Whats depth of field all about?

Depth of field is an indispensable creative tool — but what exactly is it, and how can you control it?

Depth of field, or DoF for short, is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in a photo. We say ‘acceptably sharp’ because only one point will be truly razor-sharp in your pictures, as your lens can only focus at a single distance. However, the sharpness falls off gradually both in front of and behind the point you’re focusing on, and the depth of field is a measure of how far this sharp area extends. Controlling the depth of field makes a real difference to how your pictures look. The less depth of field there is, the easier it is to blur backgrounds, which is often desirable for portraits. Increasing the depth of field helps you to capture more detail in a subject or scene, and you’ll generally want as much depth of field as you can get when shooting landscapes and macro subjects.

Your camera doesn’t have a depth of field control as such; rather it’s governed by a number of factors and camera settings. The distance to the subject (and the distance between subject and background), the focal length of the lens and the size of the camera’s sensor all play a part, although it’s the choice of aperture that’s generally seen as having the biggest impact.

For those new to photography, the aperture is the hole in the lens that light has to pass through to reach the camera’s imaging sensor. It’s measured in f-stops, and can be adjusted from a wide aperture (represented by a low f-stop number like f/2.8 or f/4) to a narrow aperture (with a high number, such as f/22 and f/32). The choice of aperture is key to controlling exposure, as wide apertures let in more light and narrower apertures let in less, and it also has a big bearing on depth of field, with wide apertures reducing it and narrower ones increasing it.

Distance and focal length

When it comes to selecting an aperture you also need to consider your distance from the subject, as the closer you are the shallower the depth of field will be — when you’re focusing close-up and using a wide aperture, the depth of field may only measure a few millimetres. Imagine, for instance, that you’re using a 70mm lens on a 7D to photograph a subject one metre away. With a wide aperture of f/2.8, the depth of field may only be 2cm at this close distance. However, if the lens was refocused on a subject 10 metres away, that same aperture may give a depth of field of almost 170cm. Consequently, your focusing needs to be bang-on when you use extremely wide apertures at close distances, otherwise important details may look soft.

Before you set the optimum aperture and position yourself at the perfect distance to provide the depth of field you want for a shot, you need to think about the best focal length to use. The lens doesn’t affect the depth of field per se — when it comes to aperture selection, f/5.6 on a 20mm lens offers the same depth of field as f/5.6 on a 200mm lens — but the magnification offered by the focal length does play a part. Longer lenses have a narrower field of view than wider ones, so they take in less of a scene, effectively making everything appear bigger in the frame. This includes the background, with any blur becoming magnified too.

Sensor size has a similar effect on the depth of field. The larger the imaging sensor inside the camera is, the easier it is to create shallow depth of field effects — this is one of the reasons many pros reach for full-frame cameras. The smaller APS-C sensor that you find in the majority of EOS bodies records a smaller area of the image projected by the lens, so everything appears larger in the picture than it does using the same focal length on a full-frame camera; it’s a bit like cropping the shot in Photoshop. Attach a 50mm lens to an APS-C D-SLR like the 700D, for instance, and its effective view will be the same as that of an 80mm lens (50mm x 1.6 crop factor of the smaller sensor). On a full-frame camera such as the 6D, there’s no crop factor — it captures the full view of the lens. In order to get the same image size as on the 700D, you’d need to be closer to the subject, and, as we’ve seen, the closer you are to a subject the shallower the depth of field becomes.

Of course, cameras with smaller sensors can be useful if you’re looking to maximise the depth of field in your shots, and a technique called ‘hyperfocal focusing’ will also help you to maximise front-to-back sharpness in a scene. This involves manually focusing the lens at a distance that will capture sharp detail from half the hyperfocal distance to infinity — see our step-by-step walkthrough.

How aperture affects depth of field

Wide apertures reduce the depth of field and help to separate the focal point from the rest of the picture, whereas narrow apertures bring more of a scene into apparent sharp focus. Although apertures are consistent across lenses (an aperture of f/8 has the same effect whether it’s used on a 50mm lens or a 500mm lens), lenses don’t offer the same range of settings. For instance, expensive ‘fast’ lenses offer very large maximum apertures for their focal lengths, such as f/2.8 on a professional 300mm lens, compared to f/5.6 on a cheaper equivalent. As well as letting more light in (and enabling the faster shutter speeds that give them their name), faster lenses enable you to capture a shallower depth of field.

Wide aperture: f/2.8

+ A wide aperture offers a shallow band of sharpness. Both close-up and distant background details will disappear into blur

– Such a shallow depth of field demands accurate focusing: here, the mallard has moved its head, so it’s no longer sharp

Narrow aperture: f/16

+ Choosing a narrow aperture extends the depth of field, and this can be more forgiving when it comes to focusing errors

– Narrow apertures can result in slower shutter speeds — and blurred pictures caused by camera shake, as seen here

STEP BY STEP

How to use hyperfocal focusing

Use this technique to get the maximum depth of field possible when shooting landscapes

Set the aperture

1.- For maximum sharpness you’ll need to use relatively narrow apertures. Select Aperture Priority and set an aperture of around f/16 (higher f-stops can lead to soft pictures).

Turn off autofocus

2. Switch your lens to the manual focus setting: you need to set the lens at a precise distance with hyperfocal focusing, and the lens would refocus if left in AF mode.

Find the distance

3. Visit www.dofmaster.com (or use its smartphone app) and input your camera and lens info. You’ll be given a table of depth of field measurements and a hyperfocal distance setting.

Set the distance

4. Focus the lens at the ■ distance suggested for the aperture being used. If the image appears blurred, use the camera’s depth of field preview button to show the true picture.

Up the ISO to get more DoF

For landscape photography, or any other situation that requires an extended depth of field to keep everything sharp from the foreground to the background, you’ll want to use narrow apertures. However, as we’ve seen this leads to slower shutter speeds; this is no problem if you’re shooting with a tripod, but if you’re shooting handheld sharpness can be lost through camera shake. The solution here is to increase the ISO to compensate. First, set the aperture, then set an ISO that brings the shutter speed within the ‘safe’ handheld shooting speed of the lens (aim for 1/focal length, so 1/50 sec for a 50mm lens).

Previewing depth of field

It’s difficult to gauge the depth of field when looking through the viewfinder, as the image you see is shown at the lens’s widest aperture setting; the aperture you’ve dialled in isn’t actually set until just before the exposure is made. Most EOS D-SLRs have a depth of field preview button, which you’ll find near the lens mount: holding this down sets the lens to the selected aperture, so you can gauge what will be sharp; the downside is that the narrower the aperture, the darker the image in the viewfinder. The alternative is to use Live View mode: if you hold down the depth of field button the bright image on the LCD will enable you to judge the depth of field accurately, even when using narrow apertures in low-light conditions.

Why distance matters

Although wide apertures reduce the depth of field, the effect depends on how far the subject is from the lens. The farther away the subject is, the greater the depth of field — even when a lens is used at its widest aperture. Take these two examples: both were shot at f/2.8, but at different distances from the subject. The shot of the goose doesn’t work because the background still looks comparatively sharp, and the bird blends into it. The dandelion image is more successful: the close focusing and wide aperture combine to produce a shallow depth of field that separates the flower from its surroundings.

Specialist lenses

The problem with using very narrow apertures to extend the depth of field is that it can reduce sharpness due to a phenomenon called diffraction. A wider aperture will produce a crisper image but with less DoF, so you’ll have to decide which is more important, or set an aperture somewhere in the middle. Not so if you use a tilt-shift lens: these pricey chunks of glass enable you to use a wide aperture for maximum sharpness, and then tilt the lens to adjust the plane of focus and maximise the depth of field.

4 hints and tips for…

Fisheye lenses

1.Choose your view

Each month we highlight a Canon EOS D-SLR or type of lens and provide handy advice to help you get more from your gear

Fisheyes generally come in wo flavours: full-frame or circular. The effect depends on the lens design and the size of the sensor inside the camera, so check the specs to make sure the results will be what you’re expecting.

2. Get close

Because of the incredibly wide view of a fisheye lens (up to 180 degrees, corner to corner), you’ll need to get much closer to a subject than would ordinarily be the case in order to prevent it from looking too small in the picture.

3. Exposure issues

The inclusion of such a wide expanse of scenery can cause severe contrast problems if you have a bright sky and a darker foreground. You may need to shoot several bracketed exposures, and combine the images in post-processing.

4. Check the edges

It’s all too easy for your feet or your shadow, or a set of tripod legs, to appear in the bottom of the picture in fisheye shots. Make a quick check of the edges of the frame before you press the shutter button.

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