Exposure determination in modern cameras is largely automated, although advanced cameras let photographers override the camera settings and also set exposures manually. An exposure setting has two components: the lens aperture and the shutter speed (how long the light is allowed into the camera). A correct balance between them creates pictures in which all tones in the subject are recorded correctly.
All cameras include exposure meters, which measure the tones in the subject according to a selected pattern. Understanding how metering patterns work will help you to decide which one to use in different situations. Three metering patterns are used in DSLR cameras: multi-pattern evaluative (or matrix), centre-weighted average and spot. Selecting the correct pattern for the subject makes it easier to obtain the correct exposure settings.
Multi-pattern metering divides the subject area into five or more segments and individually evaluates the light levels within each segment. A microprocessor in the camera takes the readings from each segment and biases them according to the difference in overall brightness (and often contrast) within each segment and between adjacent segments. It then calculates which aperture and shutter speed settings will deliver an optimum exposure.
Some cameras also include distance information from the autofocus system and/or colour data. Multi-pattern systems are good all-rounders, providing optimal exposure settings for most types of scenes (including backlit subjects). However, because they deliver an ‘averaged’ exposure setting, they may not provide the best exposure for subjects with a wide dynamic range.
Multi-pattern metering divides the subject into a number of segments and individually measures light levels in each before integrating the data to provide a correct exposure.
Centre-weighted average metering integrates readings from all over the field of view, placing more emphasis on the centre of the field. It’s effective for subjects with an average brightness range where the main area of interest is central. It’s not suitable for shooting bright, contrasty scenes with sand or snow or low-contrast subjects with a limited tonal range. In bright conditions, there’s a tendency towards under-exposure, while poorly-lit subjects with a reduced brightness range are often over-exposed.
Centre-weighted average metering biases exposure levels towards the centre of the frame but takes the remainder of the frame into account.
Spot metering takes a single reading from a small section of the field of view. In most cases, the size of the spot is expressed as a percentage of the field of view, with typical spot sizes ranging from 1% to about 4%. So-called ‘partial’ metering systems have slightly larger metering areas but work on the same principle.) Because areas outside the selected spot are ignored, spot and partial metering are ideal for backlit subjects.
Spot metering takes a single reading from the centre of the frame. It is ideal for backlit subjects.
To use a spot meter, simply centre the spot on the area you want to measure and press the AE lock button and/or press the shutter release half way down. This locks the exposure (and focus), allowing you to re-compose and take the shot by pressing the shutter all the way down. A spot meter can also be used to gauge the brightness range in the subject. Simply measure the brightest and darkest areas and calculate the number of stops between them. These factors make multiple spot metering the best option for digital photographers when shooting wide brightness range subjects.
Exposure compensation allows photographers to alter the settings used by the camera to reduce or increase the overall exposure value. It can be used to make images look lighter or darker and also to ensure highlight or shadow detail are recorded. The control is indicated by a +/- icon, either on a button or in a menu. Pressing the button and turning the command dial adjusts the exposure level.
In most cameras up to two exposure value (EV) steps of adjustment are provided for both over- and under-exposure; usually in 1/3EV increments, although sometimes also in 1/2EV steps. To cancel exposure compensation you must re-set the control to zero.
Auto Exposure (AE) Bracketing
The technique of bracketing, which involves taking a series of shots with slightly different camera settings from those determined by the camera’s automatic measurements, is used to cover uncertainties in exposure and colour balance settings. In auto exposure bracketing (AEB), the camera produces between three and five shots, with the middle shot at the metered values and the others above and below them. The photographer then chooses which of the shots in the series looks best and can delete the others.
Most cameras allow photographers to adjust the size of the AEB steps between +/-2EV but you are usually required to apply the same adjustment to both over- and under-exposure. In burst or self-timer mode, the bracketed exposures are taken continuously; in single shot mode, the photographer may be required to press the shutter button for each shot. Some cameras prevent AEB with flash exposures and AEB is blocked when the shutter is set for Bulb.
AE and AF Locks
Most DSLRs have an AE lock button that lets you lock the exposure on a different part of the subject from the point of focus. This control comes in handy with backlit subjects, particularly when the main subject is off-centre. In most cameras, pressing the shutter button halfway down locks both the AE (auto exposure) and AF (autofocus) settings. So, when you focus on a subject and half-press the shutter button, both focus and exposure are locked and an indicator in the viewfinder should show this to be the case.
To use AE or AF lock with off-centre subjects half-press the shutter button to capture the focus and exposure settings and hold down the AE Lock button. Then, without lifting your fingers, recompose the shot and capture it by pressing the shutter button all the way down. If you want to take more shots with the same setting, keep the AE Lock button down.
In some cameras, the position of the AE lock varies with different metering and focusing modes. With multi-pattern metering it is commonly linked with the selected focus point. With centre-weighted, partial and spot metering, it is applied at the central AF point.
Regardless of whether you have a built-in flash or hot shoe for accessory flash units, there are times when shooting with flash is either necessary or advisable. Flash is equally useful for ‘freezing’ moving objects, capturing shots in dim lighting and balancing the illumination on backlit subjects.
The picture at the top was taken without flash, leaving shadowed areas deficient in detail. In the shot below, fill-in flash adds detail and impact to the photograph. (Images supplied by Canon.)
Most cameras have TTL (through-the-lens) exposure metering systems that measure the amount of flash light reflected off the subject and shut down the flash when enough has been delivered. Consequently, it’s easy for photographers to simply use the auto flash setting for all flash shots. However, better results can be obtained with a few simple strategies.
1. Reduce the flash output. Most DSLRs have a flash exposure compensation setting that lets you adjust the amount of light the flash emits. In many cases, the default setting produces harsh results so, as with exposure, we recommend setting the flash output level to -0.7 or even -1.0 EV. Greater reduction may be required for close subjects and some portraits, especially where softer lighting is required.
2. Bounce the flash. Many add-on flash units have adjustable heads that let you direct the light from the flash towards a ceiling or wall – or hand-held reflector card. The larger the bounce surface, the more it will diffuse the light, making it much softer when it reaches the subject. (Note that the colour of the bounce surface will affect the colour of the light hitting the subject.)
3. Use a diffuser. Many flash units can be fitted with diffusers that cover and soften the light. The effect is not as strong as bouncing but it’s more controllable and there’s less risk of colour casts affecting the shot.
Flash can provide some interesting effects when shooting movement. Most DSLR cameras provide both first-curtain and second-curtain flash synchronisation. First-curtain synch triggers the flash straight after the shutter opens. If the shutter remains open for a second or two and the subject is moving, although the subject’s motion will be ‘frozen’ by the flash, any other moving light in the scene will leave trails in front of the subject as shown in the illustration on the next page.
The top picture was taken with direct flash on the camera, whereas bounce flash was used for the one above.
To make the light trails appear as if they are following the moving subject, you should set the flash to second-curtain synchronisation. This fires the flash just before the shutter closes. The result is a more natural looking depiction of the motion, as shown in the illustration on the next page. The flash exposure ‘freezes’ the movement of the subject, regardless of how long the shutter remains open.
First-curtain synchronisation can cause moving lights to leave trails in front of the subject.
Second curtain synchronisation provides a more natural-looking depiction of the subject’s motion. (Image supplied by Canon.)
Most people have seen red eyes in subjects photographed with flash. The phenomenon occurs because light is emitted from electronic flash units as a very brief burst (typically milliseconds), which is too fast for the iris in the eye to contract the pupil. Consequently the flash light is focused by the lens in the eye onto the blood-rich retina at the back of the eye and reflected back to the camera. (Animals have different coloured retinas so flash shots of dogs often show green eyes, while cats can be blue, yellow or pink.)
The red-eye effect can be prevented by:
1. Using ambient light for the shot.
2. Bouncing the flash.
3. Moving the flash away from the camera’s optical axis so the light hits the eye at an angle.
4. Increasing the ambient lighting so the subject’s pupils close down. (This is the underlying principle behind the red-eye reduction systems built into many cameras.)
5. Having the subject look away from the camera.
6. Processing the image post-capture. Many image editors include easy red-eye removal tools. A few DSLR cameras are supplied with built-in software that can detect and correct red eyes in flash shots. This software is relatively common in compact digicams.
An example of the red-eye effect produced by flash light reflecting off the subject’s retinas.
Article by Margaret Brown – see Margaret’s photography pocket guides