Control depth of field in photography composition for background blur

Depth of field in photography composition

There’s a lot of talk about blurred backgrounds and how to blur the background. What people are actually talking about is Depth of Field (DoF).

Depth of field is one of the first compositional elements that a beginner photographer should master.

In fact, getting a handle on it is more urgent than learning about the other composition techniques, such as subject placement and deciding what to include or exclude from an image.

Read on to discover the gateway to a blurry background!

What is depth of field (DoF) in photography?

Unlike our eyes, a lens can focus on only one distance at a time. Although an area around the in-focus subject appears sharp, the focus falls off and gradually becomes blurry.

Put simply, depth of field is the sharpness in the image in front of and behind the subject. Or, to be technically accurate, it’s the distance between the furthest and nearest points that are in acceptable focus. This is the area of focus.

As the use of the word depth implies, we have:

• Deep (or wide) depth of field
• Shallow (or narrow) depth of field

Let’s start at the deep end.

Deep (wide) depth of field

Landscape photographers generally favor a deep depth of field.

In other words, they want front to back sharpness in their images – both the foreground and the background are as in focus as possible. Imagine what a blurry mountain would look like in the distance – it would just be a big solid lump.

Shallow (narrow) depth of field

When photographers want a blurred background, they don’t want the background to be in sharp focus. So, they want a shallow depth of field.

This is particularly popular with portrait photographers. A blurred background makes the subject really stand out in the image as it separates them from the background.

Also, by blurring the background, you eliminate distractions and the viewer is automatically drawn to the sharpest element in the image, the subject. This ties in with other composition techniques of isolation and differential focus.

Further reading:

 4 ways to use isolation in photography composition

How to use differential focus to make your composition pop

How do you control depth of field (DoF)?

When taking a photo, to increase or decrease the area in focus, you need to know what controls depth of field:

• Aperture
• Magnification

There are 4 ways to adjust DoF through aperture and magnification:

1. Aperture
2. Distance
3. Lens focal length
4. Camera sensor

Let’s take a closer look at these four methods.

1. Aperture – the most well known factor affecting DoF

If you shoot “wide open”, in other words with a small f-number such as f1.8, f2.8 or f4, you’ll have a shallow DoF. In fact at f1.8 your depth of field will be so shallow that, if you were standing close enough to your subject, their eyes could be sharp, while the tip of their nose and their ears aren’t.

Conversely, if you “stop down” (reduce the size of the aperture) to larger f-numbers, such as f11, f16 and f22, you’ll have a deep DoF.

Further reading: The Exposure Triangle – what role does Aperture play?

 

2. Distance between photographer and subject

The area of focus either side of your subject (depth of field) is a proportion of the distance between you and your subject. This is roughly 1/3 of the distance in front of your subject and 2/3 of the distance behind your subject.

In other words, if your subject is 2 metres from you and 6 meters from the background, the background will be more blurred than if your subject was 2 meters from the background.

Here’s a quick sketch to demonstrate.

 

For a detailed tutorial, with examples, on using distance to control DoF read:  The easy way to a beautifully blurry background

Shallow depth of field is not just for creating a blurred background. Sometimes the subject is in the background and we want to blur the foreground to separate the subject from the foreground.

So, we use a shallow depth of field to blur the foreground, as in the photo below.

3. Lens focal length affects depth of field

When we talk about the focal length of a lens, we’re talking about the ability of the lens to magnify. As we’ve seen, magnification is one of the factors that controls depth of field.

So, if you’re shooting at 200mm, there’s considerably more magnification than if you’re shooting at 55mm. Therefore the background will be blurred more when using a focal length of 200mm, adding to a shallow depth of field.

This is another reason why landscape photographers shoot with wide angle lenses for a deep depth of field.

Further reading: 5 focal length facts you need to know for good photos

4. Camera sensor and depth of field

Remember that magnification affects depth of field?

So the size of your camera’s sensor plays a role when comparing full frame sensors and crop sensors ONLY when you’re trying to take exactly the same photo with both cameras. It all comes down to field of view.

A camera with a crop sensor has a smaller field of view –  it takes in less of a scene. A full frame camera has a larger sensor, so has a larger field of view and takes in more of a scene than a crop frame. To fill the frame with a full frame sensor you need to get closer than you would with a crop sensor camera.

Because you have to get closer to the subject, you:

• Reduce the distance between you and the subject, so you get a shallower depth of field

OR

• Zoom in, so change the focal length of the lens, and as a result get a shallower depth of field. Because of magnification.

REALLY IMPORTANT NOTE: You can achieve exactly the same depth of field with both types of sensors where neither you nor your subject move. You just have to change the lens (focal length) and camera settings (aperture).

Summary of how to get a blurred background (or foreground):

So, there you have it. If you want a blurred background you can:

• Increase aperture (smaller f-number)
• Move your subject away from the background
• Use a longer focal length
• Get closer to your subject

For a greater appreciation of why a blurry background is desirable in photography composition, read about the Gestalt principle of figure to ground.

Further reading: Harness the power Gestalt theory in photography

Summary of how to get front to back sharpness:

If you want front to back sharpness, do the reverse:

• Decrease aperture (higher f-number)
• Get your subject closer to the background
• Use a shorter lens focal length
• Move away from your subject

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