out on a weekly basis about the science behind photography. For more information (and to subscribe) click here.

(#43 of 366 X 2012 project)

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