Depth of Field. Focus on the big picture.

February 17, 2012

If you use a point-and-shoot camera or camphone, it’s often almost impossible not to get everything from your feet to the distant horizon in focus. But the large sensors that are built into D-SLRs means that it can often be surprisingly difficult to get everything in the frame looking sharp.

That’s because the bigger sensors used on SLR cameras mean less depth of field (DOF). While blurred backgrounds can be a real bonus for subjects such as portraits, the limited zone of sharpness can be a problem for other types of photography.

Outdoors, you need to set up your camera carefully if you’re going to get the boulder in the foreground and the mountain in the distance both appearing in focus in the shot. However, the same difficulties can also present themselves when you’re shooting a set of subjects that are close together, as they would be in a still life on your kitchen table.

This is where you need a proper understanding of depth of field. In theory, only the object you focus on will be sharp, but in practice there’s a zone of apparent sharpness that extends behind the point you’ve focused on and in front of it. So you need to start thinking in terms of zones of sharpness rather than fixed focus points, and plan your camera settings accordingly.

Several factors affect depth of field, including the focal length you’re using, the lens aperture, your distance from the subject and where you focus.

Our walkthrough shows how it’s done. It’s a tricky subject, because our peppers are at different distances from the camera -so we need to make sure that we have enough depth of field to cover all of them.

STEP BY STEP. Control sharpness for still lifes.

The autofocus system in your D-SLR is incredibly sophisticated, but it has a drawback – it can only pick a single thing to focus on at any one moment. Not only that, it will generally choose the thing nearest the camera. This is hopeless when you want to maximise depth of field, need to select the focus point more carefully and sometimesfocus between two objects.

 

1. Choose Aperture Priority mode.

To control depth of field, you need a mode that enables you to choose the aperture setting directly. Aperture Priority (A) mode is best for this because the camera will then set the shutter speed automatically to produce the correct exposure. This leaves you free to concentrate on focusing and depth of field.

2. Select a small aperture.

You can now use the command dial on your camera to close down the lens aperture to the smallest available setting. For most lenses, this is likely to be about f/22, although some macro lenses will stop down even further. This will increase the exposure time, which goes up to 1.6 seconds for the shot we’re taking here.

 

3. Use a tripod.

You can’t get shake-free handheld shots with exposures this long, so a tripod is essential. It will also fix the camera position so that you can focus precisely. This is crucial for careful control of depth of field, because any slight shift in the camera position will affect the focus point.

 

4. Choose the focus mode.

You also need to take control of the camera’s focus point, so open the menus to check the AF-area mode setting. In Auto-area mode, the camera sets the focus point according to what’s nearest, so make sure you swap to ‘Single point’ mode.

 

 

5. Set the focus point.

With landscapes, you should aim to focus about a third of the way into the frame, but with close-ups such as this, pick a point nearer to the centre of the frame. You can use the four-way controller to move the focus point, which shows up in both the viewfinder and on the LCD.

 

 

6. Use Live View.

You might find this much easier to do in Live View, where you can position the focus point more precisely. Here, for example, we’ve placed it right over the stalk of the second, orange pepper. This is the perfect position to maximise the available depth of field, almost halfway up the frame.

 

 

7. Precise manual focus.

The alternative is to zoom in on the LCD in Live View mode, and swap to manual focus using the switch on the side of the lens. Turn the focus ring manually, judging when the area you want looks sharpest on the screen. This stops any tendency for the camera to refocus between shots.

 

 

8. Depth of field calculations.

You can look up precise distances with online tables, or use an app such as Depth of Field Calculator. Here, we type in the focus distance, focal length and aperture, and it calculates that the depth of field will be 6.5cm.

 

 

DEPTH OF FIELD FACTORS

A quick way to increase depth of field is simply to zoom out, or to move the camera further away from a subject; you can then crop the picture later if necessary. You can also raise the camera so that you’re looking down on your subject, placing everything in the same plane of focus. Alternatively, you can give in and exaggerate a shallow depth of field-get closer, use a longer focal length and choose a low viewpoint.

Hyperfocal landscapes.

Get everything in your scene sharp, from the foreground to infinity!

Hyperfocal focusing is a specialised application of depth of field theory that’s perfectly suited to landscape photography. It’s actually quite simple to get your head around. When managing depth of field, you need to think in terms of the zone of sharp focus as a distance range, from the near limit (the closest object that will appear sharp) to the far limit (the farthest). With hyperfocal focusing, you place the far limit at infinity, and this automatically maximises the depth of field available.

The hyperfocal distance will depend on the focal length of the lens and the aperture setting. However, once you’ve worked it out, and as long as you don’t change the settings, you’ll know that everything from a fixed distance in front of the camera right up to infinity will come out sharp. The near limit for depth of field works out to be exactly half the hyperfocal distance.

Hyperfocal distances.

You’ll need a lens with a distance┬áscale. Or, if you have a D-type lens, you’ll see depth of field markers on either side of the main focusing index. Align one of the left markers with infinity and set the aperture to f/22 or f/11 – that’s your hyperfocal distance sorted!

 

 

Handheld calculators.

G-type lenses and zooms don’t have depth of field markings, so you’ll need some help to work out hyperfocal distances here. The ExpoAperture2 ($40, www.expoimaging.com) covers a range of sensor sizes, focal lengths and apertures.

 

 

Phone apps.

If you’ve got a smartphone, you’ll find that using an app such as Depth of Field Calculator ($0.99) is much easierthan juggling dialsand lining up numbers. Once you’ve selected the camera, focal length and lens aperture, it tells you the distance to focus on.

 

 

In practice.

Once you know the hyperfocal distance, you don’t need to worry about focusing. This shot was taken at a focal length of 31mm and an aperture of f/8, so the hyperfocal distance works out at 6.2m. When we focus on that, everything from 3.1m to infinity comes out sharp.

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