You can’t always rely on fully automatic exposures, so it pays to learn how to adjust two important controls that determine how much light reaches the image sensor in a camera.

To obtain a ‘correct exposure’, the lens aperture and shutter speed settings must be balanced to deliver the exact amount of light that recreates the scene you’re photographing.

This is how they work: the lens aperture determines the size of the hole in the iris diaphragm that lets the light through, and the shutter speed controls how long this light is allowed to fall on the sensor.

Most cameras provide some controls for both functions. They’re often located on the mode dial on the top panel, shown in the illustration on this page – although some cameras put them in the menu and Fujifilm’s cameras have separate adjustments on the lens for apertures and on the top of the camera for shutter speeds. (Both have ‘auto’ positions).

A typical mode dial on an interchangeable-lens camera, shown here, has four key settings for exposure control – P, A, S and M – plus a green Auto position. (Source: OM Digital Solutions.)

Getting exposures right is a complex balancing act. In fully automatic cameras, the image processor does all the work, using algorithms developed by the camera manufacturer. The end result may be satisfactory – but there’s a chance it won’t; so you may need to make some additional assessments.

A typical sunlit scene with a wide brightness range. Note how the brightest areas have no discernible details in them – a classic case of blown-out highlights resulting from auto exposure metering. A similar lack of details can be found in the man’s dark hair, where all the dark tones have ‘blocked up’ to solid black.

Fully automatic exposures often fail with scenes that have very wide brightness ranges – such as beach and snow scenes. Unfortunately, you can’t normally over-ride them. Instead, some cameras include ‘scene’ pre-sets, which provide a quick way to set the camera for special tasks like photographing sports action, portraits, landscapes, night shots and close-ups, to name a few. But even they may not deliver the result you want. When you need to take control it helps to have a camera with selectable exposure modes – and know how to use them.

Shooting modes

The easiest mode to use is the ‘P’ mode, which stands for Programmed auto exposure and is a step up from auto exposure. This setting automatically sets both lens aperture and shutter speed but, unlike the full auto mode, doesn’t stop you from changing other parameters so you can easily make further adjustments.

The P mode should give you correct exposures in most kinds of lighting as well as a good depth of focus that recreates the scene. Further adjustments are available if you need them.

The ‘A’ mode stands for Aperture priority auto exposure. In this mode, you set the lens aperture, while the camera determines the shutter speed. As with the P mode, other parameters remain adjustable. Aperture adjustments allow you to control how much depth in the scene remains sharp.

The A mode works best for blurring out distracting backgrounds when you want to isolate a particular element in the scene.

When you want to blur out distracting backgrounds, use the widest lens aperture possible – it’s indicated by the smallest f-number, usually between f/1.2 and f/4. When photographing a landscape or city scene, to make everything appear sharp from near the camera to the horizon, stop the aperture down to f/8 or smaller.

Choose the S mode and a fast shutter speed when you want to ‘freeze’ moving subjects, particularly if they are passing in front of you.

The ‘S’ mode gives you control over shutter speeds and the ability to decide between ‘freezing’ motion and blurring it. To ‘freeze’ action, select the fastest available shutter speed – typically 1/500 second or faster.

Using a slow shutter speed of one second or more will blur motion. This mode is used for photographs of waterfalls in which the water takes on a silky smoothness. Note: you’ll need a tripod to get those shots.

In Manual mode, which is marked by an ‘M’, you must control all the settings. It is best used when you know what all of the settings do and have a particular objective in mind. On the downside, it requires constant readjustments.

Some manufacturers add Custom memory banks (shown on the mode dial by the letter ‘C’ plus a number). These let you store combinations of camera settings you use a lot so they are quick to access when you need them. Many also provide a ‘B’ (‘bulb’) mode that sets up the camera for very long exposures outside of the default shutter speed range.

Other modes you might find are dedicated movie modes, scene pre-set modes and filter modes that access a library of pre-programmed special effects. Your camera’s user manual outlines what’s available.

Using a histogram

Usually found in the Display sub-menu, the histogram display is a handy tool for checking exposures. This little graph can be overlaid on your camera’s monitor or viewfinder to show if the exposure you’ve set fits into the recording range of the image sensor.

You can see the tonal bias in a scene reflected in the histogram, which is outlined in red for each example.

For evenly-lit scenes, the graph should extend across the base line with the peaks spread out fairly evenly. For dark scenes, the tonal peaks are clustered to the left of the graph, while if you want the scene to look light and bright, they should be towards the right hand end.

Watch out for histograms in which the peaks rise up along the left or right hand ends of the baseline. These indicate ‘clipping’ where tones will not be recorded. When the peak is on the left, shadows will be clipped to record only black; on the right the highlights will be clipped to pure white. Both situations should be avoided.

Metering exposures

All modern cameras come with integrated light meters that automatically measure the light reflected from the scene. Data collected is used for setting the optimal exposure. Most cameras’ meters measure this light using one of three patterns:

1. Evaluative, Matrix or Multi-pattern metering splits the frame into a series of ‘zones’, which are analysed to detect the balance of dark and light tones in each. Some cameras include measurements for colour, subject distances, highlights and shadows in their calculations. Many also use focusing information to prioritise the area you focused on.

This mode is normally the default metering mode because it can work well for a wide variety of different subjects and takes account of differences between a bright sky and darker foreground. But it doesn’t necessarily ensure you’ll get the best results.

Evaluative metering works best when image tones and colours are evenly distributed in the frame. 

Evaluative metering can also be effective for landscape shots with an almost equal balance between sky and foreground.

2. Centre-weighted average metering prioritises measurements from the centre of the frame and largely ignores the corners. It’s a good mode to choose for relatively large subjects that are in the middle of the frame, such as sports action and close-up portraits of humans and animals. It may fall short with backlit subjects, unless they occupy most of the frame.

Centre-weighted average metering works best with subjects that take up most of the frame and are centrally located. However, it should only be used in fairly even lighting as it won’t work well with strongly backlit subjects.

3. Spot metering only measures roughly 2% of the frame around your focus point and ignores everything else. Canon cameras include a Partial metering mode that measures roughly 6% of the frame. Both modes are ideal for shooting backlit subjects and scenes where the background is significantly brighter or darker than the subject.

Spot metering is the only way to obtain a correct exposure of this owl, which is partially obscured by foliage and against a brighter sky in the background.

Spot metering can also be good for photographing birds, wildlife and sports action and close-up shots of flowers and other small subjects. It also works well for bright subjects against a dark background, such as performers on a stage.

Many cameras also provide a Highlight priority metering mode, which is designed to prevent the brighter areas (highlights) from being over-exposed. It’s usually indicated by an icon like the spot metering icon but with a star beside it. Use it carefully as it may cause the rest of the scene to be under-exposed. Blocked-up shadows would be the inevitable result.

When shooting with the sun behind the subject, fill-in flash can be useful if you don’t want a silhouette. You may need to reduce the intensity of the flash by either adjusting it on the flashgun or in the camera. An alternative is to cover the flash head with a diffuser (like a single sheet of tissue), which also softens the light.

Soft fill-in light from a diffused flash prevents this pinnacle from being recorded as a silhouette by balancing out the light on the surface that would otherwise be in deep shade. 

Remember the intensity of the light from a flash decreases with distance. When you double the distance between the flash and the subject, only one quarter of the flash light will reach it.

Overriding metering

While selecting the right metering mode gets you close to the exposure you want, there are times when it’s not quite right. To address this issue, your camera has an exposure compensation adjustment. It’s indicated by a ± icon, which may be on a separate button or dial to make it easy to access.

Moving the compensation towards the + direction increases the brightness of the picture, while adjusting it in the – direction makes the picture darker. In each case, the exposure changes in one third of an f-stop increments, so it’s quite subtle.

While this control is best used to fine-tune exposure levels, some photographers routinely turn the exposure compensation down by one step to keep highlights from blowing out in landscape shots. This technique works best when the scene is generally quite bright. It’s not suitable for dark scenes, where you may need a little more exposure.

Useful links

Understanding exposure

Exposure in low light

This article by Margaret Brown is an excerpt from Take Better Photos pocket guide.

Pocket guide Partner: Camera House