Aperture priority – which is usually denoted by A or Av – is one of the most useful shooting modes on any camera, analogue or digital, because it’s the best tool for controlling depth of field. In this mode, the photographer sets the lens aperture, leaving the camera’s auto exposure (AE) system to set the shutter speed. By selecting an appropriate aperture setting you can change the appearance of a picture from having everything in it sharp, no matter how far it is from the lens, to having a narrow plane of sharpness that isolates your subject from background details.

 

Aperture priority – which is usually denoted by A or Av – is one of the most useful shooting modes on any camera, analogue or digital, because it’s the best tool for controlling depth of field. In this mode, the photographer sets the lens aperture, leaving the camera’s auto exposure (AE) system to set the shutter speed. By selecting an appropriate aperture setting you can change the appearance of a picture from having everything in it sharp, no matter how far it is from the lens, to having a narrow plane of sharpness that isolates your subject from background details.

However, there are some special limitations imposed by digital cameras that may restrict your shooting options in this versatile mode. We’ll outline them in this feature.

Depth of Field

When a lens is focused on a subject, only one plane in that subject can be in focus. Sharpness begins to fall off gradually on either side of this plane, giving a zone in which the subject appears to be in focus before it becomes obviously soft and out-of-focus. This zone, which defines the depth of field, is wider behind the subject than in front of it. What is and isn’t ‘acceptably sharp’ will depend on your viewing distance. While you’re likely to be very picky about sharpness in a 10 x 15cm print, an image on a billboard can look just as sharp when viewed from across the street, even though the dots that make up the image are actually the size of 10-cent pieces.

As a general rule, the wider the lens aperture and the closer the subject, the shallower the depth of field. In contrast, the smaller the lens aperture and more distant the subject, the more likely it is to be sharply focused over a wide range of distances. However, with digital cameras, the size of the image sensor also plays a role: the smaller the image sensor, the wider the depth of field for a particular aperture setting.

Portrait photographers use depth of field controls to isolate their subjects with a technique known as ‘selective (or differential) focusing’ in which the camera’s lens is set to a wide aperture and focused on a key feature of the subject; normally the eyes. The use of a wide aperture narrows the depth of field, causing subject elements in front of and behind the plane of focus to appear blurred.

Digicams
The first issue to contend with when looking at using aperture priority AE on compact digicams is the size of the image sensor – which is generally very small. In the overwhelming majority of compact digicams, the sensor’s dimensions are between 4.8 x 3.6mm and 8.8 x 6.6mm. You could fit between 23 and 50 of them on a 35mm film frame!

The size of the sensor influences the camera’s depth of field by a factor known as the lens multiplier (LMF) or sensor crop factor. This allows you to calculate how much greater the camera’s depth of field is than a 35mm camera produces when the same lens aperture is used. The table below shows the relationship between sensor area and LMF for compact digicams and digital SLR cameras (shown in red). Note how much larger the DSLR sensors are and how much closer their LMFs are to a 36mm frame.

Sensor “Type”

Width (mm)

Height (mm)

LMF

1/2.7-inch

5.23

3.95

6.5x

1/2.5-inch

5.34

4.01

6.1x

1/2.4-inch

5.8

4.35

5.67x

1/2-inch

6.4

4.8

5.6x

1/1.8-inch

7.18

5.32

4.9x

2/3-inch

8.8

6.6

3.8x

4/3-inch

18.0

13.5

2x

“APS-C type”

25.1

16.7

1.5x

35mm frame

36

24

1x

From the table you can see that most compact digicams have enough depth of field to ensure thatall but the closest subjects will look ‘acceptably sharp’ unless the subject is very close. It also means it can be difficult to achieve out-of-focus backgrounds with the normal lens setting.

To complicate matters, more than half of all compact digital cameras lack aperture adjustment and, of those that offer it, many only provide two aperture settings, usually wide open and stopped down. So the photographer’s control of depth of field is severely limited.

Where a range of aperture adjustments is provided – as often happens in enthusiasts’ cameras – it often stops at f8 because beyond that point diffraction can affect image resolution. And, because the aperture settings in zoom lenses commonly become smaller as the zoom is extended, many of the zoom lenses on these cameras still only support two aperture settings.

One of the few ways to focus selectively with a compact digicam is to shoot close-ups, where the camera-to-subject distance will reduce inherent depth of field. But this doesn’t work for portraits. Digicams with long zoom lenses may allow you to combine a wide aperture setting with a long focal length to achieve some degree of background blurring; but it is often necessary to ensure a much wider distance between the subject and the background than you would require when shooting on 35mm film.

DSLR Cameras

The situation changes when you use a digital SLR because its sensor is so much larger – although few sensors in consumer models are 35mm frame sized. The Olympus Four Thirds System models have sensors with double the depth of field of a 35mm lens. Models with the so-called ‘APS-C’ sized sensors have LMFs ranging from 1.5 to 1.6 times. But only a camera with a 35mm film-sized sensor chip will provide the same depth of field at the same lens settings as a 35mm camera.

So what can you do if you have a DSLR with a 1.5x LMF and you want to use selective focusing for portraiture? Here are some possible strategies, many of which will also work (at least to some degree) with most enthusiast digicams:

1. Look for lenses with wide maximum apertures (‘fast’ lenses). They will cost more but the benefit will be a narrow depth of field at the widest setting.

2. Shoot at maximum aperture and allow as much distance between the subject and the background as possible. As the ‘zone of acceptable sharpness’ is wider behind the subject than in front of it, the greater the distance between the subject and the background the more likely the background is to be blurred.

3. Use a tele lens. Although it won’t change the inherent depth of field, it will change the angle of view that is captured and make it easier to isolate key elements, like the subject’s face. Note: Filling the frame with the subject will make the few more in-focus background elements less noticeable.

Maximum Depth of Field

Although selective focusing can be challenging with a digital camera, obtaining maximum depth of field is usually very easy – provided you can set the lens aperture to f8 or smaller. The main problem photographers are likely to face when shooting with the lens stopped down is camera shake, due to the associated slow shutter speeds. While it may be tempting to increase the camera’s ISO setting to counteract the problem, be cautious as high ISO settings often produce visible image noise.

It’s better to put the camera on a tripod and trigger the shutter with the self-timer. This works well in still conditions – but not when it’s windy, when using a high ISO setting may be the only way to obtain the shot. Subjects further than 1.5 metres from the camera give you a better chance of image sharpness.