The depth of field in an image refers to the amount of the picture you choose to have in focus. By choosing how much to have in your shot, you can focus the viewer’s attention in a specific place. This makes depth-of-field control one of the most important creative decisions a photographer will make.
How it works Depth of field is centred around your point of focus. So if you have 10ft of depth of field, some will be in front of the point of focus, and some will be behind it. Everything outside of this range will be out of focus.
You control depth of field by changing the aperture setting on your camera. Like your eye, a camera lens has an ‘iris’ inside that can open or close to let in more or less light. You control the size of this hole by changing the aperture setting, which you simply measure by using a scale of f-stops.
The larger the aperture, the shallower your depth of field will be, meaning a smaller portion of your image will be in focus. The smaller the aperture, the deeper your depth of field will be, resulting in more of your image being in focus. Now here’s the tricky part: aperture size is denoted by a number, and the smaller the number, the larger the aperture. Setting your aperture setting to f2.8 will yield shallower depth of field than setting it to f11. A helpful way to think of it is that a smaller number means less depth of field.
Manual controls To change your aperture, you must have a camera with an aperture-priority or manual mode. When in aperture-priority mode, you choose the aperture you want, and the camera automatically picks a corresponding shutter speed that will yield an image that is neither too light nor too dark. Some lenses (more expensive ones) can open to a wider aperture than others.
A camera with a smaller sensor has inherently deeper depth of field. That means it’s difficult to achieve extremely shallow depth of field with a point-and-shoot, even if you have manual controls.
The right camera position In a portrait it’s nice to have a shallow depth of field so that the focus is on the person’s face. But shallow depth of field is also handy any time you need a way to separate a subject from a busy background – in the street or at an event – and many other situations can benefit from a reduction in depth of field.
If you open a lens to its largest aperture, you might have a difficult time focusing. This will make your backgrounds go too soft and be difficult to keep the correct parts of your image in focus. Camera position is also critical for achieving a shallow depth of field. While you might have your aperture set to a nice wide opening, if there’s nothing visible in the background, then you won’t be able to tell that the image has a shallow depth of field. The perception of the depth of field in an image depends on the size of the objects in the background, so be sure to compose your shot so that there’s something big in the background to reveal the shallow focus.
Landscapes and still-lifes There are also times when you’ll want to ensure a deep depth of field. Landscape images, for example, can work better when everything in the frame is in focus. Product shots, some still-life images, and any shot where you need to see both foreground and background details will also benefit from a deep depth of field.
There are two important factors to keep in mind for deep-focus shots. First, though it’s tempting to dial in the largest f-number you can, this is usually a bad idea. As the aperture size gets smaller, your image can suffer from an optical effect called diffraction, which reduces image sharpness. And if you go smaller than f11, you will see a noticeable drop in sharpness.
Second, remember that depth of field is centred around your point of focus, with some of the range in front, and some behind. When shooting a landscape, if you focus on the horizon, then some of your depth of field will actually fall behind the horizon. In other words, you won’t be making the best use of the depth of field that’s available, so your foreground might end up out of focus.
So a good rule of thumb is to focus about one-third of the way back from the horizon. This will include more of the foreground in your depth-of-field range.
Hone your technique Depth-of-field control is one of the most important tools in your creative toolbox. Spend plenty of time experimenting and practising with different apertures, lenses, and focusing techniques, and before long, it will soon become second nature to you.