What is depth of field in photography? How aperture, focal length and focus control sharpness

What is Depth of Field? How aperture, focal length and focus control what's sharp

What is depth of field in photography? Understanding how aperture, focal length and focus control sharpness is one of the first big hurdles in photography and will give you incredible confidence as a photographer.

In this depth of field tutorial we answer some of the common questions people have about controlling what’s sharp in your photos, and show you a few tricks for getting the images you want.

What is Depth of Field? How aperture, focal length and focus control sharpness

What is ‘depth of field’?

A camera can only focus its lens at a single point, but there will be an area that stretches in front of and behind this focus point that still appears sharp.

This zone is known as the depth of field. It’s not a fixed distance, it changes in size and can be described as either ‘shallow’ (where only a narrow zone appears sharp) or deep (where more of the picture appears sharp).

Why do I need to know about it?

Because depth of field has an impact on both the aesthetic and technical quality of a picture. Sometimes you’ll want to use an extensive depth of field in order to keep everything sharp.

A classic example is when you’re photographing a landscape, where generally the most desirable outcome is to capture detail from the foreground to the horizon.

Other times, a shallow depth of field will be preferable. It enables you to blur background and foreground details, causing distractions to melt away and allowing you to direct viewers to the focal point in a picture.

SEE MORE: What is focal length: definition, comparison, every question answered

OK, so where do I find the depth of field control on my camera?

Many digital cameras come with a Depth of Field Preview button near the lens mount (see Page 3 on how to use this!), or enable you to assign the same function to one of the other buttons. However, this doesn’t have any effect on the depth of field.

The image you normally see through the viewfinder or on the Live View screen is displayed at the lens’s maximum, or widest, aperture; the aperture you dial in on the camera body will only be set when you take a picture.

However, pressing the Depth of Field Preview button allows you to view the scene at the working aperture, so that you can see what areas will appear sharp.

There’s a range of ways to control the depth of field – the choice of aperture, focus distance and the type of camera. In a nutshell, wider apertures and closer focusing distances lead to a shallower depth of field.

SEE MORE: Master your camera’s autofocus: which AF points to use and when to use them

How to control depth of field: f/2.8


Remind me what you mean by ‘wide’ aperture…

Wide or large apertures correspond with the small f-stop numbers available on your camera. So an aperture of f/2.8 is wide, while an aperture of f/22 is small.

Again, focusing distance plays a part on the overall effect, with wide apertures offering considerably more depth of field when focused on a subject far away than they do when focused on a subject that’s close to the lens.

However, changing the focusing distance is often the least convenient way to control depth of field – it’s much easier to simply select an alternative aperture setting.

The only thing you need to be aware of is that shifting from a large aperture to a small one can lead to blurred photos.

I thought small apertures made more of a shot appear sharp?

They can do, but the choice of aperture has to be balanced with the shutter speed and ISO in order to maintain a consistent exposure.

Larger apertures let in more light, so faster shutter speeds can be used to freeze movement. Switch to a smaller aperture, and the amount of light passing through the lens is reduced.

Consequently, the shutter speed has to become slower, increasing the risk of camera shake and subject movement. To get round this, you could increase the ISO (What is ISO? Click here!). This allows you to use smaller apertures to increase the depth of field and use faster shutter speeds.

SEE MORE: 10 reasons your photos are blurry (and what you can do about it)

OK, so how does the type of camera affect depth of field?

It’s the size of the imaging sensor inside the camera that makes the difference. The larger the sensor, the shallower the depth of field will be at a given aperture.

This is because you’ll need to use a longer focal length or be physically closer to a subject in order to achieve the same image size as you get using a camera with a smaller sensor – and remember the effect that focusing closer has on depth of field.

This is why a full-frame camera produces a much shallower depth of field than an APS-C SLR or compact system camera (CSC) at equivalent focal lengths and apertures.

Is it true that longer lenses produce a shallower depth of field?

The focal length of the lens does appear to have a significant impact on depth of field, with longer lenses producing much more blur. A 200mm lens focused at 12ft will have a wafer-thin depth of field compared to a 20mm lens focused at 12ft.

However, if the subject occupies the same proportion of the frame, the depth of field (the area that appears sharp) is essentially the same whether you’re shooting with a wide-angle lens or a telephoto!

You would, of course, have to move closer with a wide lens or further away with a telephoto lens to maintain the same subject size.

The reason longer lenses appear to produce a shallower depth of field is thanks to their narrow angle of view: compared to a wide lens, a telephoto will fill the frame with a much smaller area of background, so any blur appears magnified too. Use this characteristic to add a professional sheen to your portraits.

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3 situations when you should take control of depth of field

3 situations when you should take control of depth of field: Landscapes

For maximum front-to-back sharpness in a landscape or cityscape, use short focal lengths and apertures of around f/16 or smaller, and focus about a third of the way into the scene. To keep the camera steady during the longer exposure, use a tripod or increase the ISO instead.


3 situations when you should take control of depth of field: Portraits

Whether you’re shooting people or animal portraits, the most successful shots are frequently those where the background is beyond the depth of field and consequently blurred. Longer focal lengths and wide aperture settings are a good choice here, although focusing needs to be bang-on.


3 situations when you should take control of depth of field: Close-up

Depth of field decreases the closer you focus, so when it comes to photographing miniature subjects the choice of aperture becomes crucial. Even the smallest aperture available on a lens may only give a depth of field measured in millimetres when the lens is used at its closest focusing distance.

SEE MORE: 3 photos made easier by a tripod and remote release

How to choose the right aperture

How to choose the right aperture

Click on the infographic to see the larger version

Set the camera to Aperture Priority or, if time allows, Manual mode. These two exposure modes give you full control over the choice of aperture, enabling you to change from a wide aperture to a small one in seconds.

We’ve used the near extremes of this lens’s aperture range to illustrate the impact that aperture has on depth of field. In reality, the change as you move through the range isn’t so abrupt.

SEE MORE: 10 reasons your pictures aren’t sharp (and how to fix them)

What is Depth of Field Preview: how to get it right in camera

What is Depth of Field Preview? It’s an incredibly useful feature found on many DSLRs that allows you to fine tune your depth of field in camera. Here we explain how to make best use of your camera’s Depth of Field Preview feature.

What is Depth of Field Preview: step 1

Activate Live View
Your DSLR’s Depth of Field Preview function can be used for viewfinder or Live View shooting, but the larger, brighter Live View image makes it easier to judge what is sharp and what is soft. Here, a wide f/2.8 aperture has been selected.

SEE MORE: Fine tune image sharpness using Live View

What is Depth of Field Preview: step 2

Press DOF Preview
The Depth of Field Preview control is usually found on the front of the camera, next to the lens mount. Press and hold this button and then rotate the camera’s control dial to cycle through the apertures available on the lens.


What is Depth of Field Preview: step 3

Preview and shoot
The image will become very dark when smaller apertures are selected, although the backlit Live View screen makes it easy to see the effect. Once you’re happy with the depth of field, release the DOF preview button and take the shot.


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